TECHNICAL FIELD

[0001] The present invention relates to power electronics. More specifically, the present invention relates to the control of a power electronic converter device.

BACKGROUND

[0002] Power electronics is the application of solid-state electronics to control and convert one form of electrical power to another form such as converting between AC and DC or changing the magnitude and phase of voltage and current or frequency or combination of these. Power electronic devices are used to couple sources of electrical power generation to electrical transmission and/or distribution networks, also referred to herein as simply ‘electrical grids’ or ‘grids’, thus supplying the electrical grid with electrical power.

[0003] In order for a source (e.g. wind turbines, solar panels, HVDC systems and links, and the like) to provide electrical power to the grid, first their generated power needs to be converted to an appropriate form (e.g., in continental Europe, the electrical grid is alternating current (AC) at a frequency of 50 Hz) and there needs to be a degree of alignment between this generated power and the grid (i.e. phase of the AC signal, voltage, etc.).

[0004] Power electronic converter devices (PECDs) are used to provide power to the electrical grid in such a synchronous manner. Power electronic converter devices can comprise a number of converter cells, each converter cell having a number of converter valves. Each valve comprises a transistor switch (e.g., an insulated gate bipolar transistor (IGBT), a gate turn-off transistor (GTO), an integrated gate- commutated thyristor (IGCT), or the like) which are arranged and fired in a particular way to generate an AC signal that is suitably aligned to the electrical grid to which it is supplying power.

[0005] In order to co-ordinate the firing of these switches, control signals are provided thereto. Thus, it is seen that a proper generation of these control signals is important for ensuring an output from the PECD that is aligned to the grid. There exist various methods for controlling PECDs.

[0006] Many PECDs connected to the grid today are operated using grid following control (GFL). They require a firm available grid voltage, for example to accurately establish the phase of the voltage signal, and basically act as current sources. An alternative way of operating a power electronic converter is grid forming control (GFM), also known as ‘grid-leading’ or ‘virtual synchronous machines’.

[0007] GFM is characterized as a voltage source behind an impedance, such that it can create and maintain voltage. GFM is intended to mimic the beneficial behaviour of a synchronous generator, but PECDs can have superior performance to synchronous generators because the GFM can overcome some shortcomings of the synchronous generators, for example their tendency for low-frequency oscillation. At the same time, a PECD may have different limitations compared to a synchronous machine, and the GFM control structure is incomplete without addressing these limitations.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide GFM operation for a PECD in a robust manner and to address at least some limitations of PECDs. Specific embodiments of the present invention are laid out in the appended independent claims. Optional modifications and enhancements are laid out in the appended dependent claims.

[0009] Viewed from a first aspect, there is provided a method for generating a current reference signal for a power electronic converter device. The method comprises providing, to a calculation module, an input reference signal comprising a grid voltage reference signal and generating, by the calculation module, a voltage reference signal, based at least in part on the input reference signal. The method further comprises providing, from the calculation module, the voltage reference signal to a virtual admittance module and generating, by the virtual admittance module and based on the voltage reference signal, the current reference signal. The input reference signal comprises at least a voltage reference signal, for example from a human user interface.

[0010] The power electronic converter device may be a power electronic inverter device, a static compensator (‘STATCOM’), an energy storage enhanced STATCOM (‘E-STATCOM’), or any other device requiring stable voltage source behaviour.

[0011] The calculation module may take the form of a number of operation blocks, such as sum blocks, difference blocks, and/or other mathematical or logical operation blocks arranged so as to enable calculations on input signals and the generation of output signals therefrom. The calculation module may be implemented as a standalone module for connection to a PECD or it may be incorporated within the PECD device.

[0012] The input reference signal comprises a grid voltage reference signal and may further comprise any signal or combination of signals measured from the electrical grid that the PECD is associated with, measured from an internal process of the PECD, and/or provided to the PECD from an operator interface (i.e., a human user interface that may be provided for a transmission system operator). For example, the grid voltage reference signal may be provided from the operator interface, e.g. depending on how the transmission system operator, or some equivalent automated system, would like to regulate the grid’s voltage.

