Field of the invention

[0001] The present invention relates to a renewable energy converter. The present invention further relates to a method for inverting power associated with renewable energy.

Background of the invention

[0002] A fundamental task of renewable energy facilities is the adaption of generated electrical energy into AC power which is suitable for a domestic power grid, energy storage etc.. This task is often realized by employing a DC-to-AC inverter.

[0003] One common approach for inverting power is two use sets of power modules which can be controlled to invert DC power into AC power. Different procedures for operating sets of power modules exist. However, a common problem is reducing various loss types, such as switching losses and conduction losses.

[0004] In addition, generation of electrical renewable energy can be subject to unpredictable and variable circumstances, such as changing winds. Accordingly, renewable energy DC-to-AC inverters and their operation procedures should preferably be able to handle these conditions.

Summary of the invention

[0005] The inventors have identified the above-mentioned problems and challenges related to power inversion in relation to renewable energy, and subsequently made the below-described invention which may improve systems and methods within this field.

An aspect of the invention relates to a renewable energy converter comprising at least one phase leg, said at least one phase leg comprising: two DC connections; at least two branches; and a converter controller arrangement configured to control a phase output; wherein each of said at least two branches comprises: a phase output, such that said at least two branches comprises at least two separate phase outputs together forming said phase output; wherein a first of said at least two branches comprises a first set of power modules comprising at least a first power module comprising a switching arrangement connected to said two DC connections and to said phase output; wherein a second of said at least two branches comprises a second set of power modules comprising at least a first power module comprising a switching arrangement connected to said two DC connections and to said phase output; wherein, during a normal cycle, said converter controller arrangement separately controls each of said at least two branches according to an alternating switching procedure which comprises alternatingly coupling said phase output to any of said two DC connections through said first set of power modules and through said second set of power modules, wherein said alternating switching procedure, within said normal cycle, further comprises: an alternating control mode during which said power modules of both of said first and second branches are in a switching stage, and an active control mode during which said power modules of both of said first and second branches are in a non-switching stage, wherein said converter controller arrangement is configured to couple said phase output to any of said two DC connections through said first set of power modules and said second set of power modules parallelly upon occurrence of said non-switching stage.

[0006] In an embodiment of the invention said renewable energy converter is a DC to AC inverter, wherein said converter controller arrangement is an inverter controller arrangement, wherein said cycle is an AC cycle, and wherein said phase output is an AC phase output.

[0007] Normal AC cycle should be understood as an AC cycle established by the DC-to-AC inverter during normal operation thereof. An AC cycle may also be referred to as an AC period having a positive half cycle and a negative half cycle. The inverter controller arrangement / inverter controller may also be referred to as a converter controller arrangement / converter controller.

[0008] In an embodiment of the invention said renewable energy converter is a four quadrant inverter. A four quadrant DC-AC inverter can process power in all four quadrants of voltage and current products, so that any combination of positive/negative voltage and positive/negative current can be obtained. [0009] In an embodiment of the invention, said first branch (5a) and said second branch (5b) are connected to the same of said two DC connection (2,3) during the occurrence of said non-switching stage (16).

[0010] In an embodiment of the invention, said control according to said alternating switching procedure is repeated in both a positive half cycle and a negative half cycle of said normal AC cycle (29).

[0011] In an embodiment of the invention, during a negative power period of said active control mode is substituted with one control mode from the list comprising: alternating non-switching control mode, passive control mode and active switching control mode. The active and passive control modes may also be referred to as both active control mode and both passive control modes.

[0012] During periods with negative power, the passive rectification unit of the switching arrangement may allow current to run through the power module independent of the control of the active switching unit of the switching arrangement.

[0013] More specific with reference to the four quadrants, quadrant 1 (where the voltage is positive and the current is positive (sourcing)) and quadrant 3 (where the voltage is negative and the current is negative (sinking)) the switches are predominantly conducting. Whereas the diodes are predominantly conducting in quadrant 2 (where the voltage is negative and the current is positive (sourcing)) and quadrant 4 (where the voltage is positive and the current is negative (sinking)) (in nonswitching mode). In non-switching mode In quadrant 2 and 4, the system will automatically share the current

[0014] Accordingly, a first half of the AC cycle during normal operation is established by controlling power modules of the first and second branches according to the alternating control mode follow by the active control mode which again is followed by the alternating control mode. Likewise, a second half of the AC cycle during normal operation is established by controlling power modules of the first and second branches according to the alternating control mode follow by the active control mode which again is followed by the alternating control mode. [0015] Controlling the first and second half s of the AC cycle during normal operation according to the alternating / active / alternating control mode sequence is advantageous in that it has the effect, that when controlled according to the active control mode where the two branches share the full current through the DC-to-AC inverter, the total branch current the DC-to-AC inverter is able to handle is increased to twice the size compared to the maximum current allowable during the alternating control mode. Thereby the expected failure rate for the DC-to-AC inverter due to over current is reduced.

[0016] When using an alternating switching procedure, current to be inverted may be alternately passed through two sets of power modules. For example, such that first, current is primarily passed through a first set of power modules, and next, current is primarily passed through a second set of power modules. In this manner, an inverter controller arrangement can thus consecutively switch between using these the two sets of power modules.

[0017] Such an alternating switching procedure typically deviates from, for example, simultaneously passing current through all power modules. Actually, by only coupling one of two sets of power modules, typically only half of the power modules are fully utilized most of the time.

[0018] Typically, a central aspect of alternating switching procedures is thus actively avoiding the use of a large fraction (typically half) of the available power modules.

[0019] Despite this apparent restriction, employing an alternating switching procedure comes with various advantages, making such procedures attractive to use. Switching may permit improved establishment of a sinusoidal signal, in comparison with other methods. Particularly, establishment of a sinusoidal signal with minimized switching losses, for example due to zero-current switching.

[0020] Nevertheless, since current is typically primarily sent through one of two sets of power modules, the current in the individual components, such as switches, is typically much larger than in inverters not relying on an alternating switching procedure. This may result in various problems. As an example, the loss through, e.g., a resistor is proportional to the current squared. Hence, despite other advantages of alternating switching procedures, conduction losses may typically be increased. Furthermore, the individual components, such as switches, typically have specifications such as upper boundaries within which they can be safely operated in relation to currents, temperature, and potentially even durations at particular currents/temperatures. By employing an alternating switching procedure, such upper boundaries may generally pose more significant restrictions than in conventional, nonalternating switching procedures.

[0021] Some alternating switching procedures comprises both stages with and without switching, e.g. a switching stage and a non-switching stage, within each AC cycle. In another example, in a three-phase system without a neutral return, the sum of the three-phase voltages and three currents is zero at any instant. One can therefore deliberately clamp the output AC voltage of one phase to either of the two DC connections. By offsetting the modulated voltages of the two remaining phases accordingly, keeping the sum of output voltages zero, all three phase currents can be controlled to provide a desired output. Typically, such clamping is most beneficial to perform in the peak of the AC current to reduce the switching losses at the high AC current levels. A typical clamping scheme is clamping in within plus/minus 30 degrees around the peak current, shifting between the phases and positive/negative polarity. Such methods are typically referred to as “flat-top modulation”, “discontinuous PWM modulation”, or “clamp PWM”.

[0022] According to embodiments of the invention, the inverter controller arrangement is configured to couple any of the two DC connections through the first set of power modules and the second set of power modules parallelly upon occurrence of such a non-switching stage.

[0023] To parallelly couple both sets of power modules goes directly against the teachings of conventional alternating switching procedure, where the entire aim is to actively avoid using some of the power modules. Otherwise, any advantages offered by alternating switching procedures may not be available in the first place. [0024] Nevertheless, since the solutions and advantages offered by alternating switching procedures may temporarily be halted due to the occurrence of a nonswitching stage, this temporary window may be utilized to perform parallel coupling of both sets of power modules. By parallel should be understood simultaneously.

[0025] This conditional initiation of parallel coupling provides several advantages. In particular, the conduction losses may be significantly reduced. These are typically proportional to the current squared in busbars and MosFETs, and thus, distributing the current to several sets of power modules advantageously reduces the conduction losses. When using IGBTs the current is more often closer to be lifted in the 1.6 potence than in the second potence.

[0026] Furthermore, since non-switching stages may often occur during peak currents of the AC cycle, a parallel coupling may extend the maximum currents under which an alternating switching-based renewable energy DC-to-AC converter may be operated. In contrast, conventional solutions may often be significantly restricted by specifications of the components, such as an upper current. For a given type of component with a given set of specifications, embodiments of the invention may thus enable operation at higher currents. Additionally, embodiments of the invention may permit selecting cheaper or smaller components, since the restrictions set by specifications are less of a problem.

[0027] In addition, a parallel coupling via both sets of power modules automatically reduces risk of malfunctions such as breakdowns due to over-current and reduces the impact of such malfunctions if they occur, which is advantageous. However, note that embodiments of the invention are typically not implemented by parallelly coupling the two sets of power modules on a condition based on a current threshold, which could be one way of implementing a dedicated safety procedure. Currents inverted by renewable energy DC-to-AC inverters may typically vary, for example due to varying winds. Thus, using the occurrence of a non-switching stage as a condition, in contrast to using a particular current is more suitable. Nevertheless, embodiments of the invention may be implemented with both a non-switching stage and a current threshold as conditions for initiating a parallel coupling of the two sets of inverted legs, for example to ensure an advantageous safety procedure in addition to other solutions provided by the invention.

[0028] Generally, a DC-to-AC inverter may be understood as a power electronic device or circuitry that changes/inverts direct current into alternating current. And a renewable energy DC-to-AC inverter may be understood as a DC-to-AC inverter arranged to invert direct current into alternating current in association with a renewable energy facility, such as a wind turbine. Renewable energy facilities pose unique challenges in relation to conversion and inversion of power. In particular, the power which has to be inverted may typically be variable in an unpredictable manner, for example since wind powering wind turbines is variable, or since sunlight powering photovoltaic cells is variable. Hence, solutions provided by conventional inverters and converters may not be applicable within the field of renewable DC-to-AC inverters.

