A series resonant converter for balancing a bipolar Direct Current, DC, power grid, as well as a corresponding arrangement, method and computer program product.

Field of the invention

The present disclosure is directed to a series resonant converter and, more specifically, to using the series resonant converter for balancing a bipolar Direct Current, DC, power grid.

Background

Traditionally, Alternating Current, AC, power distribution systems are used as they have been proven to be an effective way for electricity generation and delivery. One of the reasons is that the AC voltage can easily be stepped up using power converters and it is known that AC power may be transported over long distances efficiently.

However, as technology advances of the recent years and together with the rise of a plurality of Direct Current, DC, distributed energy resources like solar photovoltaic cells and battery energy storage devices, different types of DC distribution systems arise for distributing electrical power.

Typically, a DC distribution system may be construed as a unipolar of a bipolar system. A unipolar distribution system only has one single voltage level the is distributed using two wires I terminals. A bipolar distribution system may be considered as a two-phase system having three wires I terminals. The bipolar architectures is an energy efficient DC system architecture that uses a neutral terminal for power distribution purposes. It allows for two times the power distribution with only half the additional installation costs.

Further, a bipolar architecture has a lower line-to-ground safety risk because the interfacing neutral point is grounded and this halves the maximum DC line voltage with respect to ground. Other advantages of the two phase DC bipolar architecture include the flexible selection of multiple DC voltage level for efficient operation and higher system reliability.

Bipolar DC power grid may be advantageous over regular three phase AC and unipolar DC grid for the following reasons.

First, the power transfer capability of bipolar grid is higher compared to three phase AC and unipolar DC grids, when regular voltages and cable installations are considered. Second, the pole to ground voltage is lower in bipolar DC power grids. This makes the costs for the converter much lower as the lower rated semiconductor switches may be used. Third, more voltage levels are available in a bipolar DC power grid compared to unipolar DC power grid due to the use of a neutral line.

One of the challenges of bipolar DC power grids is related to balancing of the bipolar DC power grids.

Currently, for balancing a bipolar DC power grid, buck-boost derived topologies are used. These topologies usually consists of a large inductor and filter capacitors. One of the downsides of these topologies is that these type of converters may utilize large components.

Summary

It is an object of the present disclosure to provide for means for balancing the bipolar Direct Current, DC, power grid. Further objects of the present disclosure include corresponding methods and a related computer program product.

In a first aspect of the present disclosure, there is provided a series resonant converter, wherein said series resonant converter comprises: a first and a second switch, connected in series, and arranged to be connected between a first Direct Current, DC, terminal of a bipolar DC power grid and a neutral terminal of said bipolar DC power grid; a third and a fourth switch, connected in series, and arranged to be connected between said neutral terminal of said bipolar DC power grid and a second DC terminal of said bipolar DC power grid; a resonant tank connected in parallel over said second and third switch; a controller for controlling said switches, wherein the series resonant converter is used for balancing said bipolar DC power grid by said controller controlling said switches.

It was the insight of the inventors that a series resonant converter may be effectively used for balancing a bipolar DC power grid. The present disclosure is thus directed to the use of a series resonant converter for balancing a bipolar DC power grid.

A resonant converter is a type of electric power converter that contains a network of at least one inductor and capacitor, often referred to as a resonant tank, tuned to resonate at a specific frequency.

Various different types of resonant converters are known in the art. The most popular one is the series resonant converter wherein the resonant tank is formed by an inductor placed in series with a capacitor. Typically, a rectifier load network is placed in series with the resonant tank.

Resonant converters may have multiple advantages over conventional converters like a narrow frequency variation over wide range of load and input variation and zero voltage switching even under no load conditions. Switched mode power supplies based on resonant operating converter topologies are increasingly of interest for all power levels nowadays.

Resonant topologies are typically applied when low ElectroMagnetic Interference, EMI, levers are required or when the switching losses have to be reduced in order to allow higher frequencies for miniaturization.

In contrast to the more conventional use of a series resonant converter as explained above, the present disclosure provides for solutions in which the series resonant converter is used for balancing the bipolar DC power grid.

The advantage of using a series resonant converter is that the components that may be utilized for the series resonant converter, i.e. the components for the resonant tank as well as the switches, may be smaller and may have lower switching losses when compared to traditional converters. Hence, the series resonant converter may be smaller and lighter for the same power levels as compared to these traditional converters.

The present disclosure discusses the bipolar DC power grid as having two terminals and a neutral terminal. It is noted that the first terminal may be a positive DC terminal and the second terminal may be a negative DC terminal. The neutral terminal may be considered as a neutral line. It is also possible that the first terminal has a high positive DC voltage and the second terminal has a low positive DC voltage. The neutral terminal may still be regarded as the midpoint between these two terminals.