[0013] In some examples, the input reference signal may comprise a first regulator signal from a grid- coupled AC voltage regulator. This may be particularly advantageous in the context of a STATCOM application, as there can be provided a combined ability of the PECD to offer robust voltage source behaviour as well as grid voltage regulation.

[0014] The input reference signal may further comprise any number of additional signals such as a measured grid voltage signal, a measured grid current signal; and/or a second regulator signal provided from a DC voltage regulator, or the like.

[0015] The voltage reference signal can be generated by processing the input reference signal through the logical operation blocks. The voltage reference signal may be fixed or varying, depending on the application. For example, the voltage reference signal may be output at a fixed value based on an input reference signal from an operator, or the voltage reference signal may vary depending on the state of the grid with which the PECD is associated. The voltage reference signal (in conjunction with, e.g., the virtual admittance module) can be used to counteract changes in the grid voltage, such as voltage magnitude steps, phase jumps, faults, etc. It will be appreciated that any number of contributions to the input reference signal may be factored into the generation of the voltage reference signal depending on the particular requirements of the PECDs and the configuration of the calculation module.

[0016] The generation of a voltage reference signal in this manner enables the PECD to enact GFM by emulating the behaviour of a voltage source behind an impedance. For example, according to this first aspect of the invention, the PECD may have enhanced operation even at a very low short-circuit capacity (SCC), such as 1 per unit (pu) SCC or even less. When used in this sense, 1 pu may correspond to a situation where the short-circuit power is equal to the PECD rated power - i.e., the rated PECD power is the normative base power. Additionally or alternatively, the voltage reference signal may be generated based on the determination of positive- or negative-sequence currents (as a complement to the fault current injection provided by the virtual admittance module), e.g. in the event of faults in the grid or in the system.

[0017] The inclusion of the virtual admittance module may contribute to the voltage source behaviour of the PECD. The virtual admittance thus allows the PECD to, for example, operate as a harmonic sink and/or generate fault currents automatically. The virtual admittance module may typically be inductive- ohmic, however any virtual admittance may contribute to the creation of a stable voltage source behaviour for the PECD. For example, the virtual admittance module may be considered as a series connected or parallel connected inductor and resistor, purely resistive, or purely inductive.

[0018] The virtual admittance module may be configured to generate an output current based on a voltage drop thereacross. The voltage drop may be the difference between the voltage reference signal and a measured voltage from the grid with which the PECD is associated. Thus, the manipulation of voltage reference signal provided to the virtual admittance module may result in a change in the output current from the virtual admittance module. A stable virtual admittance, as ‘seen’ by the grid allows for predictable dynamics of the PECD, for example in the event of faults or in particularly weak grid environments.

[0019] The provision of a calculation module between the grid and the generation of the current reference allows for a PECD having a robust voltage source behaviour. For example, in weak grid conditions, the voltage source behaviour can be provided and maintained with the virtual admittance module through manipulation of the voltage reference by the calculation module.

[0020] Through manipulation of this voltage reference, functions that would normally be addressed through, for example, limits or changes to the current reference via tuning of a voltage regulator can be achieved by manipulating the voltage reference instead. This may preserve GFM operation for the PECD and reduce the need for mode switches between GFM and GFL.

[0021] An example functionality that can be implemented according to the first aspect of the invention is the application of limits, i.e. current limits or voltage limits. Manipulation of voltage reference can also be used to control the current reference signal, for example based on a current or voltage limit or, as another example, maintain the DC voltage balancing requirements of the PECD.

[0022] In some examples, the method may further comprise providing, by the virtual admittance module, the current reference signal as a first feedback to the calculation module, and generating the current reference signal further based on the first feedback.

[0023] By creating a feedback loop in this way, current changes can be implemented through manipulation of the voltage reference, as opposed to, e.g., a fast acting current limiter loop or other components connected downstream. Thus, strong voltage source behaviour is preserved.

[0024] In some examples, providing the current reference as a first feedback to the calculation module further comprises providing, by the virtual admittance module, the current reference signal to one or more limiter modules. The one or more limiter modules may then apply one or more limiter functions to the current reference signal and then provide the output of the one or more limiter functions to the calculation module, as the first feedback.