[0029] Examples of renewable energy facilities are renewable energy power plants, such as wind turbines, photovoltaic power stations, and hydropower power plants. Another type of renewable energy facility is a power storage facility, such as a batterybased power storage facility.

[0030] A renewable energy DC-to-AC inverter according to the invention may be combined with an AC-to-DC converter to form an AC-to-AC converter, for example for an AC-to-AC converter of a wind turbine, arranged to convert AC power generated by an electrical generator connected to a generator of the wind turbine into AC power suitable for the mains power grid.

[0031 ] Embodiments of the invention comprises two DC connections. In this context, these two DC connections may be understood as a DC power source from which the renewable energy DC-to-AC inverter draws DC power to be inverted into AC power. The two DC connections may thus typically comprise a positive DC connection and a negative DC connection. The positive DC connection and the negative DC connection may alternatively be referred to as a DC+ connection and a DC- connection

[0032] An inverter controller arrangement may be understood as a system, an apparatus, one or more processors, specially adapted circuitry, or any combination thereof configured to control operation of at least the branches, and thereby control operation of the renewable energy DC-to-AC inverter. In particular, the inverter controller arrangement may control operation of the branches by controlling semiconductor switches of the branches. Such control may typically be performed via individual gating signals provided to switches of the semiconductor switches of the power modules. One example of an inverter controller arrangement is a controller such as a processor programmed to control operation of the branches. An inverter controller arrangement may also be facilitated by one or more field-programmable gate arrays configured to control the branches. However, an inverter controller arrangement is not necessarily limited to a single device, but may be a distributed control system of various components which collectively control the branches via their semiconductor switches. For example, each of the modules may have a separate control device of the inverter controller arrangement. Generally, a renewable energy DC-to-AC inverter having an inverter controller arrangement may be considered as a renewable energy DC-to-AC inverter system.

[0033] A phase leg may be understood as a section of the renewable energy DC-to- AC inverter configured to provide a phase of an electrical power output of the inverter. Thus, each phase leg has an AC phase output. Typical embodiments of an inverter has three phase legs, wherein each of the three phase legs provides an AC phase output, such that the phase legs collectively provide three-phase electric power, for example to a mains power grid electrolyser, energy storage, etc. The phase legs may typically be substantially similar but are preferably controlled separately (such that, e.g., different phases are established). Each of the phase legs comprises two branches

[0034] Each branch comprises power modules, and, typically, each power module comprises at least two semiconductor switches. These semiconductor switches may serially connect the two DC connections, but current flow should typically only be permitted from one of the DC connections at a time. Thus, even though a power module and its semiconductor switches may be connected to both DC connections, only one of the DC connections are actually coupled to the AC phase output at a time. The AC phase output of a branch may, for example, be connected to a node between two semiconductor switches (and between the two DC connections) of each of the power modules of that branch. The inverter controller arrangement then controls whether one or the other of the two DC connection is coupled to the AC phase output by opening one or the other of the two semiconductor switches accordingly.

[0035] In embodiments of the invention, power modules within each branch are grouped into a first set of power modules and a second set of power modules. Each set of power modules may then comprise one or more power modules, such as one inverter leg, two power modules, three power modules, four power modules, or more than four power modules. Each set of power modules may have a substantially similar topology and substantially similar sub-components (semiconductor switches, etc.) but may be controlled separately, at least temporarily. For example, during a switching stage, the first set of power modules and the second set of power modules are controlled alternatingly, such that in a given phase leg, power is provided from the DC connections to the AC phase output in alteration between primarily the first set of power modules and primarily the second set of power modules. However, the first set of power modules and the second set of power modules are not necessarily always controlled separately. For example, when a non-switching stage is identified, both the first set of power modules and the second set of power modules may simultaneously provide power from one of the DC connections to the AC phase output.

[0036] As a branch is controlled according to an alternating switching procedure, the inverter controller arrangement may alternate between conducting current primarily via the first set of power modules and via the second set of power modules, particularly during switching stages. This alteration may for example be controlled via a modulation signal with a modulation frequency upon which switching between coupling via the first and seconds sets of power modules is established. The resulting switching frequency of the alternating switching procedure during switching stages is typically significantly larger than the mains AC frequency. Although current may primarily be conducted via one set of power modules, small amounts of current may be conducted via the other set of power modules, particularly in the short time interval just after switching between the two sets of power modules. [0037] In embodiments of the invention, the alternating switching procedure comprises a switching stage and a non-switching stage within each AC cycle of the AC phase output, which are separated in time and indicative of occurrence of switching in said alternating switching procedure. The stages are separated in time in the sense that, for a given branch, at a given time, only one of the stages are occurring. In other words, the switching stage and the non-switching stage may not occur at the same time. In embodiments of the invention, during each AC cycle (for example, during a period of 20 milliseconds, given a 50 Hz mains frequency), the alternating switching procedure goes through both the switching stage and the non-switching stage. For example, during each peak period (positive and negative) of the AC cycle, the alternating switching procedure may enter a non-switching stage, while during the remainder of the AC cycle, the alternating switching procedure is in a switching stage. Such a non-switching stage may for example take place since the AC signal is substantially flat during AC cycle peaks, and, accordingly, switching may be redundant. An example of a non-switching stage is a clamping stage. An example of a modulation signal which may result in both a switching stage and a non-switching stage is flat top modulation, such as a flat top pulse width modulation.

[0038] The switching stage and the non-switching stage are indicative of occurrence of switching (in said alternating switching procedure). In other words, the switching stage and the non-switching stage are indicative of how often or how frequently switching occurs. During switching, the AC output potential (denoted 6 on fig. la) is changing between the first and second DC potentials (denoted 2, 3 on fig. la). Note that embodiments of the invention are not limited to a fixed switching frequency, but that the switching frequency may vary, for example vary during an AC cycle.

[0039] The inverter controller arrangement may be configured to couple any of the two DC connections through the first and the second set of power modules parallelly upon occurrence of the non-switching stage within the AC cycle. Such an occurrence may for example be identified by monitoring a modulation signal used to control switching of one or more of the power modules, and if this signal is constant for a predetermined period of time, such as at least 0.5 milliseconds, at least 1 millisecond, or at least 2 milliseconds, the stage of the alternating switching procedure is qualified as a non-switching stage. Or the inverter controller arrangement may use a-priori knowledge to determine the occurrence of a non-switching stage, for example in case the occurrence of a non-switching stage is imprinted or coded into the generation of the modulation signal itself. One other example of an implementation of a condition for initiating a parallel coupling of the first and second sets of power modules is a voltage threshold. Alternatively, the duration of parallelly coupling through both the first set and second set of power modules may be manually set upon installation of the renewable energy DC-to-AC inverter, for example manually set to always occur within one or more parts of the AC cycle.

[0040] Coupling any of the two DC connections through the first set of power modules and the second set of power modules parallelly may for example be understood as substantially similar gating signals being sent to power modules of said first set of power modules and power modules of said second set of power modules. Typically, during most of the switching stage, the inverter controller arrangement alternates between coupling the DC connections to the AC phase output primarily through the first set of power modules and primarily through the second set of power modules. In contrast, during the parallel coupling, which may be initiated upon occurrence of the non-switching stage, the inverter controller arrangement conduct current through the first set power modules and the second set of power modules in parallel. However, although current is conducted in parallel, the current in the first set of power modules and the current in the second set of power modules is not necessarily identical. In particular, this may be the case just after the inverter controller arrangement has switched from the alternating switching mode to the direct parallel mode, which results in a transitional period in which the currents in two sets of legs gradually approach each other in magnitude, coming from a situation in which the current was primarily in one of the two sets of legs.

[0041] One way of quantifying the act of parallelly coupling of the first set of power modules and the second set of power modules is in relation to the act of alternatingly coupling the AC phase output to any of said two DC connections through the first set of power modules and through the second set of power modules. For example, the current in the first set of power modules relatively to the current in the second set of power modules is closer to unity while said two DC connections are coupled through the first set of power modules and the second set of power modules parallelly upon occurrence of the non-switching stage than while alternately coupling the AC phase output to any of the two DC connections through the first set of power modules and through the second set of power modules. One current relatively to another current being closer to unity may be measured as an average, for example as an average during half a period of a modulation signal.

[0042] Generally, note that the duration of the parallel coupling does not necessarily match the duration of the non-switching stage. For example, in some embodiments, the inverter controller arrangement may be configured to identify the occurrence of a non-switching stage by monitoring a modulation signal, and accordingly, the parallel coupling can only be initiated after the current stage/period of the alternating switching procedure is qualified as a non-switching stage. Accordingly, the parallel coupling may be slightly shorter than the actual period in which no switching occurs. Moreover, some embodiments may have safety precautions which overwrite the alternating switching procedure and initiate parallel coupling before a non-switching stage would otherwise occur, for example due to, e.g., a transient overcurrent which could otherwise damage components such as switches. Thus, parallel coupling may occur outside of non-switching stages as well.

[0043] In embodiments of the invention, said alternating switching procedure comprises altematingly coupling said phase output to any of said two DC connections through primarily said first set of power modules and through primarily said second set of power modules.

[0044] In embodiments of the invention, said switching stage comprises conducting at least 70 percent of a total branch current via one of said first set of power modules and said second set of power modules. [0045] The total branch current is the instantaneous current from said two DC connections to said AC phase output of one of said at least two branches. Hence, the switching stage during the alternating switching procedure may comprise alternating between conducting at least 70 percent of the total branch current via the first set of power modules and 70 percent of the total branch current via the second set of power modules. In some embodiments, the switching stage comprises conducting at least 80 percent, such as at least 90 percent, of the total branch current via one of said first set of power modules and said second set of power modules.

[0046] In embodiments of the invention, each power module of said first set of power modules and said second set of power modules comprises at least two semiconductor switches arranged between said two DC connections and is connected to said phase output between said at least two semiconductor switches.

[0047] Such at least two semiconductor switches may for example be arranged in a half-bridge configuration between the two DC connections.

[0048] In embodiments of the invention, each of said at least two semiconductor switches comprises an active switch unit.

[0049] Examples of active switch units of a semiconductor switch are semiconductor devices such as insulated gate bipolar transistors (IGBTs), thyristors, and metal- oxide-semi conductor field-effect transistors (MOSFETs), such as power MOSFETs.