[0025] Use of a fast-acting current limiter ‘downstream’ in the PECD controller would be the equivalent of a change in the virtual admittance value. However, as ‘seen’ from the grid, a change in the admittance means that the voltage source behaviour of the PECD may be lost, as it would behave more like a current source. In contrast, by manipulating the back EMF instead of using the fast-acting current limiter, the grid still ‘sees’ the same virtual impedance. This means that the dynamic response of the PECD is as expected and the voltage source behaviour is preserved.

[0026] In some examples, the method may further comprise determining one or more frequencies for removal in the input reference signal, and generating the voltage reference signal further based on the one or more frequencies. [0027] It will be appreciated by those skilled in the art that the input impedance of a PECD may define the response of the PECD for different frequencies. Thus, it is also possible for the manipulation of the voltage reference to be used to adjust the input impedance of the PECD so as to provide damping at selected frequencies. This can be considered as a form of active filtering or damping.

[0028] In some example cases (e.g. wherein the input reference signal is an AC signal such as from the grid), the manipulation of the voltage reference may be used to compensate for unbalanced sequence components, e.g. caused by faults leading to an asymmetrical grid (1 -phase faults, 2-phase faults, etc.). In these example cases, the method may further comprise determining one or more unbalanced sequence components in the input reference signal, and generating the voltage reference signal further based on the one or more unbalanced sequence components.

[0029] The virtual admittance module’s response may be considered as fast-acting according to a proportional basis (i.e. proportional to the voltage drop thereacross), whilst the response in adjusting the voltage reference may advantageously correspond to an integrator and thus act to reduce the error to zero overtime.

[0030] The output of the GFM current reference signal from the virtual admittance module may be a complex signal with positive and negative sequence components. Faults in the grid, or other events, can lead to a current reference signal that is too high for the converter valves (i.e., the instantaneous value of the valve current goes above a permitted limit). In such circumstances, or if otherwise required, manipulation of voltage reference may advantageously be used to control the positive and negative sequence components in the current reference signal according to the requirements, and capabilities of the PECD.

[0031] According to a further example implementation, the method may further comprise generating an angle reference for the current reference.

[0032] By generating an angle reference for the current reference, the PECD implementing grid forming control can remain synchronous with the grid.

[0033] One way of generating an angle reference may be implementing a phase-locked loop (PLL) coupled to a grid voltage to generate a first angle error, implementing a power synchronization control (PSC) coupled to a grid power to generate a second angle error; and summing the first angle error and the second angle error to generate the angle reference.

[0034] Integration of phase-locked loop and power synchronization may integrate the benefit of both techniques, where power synchronization is an advantageous method of synchronizing based on internal states of the control (e.g. voltage reference signal) and a benefit of a phase-locked loop to synchronise without the need for active power.

[0035] Put another way, the phase-locked loop and power synchronization control can be used to define the angle reference for the PECD, and the integration of both may improve the robustness to variations in grid strength and faults in the grid. In some examples, PSC alone may not provide sufficient damping. However, advantageously, the PSC in combination with the PLL allows for selective damping behaviour of the PECD. Furthermore, the phase-locked loop and power synchronization control may be used to introduce inertia and frequency support for PEIDs with energy storage.

[0036] As used herein, the term ‘phase-locked loop’ is intended to cover feedback loops that incorporate proportional -integral control (e.g. in the context of a STATCOM) as well as those that incorporate only a proportional control (e.g. in the context of an E-STATCOM).

[0037] Viewed from a second aspect, there is provided a power electronic converter system.

[0038] The system comprises a calculation module configured to receive an input reference signal comprising a grid voltage reference signal, and generate a voltage reference signal, based at least in part on the input reference signal.

[0039] The system further comprises a virtual admittance module configured to receive, from the calculation module, the voltage reference signal, and generate, based on the voltage reference signal, a current reference signal.

[0040] The system may further comprise one or more limiter modules, and/or a phase-locked loop and a power synchronous control.

[0041] According to some examples, the system may be arranged to carry out any of the aforementioned methods.

[0042] Furthermore, in some examples, the system may be an integrated unit, for example forming part of a PECD (e.g. a controller thereof), or the system may be a collection units arranged for connection to and/or communication with one or more PECDs. For example, the system may be a STATCOM device, such as a STATCOM or an E-STATCOM.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] One or more embodiments of the invention will be described, by way of example only, with reference to the following figures, in which:

[0044] Figure 1 schematically shows an example prior art arrangement for providing limits on a reference for a power electronic converter device;

[0045] Figure 2 schematically shows a system for controlling a power electronic converter device, according to an embodiment of the present invention;

[0046] Figure 3 schematically shows a system for controlling a power electronic converter device, according to an example modification of the present invention;

[0047] Figure 4 schematically shows a system for controlling a power electronic converter device, according to another example modification of the present invention; and [0048] Figure 5 schematically shows a method for controlling a power electronic converter device with grid forming control, according to an embodiment of the invention.

[0049] Whilst the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings are herein described in detail. It should be understood, however, that the detailed description herein and the drawings attached hereto are not intended to limit the invention to the particular form disclosed. Rather, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the appended claims.

[0050] Any reference to prior art documents or comparative examples in this specification is not to be considered an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0051] As used in this specification, the words “comprise”, “comprising”, and similar words are not to be interpreted in the exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”.

DETAILED DESCRIPTION

[0052] The present invention is described in the following by way of a number of illustrative examples. It will be appreciated that these examples are provided for illustration and explanation only and are not intended to be limiting on the scope of the present invention. Instead, the scope of the present invention is to be defined by the appended claims. Furthermore, although the examples may be presented in the form of individual embodiments, it will be recognised that the invention also covers combinations of the embodiments described herein.

[0053] Figure 1 schematically shows an example prior art system 100 for providing limits on a reference for a power electronic converter device. As shown therein, a grid-coupled reference signal Ref In 102 is provided directly to a fast-acting current limiting loop 104 (also referred to as simply ‘loop 104’), comprising a reference limiter 106 and a current limiter 108. The current limiter 108 outputs a reference Ref Out 110 for sending to, e.g., a current controller for the PECD, whilst providing a feedback 112 to the reference limiter 106 so as to keep Ref Out 110 within certain operational limits. [0054] Grid-coupled reference Ref In 102 is typically provided from a grid-coupled component, such as an AC voltage regulator, a reactive power controller, a manual current reference provided from an operator interface, or other such hardware or software components. The loop 104 is referred to as being ‘fast-acting’ because the limiter components, e.g. reference limiter 106 and current limiter 108, have a response timescale on the order of fractions of a second (for example, approximately 0.1 seconds). [0055] The fast-acting current limiter loop 104 may change the virtual admittance value for the PECD, as ‘seen’ from the grid. A change in the admittance value may lead to the voltage source behaviour of the PECD being lost, as the PECD tends to behave more like a current source with this arrangement. Therefore, a PECD implementing a system such as the system 100 shown in figure 1 may not be able to provide voltage source behaviour whilst the current limiter loop 104 is active. Hence, a stable grid forming control is difficult to achieve. In situations where such instability is pronounced, existing systems will simply switch between a grid-forming control and a grid-following control, the latter being suited to PECDs behaving as a current source.

[0056] Figure 2 schematically shows a system 200 for controlling a power electronic converter device, according to an embodiment of the present invention. The system 200 comprises a calculation module 204 and a virtual admittance module 208. The system 200 may be partly or completely comprised within a PECD, for example in a controller of a PECD, or one or more of the illustrated components may be separately formed from the PECD and arranged for connection thereto.

[0057] The calculation module 204 is configured to receive an input reference signal 202, represented as Ref In 202, which comprises a grid voltage reference signal, but may further comprise other signals. The calculation module 204 is further configured to generate a voltage reference signal 206, represented as V Ref 206, based at least in part on the input reference signal 202.

[0058] The virtual admittance module 208 is configured to receive, from the calculation module 204, the voltage reference signal 206, and generate, based on the voltage reference signal 206, a current reference signal 210, represented as Ref Out 210, for controlling operations of a power electronic converter device. For example, the current reference signal 210 may be provided to a controller of a PECD to control the firing of switches in converter cells.