[0050] In embodiments of the invention, each of said at least two semiconductor switches comprises a passive rectification unit.

[0051] A passive rectification unit may for example be a diode. The passive rectification unit may have opposite directionality in comparison with the active switch unit.

[0052] In an embodiment, said alternating switching procedure further comprises an active control mode during which power modules of both said first and second set of power modules are in a switching stage enabled during zero crossing of the current. [0053] The hard paralleling of the two branches during zero crossing is advantageous in that in the practice implementation of the alternating switching control mode, the timing of when branches should conduct and not conduct depends on direction of current. Hence a risk exists, that the controller does not have any current direction if the current direction is established at the zero current crossing.

[0054] In embodiments of the invention, said at least two branches are three branches, wherein said at least two separate phase outputs are three separate phase outputs which collectively provide three-phase electric power of said renewable energy converter.

[0055] In embodiments of the invention, said renewable energy converter is a DC- to-AC inverter which is combined with an AC-to-DC converter to form a renewable energy AC-AC converter.

[0056] In embodiments of the invention, said converter controller arrangement is configured to track said alternating switching procedure and couple any of said two DC connections through said first set of power modules and said second set of power modules parallelly upon occurrence of said non-switching stage.

[0057] In embodiments of the invention, said converter controller arrangement tracks said alternating switching procedure to detect said occurrence of said non-switching stage by monitoring a voltage of said phase output.

[0058] The monitoring of voltage may include monitoring of angle and amplitude not only of voltage, but also of current. Further, it should be noted, the measurements from all phases of the inverter may form basis for the detection.

[0059] In embodiments of the invention, said converter controller arrangement tracks said alternating switching procedure to detect said occurrence of said non-switching stage by monitoring a phase angle of a current or voltage of said phase output. This is of course most relevant if the phase output is an AC phase output

[0060] In embodiments of the invention, said converter controller arrangement tracks said alternating switching procedure to detect said occurrence of said non-switching stage by monitoring a modulation signal provided to any power modules of said first set of power modules and said second set of power modules.

[0061] By tracking the alternating switching procedure (for example by monitoring voltage, phase, or modulation signal), a certainty of timely parallel coupling through both sets of power modules may be ensured, which is advantageous. In particular, any of these parameters may typically be independent of the actual currents inverted by the renewable energy DC-to-AC converter, which may otherwise vary due to varying energy inversion conditions, such as varying winds. It should be noted, that the modulation signal may be found inside a DSP, FPGA, or similar and not possible to measure physical.

[0062] In embodiments of the invention, a coupling timing of coupling said first set of power modules and said second set of power modules parallelly is predefined relatively to an AC cycle provided to said AC phase output.

[0063] By having a predefined timing of the parallel coupling of both sets of converter legs in relation to the AC cycle, the implementation of the parallel coupling may be simplified, relatively to other solutions such as by tracking, which is advantageous.

[0064] In embodiments of the invention, said converter controller arrangement is configured to decouple one of said first set of power modules and said second set of power modules from any of said two DC connections upon expiry of said nonswitching stage.

[0065] Accordingly, the procedure of altematingly coupling said AC phase output to any of said two DC connections through said first set of power modules and through said second set of power modules can be continued once again.

[0066] The decoupling may advantageously be based on the same condition as the parallel coupling, for simplified operation. [0067] In embodiments of the invention, said non-switching stage is associated with a non-switching duration of at least 0.5 milliseconds, for example at least 1.0 milliseconds, for example at least 2.0 milliseconds, such as at least 3.0 milliseconds.

[0068] In embodiments of the invention, a switching period of said alternating switching procedure in said switching stage is shorter than said non-switching duration.

[0069] A switching period may be understood as a duration of a switching cycle during the switching stage. The switching cycle may be understood as alternately coupling the first set of inverter modules and the second set of inverter modules once, i.e. coupling each set of inverter modules once.

[0070] In embodiments of the invention, a switching frequency associated with said switching stage is at least 1.0 kHz, for example at least 1.5 kHz, for example at least 2.0 kHz, such as at least 3.0 kHz.

[0071] The switching frequency is not necessarily constant during the switching stage, but may in embodiments of the invention be at least, e.g., 1.0 kHz temporarily, for example for the duration of a switching cycle. The switching frequency and the switching period may typically be inversely proportional.

[0072] In embodiments of the invention, said converter controller arrangement separately controls each of said at least two branches such that said non-switching stage extends across each peak period of each AC cycle in each individual inverter module of said at least two inverter modules.

[0073] Having a non-switching stage with parallel coupling is particularly advantageous during peak periods of the AC cycle. Note, that the peak period is either a voltage or a current peak period.

[0074] In an embodiment, said converter controller arrangement controls said nonswitching stage from 60 degrees to 120 degrees and from 240 degrees to 300 degrees of the AC output voltage or current. [0075] In embodiments of the invention, each of said at least two branches are controlled separately by said converter controller arrangement such that parallel coupling of any of said two DC connections through said first set of power modules and said second set of power modules is controlled independently in each of said at least two branches.

[0076] In embodiments of the invention, said converter controller arrangement separately controls said at least two branches such that any of said two DC connections are coupled through said first set of power modules and said second set of power modules parallelly in maximally one of said at least two branches at a time.

[0077] Having a DC connection being coupled through both the first and second sets of power modules in maximally one branch at a time may optionally be during normal operation, e.g. without including additional safety procedures in which durations of parallel coupling in the different branches may be extended.

[0078] Having a DC connection being coupled through both the first and second sets of power modules in maximally one branch at a time may ensure that the invention can be easily combined with modulations methods such as flat-top modulation, discontinuous PWM modulation, or clamp PWM, which is advantageous.

[0079] In embodiments of the invention, said direct parallel mode comprises coupling said phase output to any of said two DC connections through said first set of power modules and said second set of power modules parallelly and continuously.

[0080] In embodiments of the invention, said direct parallel mode comprises conducting at least 35 percent of a total branch current via each of said first set of power modules and said second set of power modules.

[0081 ] For example, at one point in time while a branch is being controlled according to the direct parallel mode, 45 percent of the total branch current may be conducted through the first set of power modules and 55 percent of the total branch current may be conducted through the second set of power modules. [0082] In some embodiments, the direct parallel mode comprises conducting at least 40 percent, such as at least 45 percent, of the total branch current via each of said first set of power modules and said second set of power modules.

[0083] In embodiments of the invention, each inverter leg of said first set of power modules and said second set of power modules is individually connected to said phase output via a respective single-leg inductor.

[0084] In embodiments of the invention, said respective single-leg inductor of each inverter leg of said first set of power modules and said second set of power modules is configured to reduce reverse current during said switching stage.

[0085] Thus, each inverter leg of the first and second set of power modules may have an associated output single-leg inductor. Such a single-leg inductor may preferably be a non-linear inductor having a non-saturated inductance of at least 50 microhenry. Having such inductors may advantageously reduce reverse current through power modules in which reverse current is not intended, which is advantageous.

[0086] In embodiments of the invention, each respective inverter leg of said first set of power modules is collectively connected to said phase output together with a respective inverter leg of said second set of power modules via a set-pairing inductor.

[0087] In embodiments of the invention, said set-pairing inductor is configured to stabilize the current of said phase output across said switching stage and said nonswitching stage hereunder across parallelly coupling of any of said two DC connections through said first set of power modules and said second set of power modules.

[0088] By having a set-pairing inductor, any disturbances in current flow through this inductor, occurring due to switching between sets of power modules and due to initiating/ending parallel decoupling, may be simultaneously reduced, which is advantageous. In other words, a set-pairing inductor may advantageously smoothen the output current. Noise may potentially both be introduced by the switching stage of the alternating switching procedure and be introduced by the parallel coupling. Having a set-pairing inductor is thus particularly beneficial within the framework of the since it may reduce noise from both of these sources, which is advantageous.

[0089] The inductance of the set-pairing inductors may for example be at least 50 microhenry, for example at least 200 microhenry, such as at least 1 millihenry.

[0090] An example of an inductor is a choke, such as a ferrite ring choke.

[0091] In embodiments of the invention, each respective inverter leg of said first set of power modules is collectively connected to said phase output together with a respective inverter leg of said second set of power modules via a respective current probe.

[0092] By collectively connecting power modules of the first and second sets of power modules to the AC phase output in a pairwise manner via a respective current probe, that current probe can measure the combined current coming from an inverter leg from each of the two sets of power modules. A current probe may thus, for example measure the combined current of a first inverter leg of the first set of power modules and a first inverter leg of the second set of power modules. In typical embodiments, the inverter controller arrangement aims to provide a current at such a current probe which is independent of mode.

[0093] In embodiments of the invention, an output of said current probe is basis for said converter controller arrangement used when stabilizing the current of said phase output across said switching stage and said non-switching stage hereunder across parallelly coupling of any of said two DC connections through said first set of power modules and said second set of power modules.

[0094] In embodiments of the invention, said converter controller arrangement is configured to pre-emptively couple any of said two DC connections through said first set of power modules and said second set of power modules parallelly prior to occurrence of said non-switching stage when a current output exceeds a switching current threshold. [0095] Here, pre-emptively should be understood in the context of normal operation of the renewable energy DC-to-AC inverter, in which parallel coupling through both sets of power modules typically is performed in a particular interval of each AC cycle. Pre-emptively may thus be understood as performing this parallel coupling earlier than during normal operation (determined by the switching current threshold). For example, parallel coupling may be initiated already during the switching stage.

[0096] Such a pre-emptive parallel coupling upon exceeding a switching current threshold may that over-currents occur, which is advantageous. Furthermore, not only may over-currents be avoided, but this avoidance may be implemented without reducing the output current.

[0097] Thus, parallel coupling may be employed as a safety procedure, as well as an conduction loss procedure, which is advantageous. In particular, these various types of parallel couplings based on different conditions (non-switching period and current threshold) may be facilitated by at least some of the same components and/or control procedures, which is advantageous.

[0098] In embodiments of the invention, said converter controller arrangement is configured to couple any of said two DC connections through said first set of power modules and said second set of power modules parallelly upon occurrence of said nonswitching stage and independently of said switching current threshold.

[0099] In embodiments of the invention, said converter controller arrangement is configured to restrict a current output to a parallel current threshold while any of said two DC connections are coupled through said first set of power modules and said second set of power modules parallelly.

[0100] In embodiments of the invention, said first set of power modules comprises two or more power modules and said second set of power modules comprises two or more power modules. This is advantageous in that the more power modules, the higher current each branch can conduct. [0101] In an embodiment of the invention, said energy converter is a DC to DC inverter, wherein said converter controller arrangement is a DC-DC controller arrangement, and wherein said phase output is an output to a DC-DC choke or transformer.

[0102] Accordingly, such DCDC inverter may be bidirectional or unidirectional where power can origin from DC link to output or from output to DC link. This makes the converter of the present invention suitable for use in relation to solar systems, battery storage, P2X such as electrolysers, etc.

[0103] It should be mentioned, that the arrangement of semiconductor switches in the converter can modulate both AC and DC output voltages (and this independent on type of input). Hence, the invention can be used both for AC/DC, DC/ AC and DC/DC conversion. Thus, the converter may on one side (input) be connected to a power generating plant such as a DC solar plant or an AC wind turbine. On the other side (output) the converter may be connected to a DC choke or transformer if the output is a DC and to various kinds of AC loads if the output is AX.

[0104] An aspect of the invention relates to a method for inverting power associated with renewable energy, said method comprising the steps of: providing at least two branches in a renewable energy converter, wherein each of said at least two branches comprises a phase output, wherein a first of said at least two branches comprises a first set of power modules, and wherein a second of said at least two branches comprises a second set of power modules; separately controlling each of said at least two branches according to an alternating switching procedure which comprises alternately coupling said AC phase output to any of two DC connections through said first set of power modules and through said second set of power modules, wherein said alternating switching procedure, within a normal AC cycle (29), comprises an alternating control mode during which said power modules (7a, 7b, 7c, 7d) of both of said first and second branches (5a, 5b) are in a switching stage (15), and an active control mode during which said power modules (7a, 7b, 7c, 7d) of both of said first and second branches (5a, 5b) are in a non-switching stage (16), and coupling said phase output (6) to any of said two DC connections through said first set of power modules and said second set of power modules parallelly upon and during occurrence of said non-switching stage.

[0105] Any of the steps of separately controller each of the at least two branches and coupling any of the two DC connections may for example be performed by an inverter controller arrangement.

[0106] An aspect of the invention relates to a method wherein said renewable energy converter is a DC to AC inverter, wherein said phase output is an AC phase output, and wherein said normal cycle is a normal AC cycle.

[0107] In embodiments of the invention, said renewable energy converter of said method is the renewable energy converter according to any embodiments of this disclosure.

[0108] An aspect of the invention relates to a renewable energy DC-to-AC inverter system comprising: two DC connections; at least two branches; and an inverter controller arrangement configured to control an AC cycle of an AC phase output; wherein each of said two branches comprises an AC phase output, such that said at least two branches comprises at least two separate AC phase outputs together forming said AC phase output; wherein a first of said at least two branches comprises a first set of power modules comprising at least a first power module comprising a switching arrangement connected to said two DC connections and to said AC phase output; and wherein a second of said at least two branches comprises a second set of power modules comprising at least a first power module comprising a switching arrangement connected to said two DC connections and to said AC phase output; wherein, during a normal AC cycle, said inverter controller arrangement separately controls each of said at least two branches according to an alternating switching procedure which comprises alternatingly coupling said AC phase output to any of said two DC connections through said first set of power modules and through said second set of power modules, wherein said alternating switching procedure, within said normal AC cycle, further comprises: an alternating control mode during which said power modules of both of said first and second branches are in a switching stage, and an active control mode during which said power modules of both of said first and second branches are in a non-switching stage wherein said inverter controller arrangement separately controls each of said at least two branches with respect to a first current threshold and a second current threshold greater than said first current threshold, wherein said inverter controller arrangement is configured to couple said AC phase output to any of said two DC connections through said first set of power modules and said second set of power modules parallelly when a current output of said renewable energy DC-to-AC inverter system exceeds said first current threshold, wherein said inverter controller arrangement is configured to restrict said current output to said second current threshold.

[0109] In an inverter system controlled according to an alternating switching procedure, having a first and a second current threshold which provide separate restrictions/actions upon the inverter system may provide improved protection and operation, which is advantageous. In contrast having a single current threshold may not provide both optimal flexibility and safety during operation.

[0110] During operation, different current magnitudes may be conducted through different parts of the inverter system, such as different currents through different power modules. Such power modules may typically have an upper current limit within which they are specified to operate without malfunction. If a single universal current threshold is implemented, it may not be possible to utilize all parts of a renewable energy DC-to-AC inverter system fully, up to their respective upper current limits.

[0111] For example, when performing parallel coupling, the current through a given inverter leg may be significantly less than while not performing parallel coupling, such as while alternately coupling through the first and second sets of power modules. Having both a first and a second current threshold may thus successfully handle such different circumstances, to advantageously utilize components of the inverter system in a larger range.

[0112] Alternatively, the first current threshold may also be referred to as a switching current threshold, and the second current threshold may be referred to as a parallel current threshold.

[0113] Note that embodiments of the invention having a first current threshold and a second current threshold are not necessarily restricted to having an alternating switching procedure with a switching stage and a non-switching stage.

[0114] In embodiments of the invention, said current output is a sum of at least a current of an inverter leg of said first set of power modules and a current of an inverter leg of said second set of power modules.

[0115] In embodiments of the invention, said current output is measured at a common output of an inverter leg of said first set of power modules and of an inverter leg of said second set of power modules.

[0116] By measuring the current output as a sum of currents from both sets of power modules, the alternating coupling of sets of power modules provided by the alternating switching procedure may be taken into account in the safety procedure offered by the invention, which is advantageous.

[0117] In embodiments of the invention, said second current threshold is twice as large as said first current threshold

[0118] In embodiments of the invention, said alternating switching procedure comprises alternatingly coupling said AC phase output to any of said two DC connections through primarily said first set of power modules and through primarily said second set of power modules.

[0119] In embodiments of the invention, said alternating switching procedure comprises conducting at least 70 percent of a total branch current via one of said first set of power modules and said second set of power modules.

[0120] In embodiments of the invention, each inverter leg of said first set of power modules and said second set of power modules comprises at least two semiconductor switches arranged between said two DC connections and is connected to said AC phase output between said at least two semiconductor switches.

[0121] In embodiments of the invention, each of said at least two semiconductor switches comprises an active switch unit.

[0122] In embodiments of the invention, each of said at least two semiconductor switches comprises a passive rectification unit.

[0123] In embodiments of the invention, said at least two branches are three branches, wherein said at least two separate AC phase outputs are three separate AC phase outputs which collectively provide three-phase electric power of said renewable energy DC-to-AC inverter system.

[0124] In embodiments of the invention, said renewable energy DC-to-AC inverter system is combined with an AC-to-DC converter to from a renewable energy AC-AC converter system. [0125] An aspect of the invention relates to use of a first current threshold and a second current threshold greater than said first current threshold in a renewable energy DC-to-AC inverter system, wherein said renewable energy DC-to-AC inverter system comprises: two DC connections; at least two branches; and an inverter controller arrangement configured to control an AC cycle of an AC phase output; wherein each of said two branches comprises an AC phase output, such that said at least two branches comprises at least two separate AC phase outputs together forming said AC phase output; wherein a first of said at least two branches comprises a first set of power modules comprising at least a first power module comprising a switching arrangement connected to said two DC connections and to said AC phase output; and wherein a second of said at least two branches comprises a second set of power modules comprising at least a first power module comprising a switching arrangement connected to said two DC connections and to said AC phase output; wherein, during a normal AC cycle, said inverter controller arrangement separately controls each of said at least two branches according to an alternating switching procedure which comprises alternatingly coupling said AC phase output to any of said two DC connections through said first set of power modules and through said second set of power modules, wherein said alternating switching procedure, within said normal AC cycle (29), further comprises: an alternating control mode during which said power modules (7a, 7b, 7c, 7d) of both of said first and second branches (5a, 5b) are in a switching stage (15), and an active control mode during which said power modules (7a, 7b, 7c, 7d) of both of said first and second branches (5a, 5b) are in a non-switching stage wherein said inverter controller arrangement separately controls each of said at least two branches with respect to a first current threshold and a second current threshold greater than said first current threshold, wherein said inverter controller arrangement is configured to couple said AC phase output to any of said two DC connections through said first set of power modules and said second set of power modules parallelly when a current output of said renewable energy DC-to-AC inverter system exceeds said first current threshold, wherein said inverter controller arrangement is configured to restrict said current output to said second current threshold.

[0126] In embodiments of the invention, said renewable energy DC-to-AC inverter system of said use is a renewable energy DC-to-AC inverter system according to any of the renewable energy DC-to-AC inverter systems of the disclosure.

The drawings

[0127] Various embodiments of the invention will in the following be described with reference to the drawings where fig. la-d illustrate a renewable energy DC-to-AC inverter according to an embodiment of the invention where fig. lb shows a branch and its connections and fig. 1c shows an inverter leg and its connections and fig. Id shows an inverter having three phase legs, fig. 2a-e illustrate output currents of power modules of different sets of power modules and their associated control signals and duty cycle equivalent, fig. 3 illustrates examples of inductors and current probes according to embodiments of the invention, fig. 4 illustrates method steps according to an embodiment of the invention, fig. 5 illustrates current outputs under influence of current thresholds according to embodiments of the invention, fig. 6 illustrates zero current crossing according to an embodiment of the invention, and fig. 7 illustrates dynamic safety limit control according to an embodiment of the invention.

Detailed description

[0128] Fig. la-c illustrate a phase leg 1 of a renewable energy DC-to-AC inverter according to an embodiment of the invention where fig. lb shows a branch 5 and its connections, fig. 1c shows a power module 7 and its connections and fig. Id shows a renewable energy DC-to-AC inverter 33 having three phase legs.