[0059] In examples, the current reference signal 210 is provided to a current controller of the PECD (not shown). In such examples, the current controller may generate a valve voltage reference from the current reference signal 210. The valve voltage reference may then be used by a modulator to control the switches.

[0060] Ref In 202 may be any grid voltage reference signal, such as a grid voltage reference from a point of common coupling (PCC), or a grid voltage reference from an operator interface, for example. Ref In may further comprise, in some examples, a current reference from the PCC, an output from an AC voltage regulator coupled to the PCC (e.g. similarly as with the system 100 discussed in relation to figure 1), and/or any other reference that provides information about the state of the grid signal and/or any other reference that has been automatically generated by a control function and/or any other reference that has been provided manually, e.g. from an operator interface.

[0061] That is, although the input reference signal 202 is shown as a single input, it will be appreciated that any number of combined references, signals and/or information could be used when creating the voltage reference 206. For example, additional inputs from an AC voltage regulator, a DC voltage regulator, an active filtering module, a PECD energy storage control module, or the like can all be introduced as inputs into the calculation module 204 for use in ‘building’ the voltage reference 206. [0062] The calculation module 204 may take any form as a component that is suitable to receive and process one or more input signals, apply one or more calculations thereto, and output one or more output signals. In some examples, the calculation module 204 is a number of logical operation blocks, such as sum blocks, difference blocks, and/or other logical operation blocks arranged so as to enable calculations on input reference signal 202 and the generation of the voltage reference signal 206 therefrom. The calculation module 204 may be implemented as a standalone module for connection to a PECD or it may be incorporated within the PECD device.

[0063] The voltage reference 206 output from the calculation module 204 serves as an input to the virtual admittance module 208.

[0064] The virtual admittance module 208 may be provided to contribute to the voltage source behaviour of the PECD. The virtual admittance module 208 may thus allow for the PECD to operate as a harmonic sink and generate fault currents automatically. The virtual admittance module 208 can be inductive-ohmic, such as a series- or parallel-connected inductor and resistor. It is also possible that the virtual admittance module 208 can be purely inductive or purely resistive.

[0065] The operation of the virtual admittance module 208 may follow the principles of virtual admittance in electronics. In the context of the present invention, the output current (Ref Out 210) may be based on a difference between a measurement of a grid voltage ( F _nd ) and a reference voltage (k _. ), as exemplified in the below equation (where Y is some scaling factor).

Ref_Out = Y · (y _grid - y _ref .)

[0066] Put another way, the output current Ref Out 210 from the virtual admittance module 208 may be based at least in part on the voltage drop across the virtual admittance module 208.

[0067] As a mathematical analogy, the calculation module 204 can be seen as a generator of a ‘back EMF’ signal (e.g. V Ref 206) because it works to counteract changes in the grid voltage. Being outside of the context of rotating machines having inductive coils (e.g. synchronous machines), this ‘back EMF’ can be thought of as a virtual back EMF signal.

[0068] Thus, it is seen from the above that Ref Out 210 can be adjusted through a modification of the voltage reference, i.e. V Ref 206 from the calculation module 204. The use of the calculation module 204 and the virtual admittance module 208 thus provides robust voltage source behaviour for the PECD, allowing it to operate with a consistent and robust grid-forming control in any grid conditions.

[0069] The introduction of information from the grid (i.e. via V _Kn<i ) may allow for the virtual admittance module 208 to counteract changes in the grid. For example, the virtual admittance module 208 may act as a harmonic sink, counteracting harmonics from the grid by generating a complementary current reference signal 210.

[0070] As another example, the virtual admittance module 208 may act to provide automatic fault current generation in the case of a fault in the grid - that is, for example, an asymmetrical grid fault such as a 1-phase fault or 2-phase fault. This functionality may also be referred to as ‘fault current injection’, as the PECD comprising the virtual admittance module 208 can inject positive sequence current to bring the positive sequence current closer to 100% and bring the negative sequence current closer to 0% (i.e. closer to an ideal grid composition).