[0129] In fig. la, two DC connections 2,3 are illustrated, namely a DC+ connection 2 and a DC- connection 3. The DC+ connection 2 provides a positive voltage output in comparison with the DC- connection 3. Thus, DC current may be drawn from these connections to be inverted into AC current. The DC connections 2,3 may for example provide energy from a renewable energy facility. It should be noted, that several internal DC connections may be found in the inverter.

[0130] The figure further illustrates two branches 5 a, 5b. Each of these branches 5a, 5b are connected to both the DC+ connection 2 and the DC- connection 3. Furthermore, each of the branches 5a, 5b provide an AC phase output 6a, 6b. Namely, a first branch 5a provides a first AC phase output 6a and a second branch 5b provides a second AC phase output 6b. Some embodiments further comprise a third branch which provides a third AC phase output.

[0131] Additionally, the figure shows an inverter controller arrangement 4. As indicated by the dashed lines, the inverter controller arrangement 4 is in communication with the two branches 5a, 5b. Accordingly, the inverter controller arrangement 4 is capable of controlling these branches 5a, 5b. In particular, the inverter controller arrangement 4 is configured to separately control each of the two branches according to an alternating switching procedure.

[0132] In fig. lb, the phase leg 1 is illustrated in greater detail. Both of the two branches 5a, 5b illustrated in fig. la may have the same topology as illustrated in fig. lb. In this particular embodiment, each of the branches 5a, 5b comprises a set of power modules 8, 9 and here these sets of power modules each comprises two power modules 7a-d. The number of power modules in a branch 5 is a design choice depending on the maximum current that has to be conducted by the branch. Hence, a branch may comprise 1, 2, 3, 4, etc. power modules operating synchronously. Two or more power modules of a branch may be referred to as a set of power modules. In other embodiments, a branch 5 may comprise additional components, for example current probes, additional semiconductor elements, and/or output inductors such as air-core inductors, ferromagnetic core inductors, or chokes.

[0133] Even though communication between the inverter controller arrangement 4 and the power modules 7 are illustrated as separate, only one communication line may be established to a gate drive / gate drives for the two power modules. This is true both in case the communication is wireless, optical, via cobber or the like.

[0134] The illustrated branches 5a comprises a first set of power module 8 comprising a power module 7a and a second power module 7b. The branch 5b comprises a second set power modules 9 comprising a third power module 7c and a fourth power module 7d. Each of the four power modules 7a-d is in communication with the inverter controller arrangement 4, and accordingly, the controller arrangement can control these power modules. Further, each of the four power modules 7a-d are connected to both the DC+ connection 2 and the DC- connection 3. And additionally, each of the four power modules 7a-d is connected to the same AC phase output 6a. Thus, the output of the branch 5 may be considered as a sum of the outputs of the power modules 7a-d of the first set of power modules 8 and the second set of power modules 9.

[0135] In fig. 1c, a power module 7 is illustrated in greater detail. Each of the four power modules illustrated in fig. lb has a topology as the power module 7 illustrated in fig. 1c. The illustrated power module 7 comprises a first switching arrangement 10a and a second switching arrangement 10b which connect the DC+ connection 2 and the DC- connection 3. In between these two semiconductor switches lOa-b, the AC phase output 6 is connected to the power modules 7 of this particular illustration. In Fig. lb the output of the power module 7a is denoted 6al which corresponds to the output 6al in Fig. 1c. Accordingly, the first switching arrangement 10a is capable of coupling the DC+ connection 2 to the AC phase output 6, and the second switching arrangement 10b is capable of coupling the AC phase output 6 to the DC- connection 3. Each of the two semiconductor switches 10a, 10b comprises an active switch unit 11, and a passive rectification unit 12. The inverter controller arrangement 4 is configured to control to active switch units 11 of each of the two semiconductor switches 10a, 10b to couple the respective DC connections 2,3 to the AC phase output 6 as intended.

[0136] In fig. Id, a renewable energy DC-to-AC inverter 33 having three phase leges la, lb, 1c is illustrated. Individually, the three phase legs la, lb, 1c are similar in design, function and control as the phase leg 1 illustrated in fig. la. The renewable energy DC-to-AC inverter 33 thus provides three AC outputs all denoted 6 and established according to the described alternating switching procedure.

[0137] Referring now to all of the subfigures la-d, the inverter controller arrangement 4 is configured to separately control each of the at least two branches according to an alternating switching procedure. This alternating switching procedure comprises alternately coupling the AC phase output 6 to any of the two DC connections 2,3 through the first set of power modules 8 and through the second set of power modules 9. Whenever a positive voltage is to be supplied to the phase output 6, the inverter controller arrangement 4 alternately couples the AC phase output 6 to the DC+ connection 2 through the first set of power modules 8 and through the second set of power modules 9. And, similarly, whenever a negative voltage is to be supplied to the phase output 6, the inverter controller arrangement 4 alternately couples the AC phase output 6 to the DC- connection 3 through the first set of power modules 8 and through the second set of power modules 9. To alternately couple the output 6 via the two sets of power modules 8,9, the inverter controller arrangement alternates between supplying a gating signal to the relevant active switch units 11 of the first set of power modules 8, and the relevant active switch units 11 of the second set of power modules 9. For example, at one point in time, a gating signal is provided to the first set of power modules 8 to couple the DC+ connection 2 to the AC phase output 6 while no substantial gating signal is provided to the second set of power modules 9. And, subsequently, a gating signal is provided to the second set of power modules 9 to couple the DC+ connection 2 to the AC phase output 6 while no substantial gating signal is provided to the first set of power modules 8. [0138] In this manner, the current of the AC phase output 6 may be established, at least partially, in piecewise/segmented manner, based on current from the two sets of power modules 8,9 provided sequentially.

[0139] Some alternating switching procedures have periods in which switching is performed and periods in which no switching is performed. In embodiments of the invention, the alternating switching procedure comprises a switching stage and a nonswitching stage within each AC cycle of the AC phase output. That is, in one period of the AC cycle, switching is performed, and in another period of the same AC cycle, no switching is performed.

[0140] In the embodiment illustrated in fig. la-c, the inverter controller arrangement 4 controls each of the branches 5a, 5b according to a flat-top modulation alternating switching procedure. During each positive and negative voltage peak of the AC phase output 6, no switching is performed for approximately 3 milliseconds, such that, during each AC cycle, no switching is performed for at least 6 milliseconds. During the rest of the AC cycle, in between the voltage peaks, switching is performed.

[0141] In practice, the switching stage and the non-switching stage is established by the inverter controller arrangement 4, which provides a modulated signal with periods with switching and periods without switching, which is then used as gating signal to the relevant active switch units of the relevant power modules. During the switching stage, while one set of power modules couples a DC connection to the AC phase output, the other set of power modules is closed. That is, during the switching stage, substantial current does only flow through one of the sets of power modules at a time.

[0142] Upon the occurrence of the non-switching stage, the inverter controller arrangement 4 is configured to couple any of the DC connections 2,3 through the first set of power modules 8 and the second set of power modules 9 in parallel. Both sets of power modules 8,9 thus simultaneously couple the relevant DC connection to the AC phase output 6. This is in contrast to the control scheme applied during the switching stage, in which only one of the sets of power modules couple the relevant DC connection to the AC phase output at a time. [0143] As a result of this parallel coupling, the current through each of the sets of power modules 8,9 approach half of the current which would otherwise run through a single set of power modules, if only this set was user for coupling.

[0144] Upon the end of the non-switching stage, at which the switching stage occurs again, the parallel coupling through both sets of power modules is halted, such that the AC phase output can once again be alternately coupled to any of the two DC connections through the first set of power modules and through the second set of power modules.

[0145] In this manner, the inverter controller arrangement 4 ensures that each branch is controlled with sequential periods of switching and periods of parallel coupling.

[0146] This control scheme is applied to each of the branches 5a, 5b. However, since the branches 5 a, 5b supply separate AC phase outputs, the switching stages and nonswitching stages occur separately, and accordingly, the parallel couplings are timed differently in each branch.

[0147] Fig. 2a-e illustrate output currents of power modules of different sets of power modules and their associated control signals and duty cycle equivalent. The illustrated examples of parameters may for example be realized using the one phase leg 1 embodiment illustrated in fig. 1, having three phase legs la, lb, 1c as illustrated in fig. Id. Here, the currents and control signals illustrated in fig. 2a-e would correspond to control of one of the phase leges la. In comparison, control of other phase legs lb, 1c may be similar, merely shifted in time/phase.

[0148] The various parameters are shown in separate subfigures for clarity, but may be analyzed collectively to understand embodiments of the invention. They all have the same horizontal time axis having units of seconds. The time axis extends from 0.04 seconds to 0.08 seconds, corresponding to a duration of 40 milliseconds, corresponding to two AC cycles given a 50 Hz AC frequency. [0149] At the top of Fig. 2a, the extent of switching stages 15 and non-switching stages 16 is indicated by horizontal arrows, which applies to all of the subfigures 2a- e.

[0150] In more detail, Fig. 2a illustrates currents of power modules of different sets of power modules as well as their sum. The vertical axis is current in units of ampere. Fig. 2b illustrates a parallel coupling control signal which controls parallel coupling of DC connections through both sets of power modules. Fig. 2c illustrates a duty cycle equivalent, corresponding to a pulse width modulation level signal. The value of this signal is associated with the magnitude of the AC current which is established as illustrated in fig. 2a. Fig. 2d illustrates control signals to the switches of an inverter leg in the first set of power modules. And Fig. 2e illustrates control signals to the switches of an inverter leg in the second set of power modules.

[0151] In fig. 2a, the current of an inverter leg of the first set of power modules 17 is shown. Similarly, the current of an inverter leg of the second set of power modules 18 is also shown. During the switching stages 15, switching occurs such that the AC phase output is alternatingly coupled to a DC connection through the first set of power modules and the second set of power modules. As illustrated in the figure, the switching between conducting current primarily through the first set of power modules and conducting current primarily through the second set of power modules occurs on a time scale which is fast compared to the duration of the AC cycle. Resultingly, it is difficult to visually distinguish the currents 17,18 in the illustration during the switching stages 15. The total current 19 is also shown in the figure, which is a sum of the two individual currents 17,18. In the illustrated example, during the switching stages 15, the total current 19 trace the envelope of the two individual currents 17,18 while their corresponding power modules are alternately performs coupling.

[0152] During the AC cycle, a non-switching stage 16 occurs, in which the power modules of the different sets are not alternately coupled. Instead, as proposed by the invention, the power modules of different sets of power modules are parallelly coupled. Consequently, the currents 17,18 of the two sets of power modules gradually approach each other during the non-switching stages 16. Meanwhile, the total current 19 maintains is sinusoidal shape during this non-switching stage.

[0153] When a non-switching stage 16 ends and a new switching stage 15 occurs, the two sets of power modules are once again alternately coupled, resulting in switched/interleaved output currents 17,18 of the two sets of power modules.

[0154] In fig. 2b, the parallel coupling control signal 20 is shown. When it reaches a logic value of “1”, parallel coupling via power modules of both sets of power modules is activated. Otherwise, when the logic value is “0”, the AC phase output is coupled to a DC connection primarily through one set of power modules at a time. As shown in the figure, the parallel coupling control signal 20 is set to “1” in durations matching the non-switching stages 16. This waveform of the parallel coupling control signal 20 provides the parallel coupling of legs/ currents which is illustrated in fig. 2a during the non-switching stages. In practice, there is a small delay from the parallel coupling control signal 20 being “1” to the actual parallel coupling occurring. In this example, the parallel coupling control signal 20 has been hard-coded to be “1” a third of the time during the peak period of each half AC cycle.

[0155] In fig. 2c, the pulse width modulation level signal 21 is shown. This signal 21 determines, via pulse width modulation the output voltage/current of the power modules. The signal value is convertible into a pulse width, which in turn determines the output. During each half AC cycle, the signal is “clamped” to “1” or “-1”. Since the sum of output voltages of all branches is zero, the actual output here (current waveform in Fig. 2a) is determined by the control of the other branches in the renewable energy DC-to-AC inverter. In this example, these periods of the pulse width modulation level signal 21 being “1” or “-1” determined to non-switching stages 16, which the parallel coupling control signal 20 in fig. 2b matches.

[0156] In fig. 2d, control signals 22,23 to switches in an inverter leg of the first set of power modules is illustrated. Similarly, in fig. 2e, control signals 24,25 to switches in an inverter leg of the second set of power modules is illustrated. These control signals are pulsed signals which are established based on the pulse width modulation level signal 21 of fig. 2c.

[0157] In fig 2d, within the time duration (horizontal axis) from approximately 0.04 to 0.05, a control signal 22 of a first switch of a first inverter leg is modulated between on and off, while a control signal 23 to a second switch of that inverter leg is kept off. Similarly, in fig. 2e, within the same time duration, a control signal 24 of a first switch of a second inverter leg is modulated between being on and off, while a control signal 25 to a second switch of that inverter leg is kept off. The modulated control signals 22,24 establish, in combination, the currents 17,18,19 illustrated within the corresponding time duration in fig. 2a. During the switching stage, the control signals 22,24 are modulated, alternately, out of phase, such that the AC phase output is alternately coupled to the relevant DC connection via the first set of power modules and the second set of power modules. In this example, the first switches, to which control signals 22,24 are provided, couple the positive DC+ connection.

[0158] Within the next time duration from approximately 0.05 to 0.06, a control signal 23 of a second switch of a first inverter leg is modulated between on and off, while the control signal 22 of the first switch of that inverter leg is kept off. Similarly, in fig. 2e, within the same time duration, the control signal 25 of a second switch of a second inverter leg is modulated between being on and off, while the control signal 24 of the first switch of that inverter leg is kept off. Accordingly, the currents 17,18,19 illustrated within the corresponding time duration in fig. 2a are established. But in contrast to previously, the currents are negative, since the second switches to which control signals 23,25 are provided couple the negative DC- connection.

[0159] Fig. 3 illustrates examples of inductors 13,14 and current probes / sensors 26a-b according to embodiments of the invention.

[0160] More specifically, the illustration shows an branch 5, and examples of how single-leg inductors 13, sharing inductor 14, and current probes 26a-b are located in relation to power modules 7a-d of a first set of power modules 8 and a second set of power modules 9. Any of these examples of placements of components in branches may be used in embodiments of the invention.

[0161] The branch comprises a first set of power modules 8 and a second set of power modules 9. The first set of power modules 8 comprises a first inverter leg 7a and a second inverter leg 7b, and the second set of power modules 9 comprises a third inverter leg 7c and a fourth inverter leg 7d. In an embodiment, each of the four power modules 7a-d are individually connected to the AC phase output 6 via a respective single-leg inductor 13a-d. That is, the first inverter leg 7a is connected to the AC phase output 6 via a first single-leg inductor 13, the second inverter leg 7b is connected to the AC phase output 6 via a second single-leg inductor 13, the third inductor leg 7c is connected to the AC phase output 6 via a third single-leg inductor 1c, and the fourth inductor leg 7d is connected to the AC phase output 6 via a fourth single-leg inductor 13. These single-leg inductors 13 reduce reverse currents during the switching phase. Such reverse currents may for example be from the first inductor leg 7a to the third inductor leg 7c, or vice versa, just after alternating between coupling the AC phase output 6 to a DC connection 2 through one and through the other of the two sets of inductor legs 8,9. In the alternative embodiment illustrated in fig. 3, the single-leg inductors 13ab, 13 cd are located on the common output from the first and second sets of power modules (8,9). It should be noted, that the commutation inductor 13 may functionally be considered as one, but implemented as one or more cores.

[0162] The branch 5 further comprises one or more sharing inductors 14, which in an embodiment collectively connect an inverter leg from the first set of power modules 8 and an inverter leg from the second set of power modules 9 to the AC phase output 6. Namely, the first inverter leg 7a is collectively connected to the AC phase output 6 together with the third inverter leg 7c via a first sharing inductor 14, and the second inverter leg 7b is collectively connected to the AC phase output 6 together with the fourth inverter leg 7d via a second sharing inductor 14. Since these sharing inductors 14 connect an inverter leg from each set 8,9 to the AC phase output 6, disturbances in current flow due to switching between the sets of power modules is reduced. Further, disturbances in current flow due to parallel coupling is also reduced. In the alternative embodiment illustrated in fig. 3, the sharing inductor is located at the common AC output 6. The sharing inductor may also be referred to as a set-pairing inductor, main or load inductor.

[0163] Additionally, the branch 5 comprises current probes 26a-b, which each collectively connect an inverter leg from the first set of power modules 8 and an inverter leg from the second set of power modules 9 to the AC phase output 6. A first current probe 26a probes the collective current from the first inverter leg 7a and the third inverter leg 7c, and a second current probe 26b probes the collective current from the collective current from the second inverter leg 7b and the fourth inverter leg 7d. Since these current probes 26a-b probe current from an inverter leg from each set 8,9 to the AC phase output 6, they are capable of measuring a current resembling a sinusoidal current. A current probe only connected to an inverter leg of one of the sets of power modules 8,9 would measure a switched signal instead. The illustrated examples of current probes 26a-b are thus well-suited for performing control of the branch 5 according to an alternating switching procedure. Further, the current probes 26a-b are well-suited for monitoring current in relation to control according to current thresholds. The location of the current probes 26 relative to the inductors is not important, the current probe can be located on both sides of the inductor and also only one current sensor may be located at the common AC output 6.

[0164] Fig. 4 illustrates method steps according to an embodiment of the invention.

[0165] In a first step SI, at least two branches are provided in a renewable energy DC-to-AC inverter. Each of the at least two branches comprises an AC phase output, a first set of power modules, and a second set of power modules. Each inverter leg of the first set of power modules and the second set of power modules may be connected to two DC connections and the AC phase output.

[0166] In a next step S2, each of the at least two branches are separately controlled according to an alternating switching procedure which comprises alternately coupling the AC phase output to any of the two DC connections through the first set of power modules and through the second set of power modules. The alternating switching procedure comprises a switching stage and a non-switching stage within each AC cycle of the AC phase output. The switching stage and the non-switching stage are separated in time and indicate occurrence of switching during the alternating switching procedure.

[0167] In a next step S3, any of the two DC connections are coupled through the first set of power modules and the second set of power modules parallelly upon occurrence of the non-switching stage. In particular, any of the two DC connections may be coupled parallelly to the AC phase output through the first set of power modules and the second set of power modules.

[0168] Note that embodiments of the invention are not limited to a particular order of performing the above-described steps, and that steps of the method may be performed, at least partially, simultaneously. Further, some embodiments of the invention comprise additional method steps.

[0169] Fig. 5 illustrates current outputs 17,18,19 of AC cycles 29 of an AC output 6 during a normal AC cycle 29 and under influence of current thresholds 27,28 according to embodiments of the invention.

[0170] The currents 17,18,19 are illustrated in a coordinate system with a horizontal time axis and a vertical current axis, in similarity with fig. 2a. The numbers (Pl-Pl 1) along the time axis refers to points in time used below to refer to specifica o the control of the current output. However, in contrast to fig. 2a, the AC cycles 29a-c illustrated in fig. 5 have different amplitudes. In a first AC cycle 29a, the total current 19 has an amplitude below a first current threshold 27. In a second AC cycle 29b, the total current 19 has an amplitude larger than the first current threshold 27, but below a second current threshold 28. In a third AC cycle 29c, an unrestricted total current 30 would have an amplitude larger than the second current threshold 28, but due to control according to the thresholds, the total current 19 has an amplitude which is restricted to be maximally at the second current threshold 28. The first current threshold 27 may also be referred to as a switching current threshold, and the second current threshold 28 may also be referred to as a parallel current threshold. [0171] At the top of fig. 5, the extent of switching stages 15 and non-switching stages 16 is indicated by horizontal arrows. In addition, the extent of parallel coupling 31 is indicated by horizontal arrows.