[0071] In this way, the virtual admittance module 208 allows the PECD to act as a harmonic sink and [0072] According to one example, the input reference signal 202 may be provided from a human user interface such as an operator interface (i.e. an interface for a transmission system operator). This input reference signal 202 may correspond to a determined regulated voltage for the grid (i.e. a grid voltage reference signal), wherein this determination has been made manually or automatically. Additionally or alternatively, the grid voltage reference signal comprised in the input reference signal 202 may be measured from the PCC or determined by one or more automatic control functions, for example based on an internal state of the PECD.

[0073] According to a further example, the input reference signal 202 may further comprise a regulator output signal (e.g. a current output from an AC voltage regulator). In the example context of a STATCOM, this may advantageously provide simultaneous voltage source behaviour and grid voltage regulation.

[0074] The input reference signal 202 may further comprise any number of further signals, such as a measured grid voltage or grid current, a further regulator output (e.g. from a DC voltage regulator), or the like.

[0075] It will be appreciated that any of these signals may be provided as input in isolation or in combination, so as to generate the voltage reference signal 206 from the calculation module 204.

[0076] The generated output current reference signal Ref Out 210 can be used to control operations of the PECD. For example, the PECD may comprise a controller, such as a current controller module, arranged to control the switches of the converter cells comprised in the PECD. This controller may be arranged to receive the current reference signal 210 and control the firing of the switches (e.g. the timing thereof) based thereupon.

[0077] For example, as discussed above, the current controller may generate a voltage control signal (e.g. a valve voltage reference) from the current reference signal 210. The voltage control signal may then be used by a modulator to control the switches in the converter cells.

[0078] The current reference signal Ref Out 210 may also be provided to further components of the PECD for further processing before or after being used to control operations of the PECD.

[0079] For example, as an optional modification of the system 200, a limiting feedback loop may be introduced. Figure 3 shows an example of a system 300 including such a modification, showing limiting loop 312 added between the output of the virtual admittance module 208 and the input of the calculation module 204. This system 300 may be similarly configured as the system 200 discussed with reference to figure 2, with the addition of a limiter 316 that provides feedback 314, based on the current reference signal 210 output from the virtual admittance module 208, to the calculation module 206.

[0080] In some examples, the current reference signal Ref Out 210 may be provided to one or more limiting modules such as the limiter 316. The limiter 316 may apply one or more limiter functions to the current reference signal Ref Out 210 and consequently provide feedback 314 to the calculation module 204. These limiter functions may relate to a current limit, a voltage limit, or the like.

[0081] In contrast to the comparative system shown in figure 1, by manipulating the voltage reference to apply limits, instead of using a fast-acting current limiting loop (such as the loop 104 in figure 1), the grid may still ‘see’ the same virtual admittance (admittance being directly related to impedance) for the PECD. This means that the dynamic response of the PECD may be as expected and the voltage source behaviour thereof can be preserved.

[0082] The operation of this feedback loop may be as follows. The current reference signal 210 can be used to estimate the current through the converter valves of the PECD. This estimated current can be compared to a predetermined threshold current. This threshold current may be determined based on the physical limitations of the PECD components, the status of the grid, or by some other manual or automatic means. Furthermore, the threshold current may be static or may be dynamically determined. [0083] If the predetermined current threshold is exceeded, the limiter 316 may apply one or more limiter functions to the current reference signal 210. For example, the one or more limiter functions may involve the creation of a ‘limiter coefficient’, which for the sake of the present illustration can be represented by K Limit. The limiter co-efficient can be created, for example, by an integrator, as part of the one or more limiter functions applied by the limiter 316.

[0084] The relative contribution of the original voltage reference signal 206 being output from the calculation module 204 can then be balanced relative to a contribution from a grid-coupled voltage reference. For example, the grid-coupled voltage reference may be the same as that discussed above, i.e. F _nd . This function may take the form of, for example:

V_Ref ® V_Ref (1 — K_Lim.it ) + F _gri K_Limit

[0085] It is seen from this example formulation of a limiter function that, in normal operation (i.e. K Limit is zero), the voltage reference V Ref 206 may be unchanged as a result of this limiter function. However, when K Limit increases and is greater than zero, the voltage reference 206 may be gradually replaced with the measured grid voltage F _grid .

[0086] This may have the effect of reducing the voltage drop across the virtual admittance module 208 ( _g ri _d ^— V-Ref), thus reducing the output current reference signal 210. Thereby, a current limit may be applied. It is evident from this explanation that, in an extreme case, where K_Limit ® 1, V_Ref ® F _g ri _d , and thus the voltage drop across the virtual admittance module 208 may drop to zero (F _gri — F _gri ). [0087] It will be appreciated that the same technique can be employed so as to enforce a voltage limit as part of the one or more limiter functions applied by the limiter block 316. That is, if the output voltage (e.g. estimated from Ref Out 210) is determined as exceeding a predetermined voltage threshold, the voltage can be adjusted so as to be reduced to the threshold, or lower.

[0088] Assuming that there are no events that would trigger a fast-acting limiter, such a fast-acting limiter loop can be left out of the system 200. Alternatively, according to some examples, the current reference signal 210 may further be provided to a fast-acting limiting loop such as loop 104 shown in figure 1, if required, before being used to control operations of the PECD.

[0089] The systems 200 or 300 may further comprise functionality for detecting undesirable frequencies (such as harmonics caused by non-linear loads on the grid) and/or negative or positive sequence currents (for example, caused by a fault in the grid) that may be present in the input reference signal Ref In 202. This functionality may be integrated into the calculation module 204 or may be separate therefrom.

[0090] In response to detecting one or more frequencies for removal in the input reference signal 202, the calculation module 204 may generate the voltage reference signal 206 based on the detected one or more frequencies, so as to remove at least a portion of the detected one or more frequencies when the virtual admittance module generates the current reference signal 208 and operations of the PECD are controlled based thereon. This functionality may be described as an active filtering module. That is, the calculation module 204 may include or be connected to an active filtering module.

[0091] According to the function of the active filtering module, selected harmonics (e.g. voltage harmonics) can be filtered out, for example, by narrow-band band-pass filters or other similar hardware or software components. These may then feed (back) into the calculation module 204 with an increased real gain with a view to causing an increased ohmic output admittance of the PECD (corresponding to a low impedance, i.e. short-circuiting the harmonic).

[0092] Similarly, in response to detecting one or more unbalanced sequence currents, the calculation module 204 may generate the voltage reference signal 206 based on the detected one or more unbalanced sequence currents, so as to remove at least a portion of the detected one or more unbalanced sequence currents. This functionality may be described as a sequence controller. That is, the calculation module 204 may include or be connected to a sequence controller.

[0093] The sequence controller differs from ‘fault current injection’, which is an inherent advantage of introducing the virtual admittance module 208. This operation of virtual admittance module 208 may be thought of as a proportional control, i.e. fast-acting in countering the changes in the grid. However, the sequence controller may correspond to an integrator (e.g. by including an integrator as part of the sequence module). Therefore, in some examples, the response of the sequence controller may work to reduce the negative sequence component of the grid voltage to (ideally) zero, whilst positive components may be controlled by an output from an AC voltage regulator such as that discussed above.

[0094] The detection of unbalanced sequence components may be carried out by a fdter that separates the positive and negative sequence components of the AC voltage. Thereafter, the removal may be carried out by injecting a positive sequence current (e.g. for the AC voltage regulator) and/or a negative sequence current (e.g. for the sequence controller). In some examples, this current injection may be implemented by separate components, such as an AC voltage regulator and a negative sequence controller module. In an alternative example, the sequence controller may control both positive and negative sequences, and may form part of the calculation module 204 or be separate thereto. These current references can be calculated, for example, based on the voltage errors (V Ref - f _g , _Ki ). e.g. using an integrator.

[0095] Put another way, the active filtering module and the sequence control (i.e. the detection and removal of unbalanced sequence components) may be thought of as a narrow-band closed loop controller which follows the input reference 202. Thus, the generation of the voltage reference 206 can be adjusted to take into account many aspects of the input reference signal 202.

[0096] It can also be useful when controlling PECDs to establish a reference angle or phase for the AC waveform, to ensure synchronicity with the rest of the electrical grid. Conventional systems employ a so-called ‘phase-locked loop’ (PLL) which probes the grid signal for its phase, establishes an ‘error’ between the grid signal and the control signal for the PECD, and adjusts the control signal for the PECD based on this error. PLLs are typically associated with grid-following control.