[0172] The figure shows the current 17 of a power module of a first set of power modules 8, the current 18 of a power module of a second set of power modules 9, and their sum which is the total current 19. During the switching stages 15, while the total current 19 is below the first current threshold, the separate currents 17,18 through the separate power modules are conducted by alternately coupling the AC phase output of an branch to DC connections through the first set of power modules and the second set of power modules. During the first AC cycle 29a, upon occurrence of a non-switching stage 16, DC connections are parallelly coupled through the first and second sets of power modules 8, 9. The time extent of the parallel coupling 31 approximately match the extent of the non-switching stage in this first AC cycle 29a.

[0173] In the second AC cycle 29b, the current amplitude has increased in comparison with in the first AC cycle 29a. As the total current 19 reaches the first current threshold 27, parallel coupling 31 is initiated. This initiation of parallel coupling 31 is pre-emptive in comparison with the regular timing of the non-switching stage 16. This is clearly demonstrated by the extend of the non-switching stage 16 and extend of the parallel coupling 31 illustrated at the top of the figure.

[0174] This parallel coupling 31 is of the same character as the parallel coupling initiated upon occurrence of non-switching stages 16 while the total current 19 is below the first current threshold 27, but the conditions for initiation are different. Under either condition, an inverter controller arrangement 4 couples any of the two DC connections 2, 3 through the first set of power modules 8 and the second set of power modules 8 parallelly.

[0175] In the third AC cycle 29c, the current amplitude has increased even further, in comparison with the previous AC cycles 29a-b. Again, as the total current 19 reaches the first current threshold 27, parallel coupling 31 is initiated. However, the total current further reaches the second current threshold 28. At this threshold 28, the total current 19 is restricted, such that, e.g. the output current does not significantly exceed the second current threshold 28. The figure illustrates an “unrestricted total current” 30, which corresponds to the total current as if no restrictions were laid upon the total current due to a second current threshold. This unrestricted total current 30 maintains its sinusoidal current throughout the AC cycle 29c, exceed the second current threshold 28. In comparison, the actual total current 19 does not significantly exceed the second current threshold 28.

[0176] The beneficial impact of control according to the first and second current thresholds 27,28 is exhibited in the figure by comparison of the first current threshold 27 with the individual currents 17,18 of power modules of the first and second sets of power modules. These currents 17,18 never exceeds the first current threshold 27, at least not by a large margin. This would otherwise occur if parallel coupling was not initiated pre-emptively at the first current threshold 27, of the total current 19 was not restricted at the second current threshold 28. The first and second current thresholds 27,28 can thus be matched to the (current) specifications of the individual components (such as active switch units of power modules) of a renewable energy DC-to-AC inverter. Or for a given target specification of a renewable energy DC-to-AC inverter, individual components can be selected accordingly, such that their individual current specifications jointly provide the target specification of the inverter when operating according to one or more of the proposed thresholds.

[0177] In some embodiments, the current of a power module of a set of power modules is restricted to the first current threshold.

[0178] The figure further illustrates an aspect of normal operation according to embodiments of the invention. Even through the amplitude of the currents lie below a current threshold 27, durations of parallel coupling 31 are nevertheless initiated, as illustrated by the first AC cycle 29a, at least according to some embodiments of the invention. This contrasts a DC-to-AC inverter where parallel coupling is only initiated based on a current threshold. [0179] The above described control according to the alternating switching procedure will now be described in further details with reference to the time points of fig. 5 and control features illustrated on fig. 6 and fig. 7.

[0180] The AC cycle 29 is illustrate during normal operation 29a, during first overcurrent operation 29b and during second overcurrent operation 29c. The switch arrangements 10 of the branch legs 7 of the first and second power modules 8, 9 are controlled differently during these modes of operation. Table 1 presents an overview of the different combinations of modes of operation and control modes of the switch arrangement 10.

[0181] The alternating control mode should be understood as a control mode where the two branches 5a, 5b of a phase leg is alternatingly connecting the AC output 6 to one of the two DC connections 2, 3 i.e. only one branch is conducting the full current.

[0182] The both active control mode should be understood as a control mode where the switching unit 11 of either the top or bottom semiconductor switches 10a, 10b of power module(s) is switched on in both branches 5a, 5b and thereby in parallel is connecting the AC output 6 to one of the two DC connections 2, 3 i.e. both branches are sharing the conducting of the full current.

[0183] The both passive control mode should be understood as a control mode where switching units 11 of both the top and bottom semiconductors 10a, 10b of power module(s) are switched off in both branches 5a, 5b and thereby preventing connecting the AC output 6 to the DC connections 2, 3. Which this said, the passive rectification unit of a semiconductor switch may conduct a current in this control mode.

[0184] The alternating and the both active control modes may be controlled in a switching stage or in a none-switching stage. In the switching stage, alternatingly the switching units 11 of the top and bottom semiconductor switches 10a, 10b is switched on and in the non-switching stage the switching unit 11 of one top or bottom semiconductor switches 10a, 10b is switched on and stays on during the non-switching stage. Note that when a reference is made to a semiconductor switch 10a, 10b that is switches it is a reference to the switching unit 11 of that semiconductor switch 10a, 10b.

[0185] The alternating switching control mode is preferred as described below because switching losses are reduced. [0186] The both active switching control mode is preferred as described below because maximum allowable instantaneous phase current is increased.

[0187] The both active non-switching control mode is preferred as described below to protect the semiconductors in an extreme overcurrent situation.

Table 1 [0188] In the top row of table 1, the different modes of operation in which the DC- to-AC inverter can be controlled are found. In the left column, the different switching control modes are found and in the row second to the top, it is specified if the semiconductor switches are in the switching stage or not. The numbers in table 1 refers to points / periods in time specified in fig. 5 where the combination of operation mode, control mode and switching mode are applied during the three AC cycles 29a-29c fo the output current 19.

[0189] During the different operation modes, the two (or more) parallel power modules of a branch are controlled to shape the desired AC phase output waveform 6. [0190] In the alternating control mode during normal operation the semiconductor switches 10 are switched with a switching frequency determined by the desired requirements to the AC output 6 in terms of amplitude, frequency, etc.

[0191] The AC cycles 29a-29c change at zero current crossings denoted 32 after one positive and negative half cycle. Only on zero current crossing point is denoted 32, the rest is only illustrated as black dots.

[0192] At the AC cycle 29a phase angles are illustrated with black dots. Only the 60, 120, 270 and 300 degrees are denoted, the 0 / 360 degrees are only marked with a dot with no degrees. At the AC cycles 29b, 29c only black dots illustrates these degrees such that e.g. P17 is 60 degrees, P19 is 120 degrees etc.

[0193] In general, the control mode implement the switching stage between 0-60 degrees, 120-270 degrees and 300-360 degrees and the non-switching stage between 60-120 degrees and 270-300 degrees. Changing stage every 60 degrees is the most optimal, however it may be possible to change stage at other degrees than 60, this would however not be as optimal.

[0194] Hence, during the normal AC output cycle 29a, the DC-to-AC inverter is operated in normal mode of operation. After the zero current crossing at time point Pl the semiconductor switches 10a, 10b of the semiconductor switches 10 of the first and second sets of power modules 8, 9 in the two branches 5a, 5b are controlled according to the alternating and switching stage control mode during time period P2 (P2, between Pl and P3). Hence, the current 19 is alternatingly conducted by power modules 7a, 7b of the first branch 5a and by the power modules 7c, 7d of the second branch 5b.

[0195] From time point P3 to time point P5 i.e. during time period P4, the control of the semiconductor switches 10a, 10b of the semiconductor switches 10 of the first and second sets of power modules 8, 9 in the two branches 5a, 5b is changed to the both active and non-switching control mode. Hence, only one of the semiconductor switches 10a, 10b of each of the semiconductor switches 10 are conducting current. At this stage in the AC cycle, this control mode is also referred to as direct paralleling. [0196] From time point P5 to just before time point P7 i.e. during time period P6, the control of the semiconductor switches 10a, 10b, are again controlled according to the alternating and switching stage control mode as during time period P2.

[0197] Just before the zero current crossing 32 at time point P7, the semiconductors switches 10a, 10b of the semiconductor switches 10 of the first and second sets of power modules 8, 9 in the two branches 5a, 5b are changed to the both active and switching control mode. At this stage in the AC cycle, this control mode is also referred to as direct paralleling. The control of the semiconductor switches 10a, 10b just before and after the zero current crossing 32 at time period P7 is illustrated on fig. 6.

[0198] It should be noted that preferably, at each of the zero current crossings of the AC output waveform, the semiconductors 10a, 10b of the power modules 7 are controlled as illustrated in fig. 6.

[0199] Just after time point P7 to time point P9 i.e. during time period P8, the control of the semiconductor switches 10a, 10b, are again controlled according to the alternating and switching stage control mode as during time period P2.

[0200] From time point P9 to time point Pl 1 i.e. during time period P10, the control of the semiconductor switches 10a, 10b, are again controlled according to the both active and non-switching control mode as during time period P4.

[0201] From time point Pl 1 to just before time point P13 i.e. during time period P12, the control of the semiconductor switches 10a, 10b, are again controlled according to the alternating and switching stage control mode as during e.g. time period P2.

[0202] At time period P13 the AC output 19 again crosses zero current and the semiconductors switches 10a, 10b are again controlled as described in fig 6.

[0203] During the first overcurrent operation mode, the total current 19 conducted by the power modules in the two branches 5a, 5b reaches the first current threshold 27 in AC cycle 29b. It is noted, that the current 19 reaches the first current threshold 27 before the 60 degrees (at Pl 7) which need to be accounted for as described below. [0204] At the time points of the zero current crossing in cycle 29b and 29c, the control is as described in relation to fig. 6 and will not be mentioned below.

[0205] After time point P13 and until the total current 19 reaches the first current threshold 27 at time period P15, i.e. during time period P14, the semiconductors are controlled according to the alternating switching control mode.

[0206] The first current threshold is determined as an upper limit for the current the set of power module of one branch may conduct. Thus, when reaching the first current threshold 27 at time period 15 and until the 60 degrees phase angle at point P17 i.e. during time period P16, the control mode is both active switching.

[0207] It is noted, that it is the current 19 in this situation that makes the control mode change from alternating switching to both active switching before the 60 degrees because of the current 19 reaches the first current threshold an both branches are needed to conduct the current.

[0208] When reaching the 60 degrees at point P17 the control mode change to both active non-switching and continues in this control mode to 120 degrees at point P19 i.e. during time period Pl 8.

[0209] Between point P19 and the point P21 i.e. during time period P20, the control mode change again to both active switching.

[0210] From point P21 to point P23 i.e. during time period P22, the power modules of the branches are controlled according to the alternating switching control mode. Note however, that around the zero current crossing, the control mode is described in relation to fig. 6.

[0211] The negative half cycle is controlled as described according to the positive half cycle.

[0212] During the second overcurrent operation mode, the total current 19 conducted by the power modules in the two branches 5a, 5b reaches the first current threshold 27 in AC cycle 29c. AC cycle 29c starts by being controlled according to the alternating switching control mode started in the last part of the negative half cycle of AC cycle 29b and continues to the first current limit 27 at point P31.

[0213] Between point P31 and point P33 (at 60 degrees) i.e. during time period P32, the control med is both active switching. From point P33 to point P35 i.e. during time period P34, the control mode is both active non-switching.

[0214] At point P35, the total current 19 reaches the maximum allowable current for power modules in both branches i.e. the second current threshold 28 and to protect e.g. the power modules, the semiconductor switches are control in the passive control mode i.e. non-conducting during the time period P36.

[0215] At time point P37, the total current 19 again drops below the second current threshold 28 and the semiconductor switches are again controlled according to the both active non-switching control mode during time period P38 between time point P37 and P39.

[0216] At time point P39 (120 degrees), the control mode change to both active switching which continues until time point P41 i.e. during time period P40. The current 19 continues to drop and when it reaches the first current threshold limit 27 at point P41, the control mode is again changed to the alternating switching during the time period P42.

[0217] During the negative half cycle of the third AC cycle 29c, the semiconductors switches of the power modules are controlled as described above during the positive half cycle.

[0218] As mentioned above, at the zero current crossing 32 (i.e. when the angle of the AC output current is 0 degrees, 180 degrees, 360degrees, etc.), the control mode has changed from alternating switching to both active switching. This is illustrated on fig. 6, where the direct parallel signal is high. Consequently, when the direct parallel signal is high, both branches are conducting the AC output current. The change in control mode is initiated just before the zero crossing and starts again just after. More specifically, the change in control mode could be made e.g. 2 degrees prior to / after the current is at zero 32 or maybe under a current threshold of e.g. 50A.

[0219] The reason for this change in control mode is that during the alternating switching control mode the change between which branch that is conducting the current is based on direction of current. At the time of zero crossing, an uncertainty of current direction occurs, thus to avoid this uncertainty, the change in control mode is made.

[0220] The control of the semiconductor switches illustrated on fig. 7 illustrates the dynamic increase of maximum current through the two branches 5a, 5b. The graphs illustrates as fig. 2, 5 and 6 controls signals and associated current flow through the power modules of the two branches of a phase leg.

[0221] On the Pulse skip level graph of fig. 7, the maximum allowable AC output current is just above 4000A such as around 4300A before the rising edge of the control signal 34. After the rising edge of the pulse skip level control signal 34, the maximum allowed current of the AC output is above 8000A such as around 8600A i.e. twice as the limit when the pulse skip level is enabled. The increase of factor 2 is only an example and can be controlled such as to a factor 1.5 or the like.

[0222] The change in control mode from alternating switching (see change in conducting branches at the PWM branch 1 and 2 graphs 22, 24) to both active nonswitching (see the non-switching branches at the PWM branch 1 and 2 graphs 23, 25 ) is performed by the inverter controller arrangement 4 as described above. The pulse skip level signal 34 is controlling the limit of allowable current which during the alternating switching control mode is equal to the current limit of one power module. This is because only one power module at the time is conducting the total current. When the inverter controller arrangement 4 are changing the control mode to the active non-switching (direct paralleling), as mentioned power modules or sets of power modules of the two branches shares the full current and thus can carry twice the current. [0223] The pulse skip level signal is delayed a bit compared to the direct parallel signal. This is because first at the time the pulse skip level signal goes high, the branches are sharing the current equally (enough) to allow an increase of the current limit. Hence, by going from single branch conducting to two branches conducting, increases the allowable current.

[0224] Hence, the pulse skip function is designed as an extra safety limit prolonging the time before tripping (by increasing maximum allowable current) of the inverter in case of e.g. a fault ride through occurrence on the utility grid to which the inverter is connected or a current peak from a wind turbine generator

[0225] If the current continue to increase above the 8600A, the control modes change to both passive i.e. switching units 11 are non-conducting before, the inverter is tripping i.e. circuit breakers to the inverter is disconnected. It should be mentioned, that it may be possible for a short period of time to conduct a current above the second threshold before tripping.

[0226] An aspect of the invention relates to renewable energy DC-to-AC inverter comprising: two DC connections; at least two inverter modules; and an inverter controller arrangement; wherein each of said at least two inverter modules comprises: an AC phase output, such that said at least two inverter modules comprises at least two separate AC phase outputs; a first set of power modules; and a second set of power modules, wherein each inverter leg of said first set of power modules and said second set of power modules is connected to said two DC connections and said AC phase output; wherein said inverter controller arrangement separately controls each of said at least two inverter modules according to an alternating switching procedure which comprises alternatingly coupling said AC phase output to any of said two DC connections through said first set of power modules and through said second set of power modules, wherein said alternating switching procedure comprises a switching stage and a non-switching stage within each AC cycle of said AC phase output, wherein said switching stage and said non-switching stage are separated in time and indicative of occurrence of switching, wherein said inverter controller arrangement is configured to couple any of said two DC connections through said first set of power modules and said second set of power modules parallelly upon occurrence of said nonswitching stage.

[0227] In embodiments of the invention, said inverter controller arrangement is configured to couple any of said two DC connections to said AC phase output through said first set of power modules and said second set of power modules parallelly upon occurrence of said non-switching stage.

[0228] In embodiments of the invention, said alternating switching procedure comprises a switching stage and a non-switching stage within each AC cycle provided to said AC phase output of each of said at least two branches.

[0229] In embodiments of the invention, said inverter controller arrangement is configured to couple any of said two DC connections through said first set of power modules and said second set of power modules parallelly upon occurrence of said nonswitching stage during normal operation of said DC-to-AC inverter.

[0230] Thus, in some embodiments of the invention, the parallel coupling may not be implemented as a safety precaution in the event of, e.g. unusually large currents, but to reduce losses in renewable energy DC-to-AC inverters and extend the conditions in which components of the renewable energy DC-to-AC inverters may be operated. A period of operation may be qualified as a period of normal operation if a transient overcurrent does not occur within that period.

[0231] In embodiments of the invention, said inverter controller arrangement separately controls said at least two branches such that any of said two DC connections are coupled through said first set of power modules and said second set of power modules parallelly for at least 33 percent of a duration of an AC period across said at least two branches.

[0232] In embodiments of the invention, said parallel current threshold is twice as large as said switching current threshold.

[0233] During parallel coupling, the current is typically distributed across a larger number of power modules, in comparison with during said switching stage. Typically, the current is distributed across twice the number of power modules while coupling a DC connection through the first and second sets of power modules parallelly. Accordingly, a parallel current threshold may be introduced, to ensure safety, which may be separate from the switching current threshold.

[0234] The safety procedure at the parallel current threshold may typically be different than at the switching current threshold. During said non-switching stage when the switching current threshold is reached, non-used power modules are available for use to avoid over-current in components. Instead, during parallel coupling, at the parallel current threshold, all power modules are typically already in use. Accordingly, the inverter controller arrangement may be configured to restrict/limit the current output to this parallel current threshold.

[0235] In embodiments of the invention, said current output is monitored based on output of said current probe.

[0236] In the context of the current output in relation to the switching current threshold and the parallel current threshold, the output current may generally be understood as a current indicative of current output of the first set of power modules and current output of the second set of power modules. Such current output may be from all power modules of each of the sets, or from a subset of power modules of each of the sets. Examples of an output currents are the current measured by the previously- mentioned respective current probe and the current at the AC phase output.

[0237] From the above, it is now clear that the invention relates to renewable energy DC-to-AC inverters, and systems and methods thereof. By performing parallel coupling through different sets of power modules during non-switching stages of an alternating switching procedure, it is possible to lower peak currents through individual components such as switches and to reduce conduction losses. Further, separate current thresholds improve safety and flexibility.

[0238] The invention has been exemplified above with the purpose of illustration rather than limitation with reference to specific examples of methods and renewable energy DC-to-AC inverters. Details such as a specific method and system structures have been provided in order to understand embodiments of the invention. Note that detailed descriptions of well-known systems, devices, circuits, and methods have been omitted so as to not obscure the description of the invention with unnecessary details. It should be understood that the invention is not limited to the particular examples described above and a person skilled in the art can also implement the invention in other embodiments without these specific details. As such, the invention may be designed and altered in a multitude of varieties within the scope of the invention as specified in the claims.

List of reference signs:

1 Phase leg of a renewable energy DC-to-AC inverter

2 DC+ connection

3 DC- connection

4 Inverter controller arrangement

5 Branch

6 AC phase output

7 Power module

8 First set of power modules,

9 Second set of power modules, 10 Semiconductor switch

11 Active switch unit

12 Passive rectification unit

13 Commutation inductor

14 Sharing inductor

15 Switching stage

16 Non-switching stage

17 Current of leg of first set of power modules

18 Current of leg of second set of power modules

19 Total current

20 Parallel coupling control signal

21 Pulse width modulation level signal

22 Control signal of first switch of leg of first set of power modules

23 Control signal of second switch of leg of first set of power modules

24 Control signal of first switch of leg of second set of power modules

25 Control signal of second switch of leg of second set of power modules

26 Current probe

27 First current threshold

28 Second current threshold

29 AC cycle

30 Unrestricted total current

31 Parallel coupling

32 Zero crossing

33 Renewable energy DC-to-AC inverter

34 Pulse skip control signal

S 1 - S 3 Method step s

P1-P29 Time points / Time signals