[0097] Typically, when implementing grid-forming controls, a so-called ‘power synchronous control’ (PSC) may be employed to maintain synchronicity between PECDs and the grid. PSC utilises the internal synchronization mechanism in AC systems, in principle, similar to the operation of a synchronous machine. Thus, PSC is typically employed in systems implementing grid-forming control, as it can provide strong voltage support to a weak AC grid.

[0098] Figure 4 schematically shows a system 400 for generating an angle reference for a PECD, according an example modification of the invention. This system 400 may be similarly configured as the system 200 discussed with reference to figure 2 and the system 300 discussed with reference to figure 3.

[0099] The system 400 may comprise a PLL control combined with a PSC control. The PLL control branch may comprise a PLL control module 420 and the PSC control branch may comprise a PSC control module 428.

[0100] A grid reference signal Grid Ref 418, such as a measured grid voltage, may be provided as input to the PLL control module 420 and the PLL control module 420 may determine the phase of the grid reference signal 418 (e.g. the phase of a sinusoidal signal measured from the grid). From this determined phase, the PLL control module 420 may be configured to determine a difference, or angle error (i.e. Q_1 shown in figure 4), between the PECD control angle and the determined phase. This angle error is provided to an addition junction 432.

[0101] A power reference signal P Ref 426 and a determined grid power P Grid 427 may be provided as input to the PSC control module 428 and the PSC control module 428 may determine an angle error therefrom (i.e. 0 2 shown in figure 4). P Grid 427 may be obtained from a product of measured grid voltage and current. The PSC control module 428 may then provide the angle error 0 2 to the addition junction 432.

[0102] The system 400 may generate an angle reference 0 Out 430 by summing, at the addition junction 432, the angle error 0 I from the PLL control module 420 and the angle error 0 2 from the PSC control module 428. This angle reference 430 can then be used, for example in combination with the generated current reference Ref Out 210, to control operations of the PECD.

[0103] To complete the control loop, the angle reference 430 may be provided, as feedback 424, to the PLL control module 420, so that the difference between the PECD control angle and the detected phase can be determined by the PLL control module 420.

[0104] In some examples, the angle reference 430 can be generated by providing the summed angle errors from the addition junction 432 to a proportional-integral (PI) regulator, a proportional regulator, or other control regulator, before providing the feedback 424 to the phase detector 420. Additionally or alternatively, these regulator functions may form part of the functionality of the PLL control module 420. furthermore, in some examples, the angle reference 430 may be provided to an oscillator before using the angle reference 430 for control of PECD operations.

[0105] Lor example, in the context of a STATCOM, the summed angle errors may be provided to a PI regulator, whilst in the context of an E-STATCOM, for example, the summed angle errors may be provided to a proportional regulator as part of the phase-locked loop.

[0106] figure 5 schematically shows a method 500 for controlling a PECD, according to an embodiment of the present invention.

[0107] As indicated by block 502, the method 500 comprises providing, to a calculation module, an input reference signal comprising a grid voltage reference signal.

[0108] As indicated by block 504, the method 500 further comprises generating, by the calculation module, a voltage reference signal, based at least in part on the input reference signal.

[0109] As indicated by block 506, the method 500 further comprises providing, from the calculation module, the voltage reference signal to a virtual admittance module.

[0110] As indicated by block 508, the method 500 further comprises generating, by the virtual admittance module and based on the voltage reference signal, the current reference signal. [0111] Although embodiments and example modifications thereof may have been presented in isolation herein, it will be appreciated that these embodiments and examples may be combinable in any form so as to achieve collective and/or synergistic advantages therefrom.

[0112] As such, there is disclosed herein a method for generating a current reference signal for a power electronic converter device, comprising providing, to a calculation module, an input reference signal, generating, by the calculation module, a voltage reference signal, based at least in part on the input reference signal, providing, from the calculation module, the voltage reference signal to a virtual admittance module, and generating, by the virtual admittance module and based on the voltage reference signal, the current reference signal.