In an example, the controller is arranged for controlling said switches such that said resonant converter is in any of the following switching patterns:

Switching Pattern 1 , SP1 , wherein said first switch and said third switch are activated;

Switching Pattern 2, SP2, wherein said first switch and said fourth switch are activated;

Switching Pattern 3, SP3, wherein said second switch and said fourth switch are activated;

Switching Pattern 4, SP4, wherein said second switch and said third switch are activated.

The Switching Patterns indicate which of the switches are activated. If a particular switch is not mentioned in a switching pattern, it is assumed that that particular switch is deactivated.

In a further example, the controller is arranged for controlling said switches such that power is flowing from said first DC terminal to said second DC terminal, or vice versa, by following any of the following patterns over time:

Group 1 :

- SP1. SP2, SP3;

- SP1 , SP2, SP3, SP4;

- SP1. SP4, SP3;

- SP1 , SP4, SP3, SP2;

Group 2:

- SP4, SP3, SP2, SP1 ;

- SP3, SP1. SP2;

- SP4, SP3, SP1 ;

- SP4, SP1 , SP2, SP3; wherein said controller is arranged to operate said switching in a capacitive region for group 1 and in an inductive region for group 2. The inventors have found that the order in which the controller is arranged to control the switches, i.e. the order of the switching patterns, may be relevant, as indicated above. The particular order may also determine whether the series resonant converter is to be used in a capacitive region or in an inductive region.

In an example, the controller is arranged for controlling said switches such that power is flowing from said second DC terminal to said first DC terminal, or vice versa, by following any of the following patterns over time:

Group 1 :

- SP1. SP2. SP3;

- SP1 , SP2, SP3, SP4;

- SP1. SP4, SP3;

- SP1 , SP4, SP3, SP2;

Group 2:

- SP4, SP3, SP2, SP1 ;

- SP3, SP1. SP2;

- SP4, SP3, SP1 ;

- SP4, SP1 , SP2, SP3; wherein said controller is arranged to operate said switching in an inductive region for group 1 and in a capacitive region for group 2.

The above provided example is directed to balancing in which the power flows from the second DC terminal to the first DC terminal, or vice versa.

In an example, the controller is further arranged for: controlling said switches such that, momentarily, said first switch and said fourth switch are activated, and said second switch and said third switch are deactivated, such that a voltage over said first DC terminal and said second DC terminal of said bipolar DC power grid is momentarily provided to said resonant tank.

The inventors have found that, typically, the voltage that the resonant tank is able to reach depends on the pole to neutral voltage. Hence, the maximum power transfer accomplished by the series power converter is limited to this particular voltage. To increase that voltage, it was found that it may be beneficial to momentarily connect the resonant tank between the two poles of the bipolar DC power grid, i.e. between the first DC terminal and the second DC terminal of the bipolar DC power grid. This leads to an increase in the voltage over the resonant tank and thus also in an increase of the maximum power transfer that may be accomplished by the series power converter.

In an example, the controller is further arranged for: controlling said switches using a switching frequency, wherein said switching frequency is lower than a resonant frequency of said resonant tank such that said series resonant converter operates in a capacitive region.

It was found that the current trough the resonant tank may increase to very high levels. This is especially the case when the resonant tank is momentarily connected between the two poles of the bipolar DC power grid as explained above. This is an undesired side effect. To mitigate this particular issue, the switching frequency of the series resonant converter may be decreased to bring the series resonant converter in the so-called capacitive region, or the switching frequency of the resonant converter may be increased to bring the series resonance converter in the so-called inductive region.

The switching frequency of the series resonant converter is controlled by the controller. The switching frequency may be set lower than the resonant frequency of the resonant tank. The switching frequency may also be set higher than the resonant frequency of the resonant tank. This depends on the desired flow of the power, as well as the order in which the switching patterns are cycled. This decreases the current through the resonant tank and also aids in balancing the bipolar DC power grid.

In a further example, the controller is arranged to control said switches with subsequent recurring cycles, wherein each cycle comprises: a first state wherein said first switch and third switch are activated; an optional second state in which said first switch is activated; a third state in which said first switch and said fourth switch are activated; a fourth state in which no switch is activated; a fifth state in which said second switch and said fourth switch are activated.

It is noted that the switches of the series resonant converter are controlled in a particular order. The order is, as an example, provided above. As shown, in the third state, the first switch and the fourth switch are activated such that the full voltage of the bipolar DC power grid is provided to the resonant tank. As an alternative, the body diode of a switch may conduct any current in stead of the switch itself. That is, the voltage between the first terminal and the second terminal is provided to the resonant tank.

More specifically, the controller may be arranged for controlling said switches using a switching frequency corresponding to a particular time period, wherein: a time duration of said third state is between 1 % - 5% of said particular time period.

The full voltage of the bipolar DC power grid may be, temporarily, provided to the resonant tank. As an example, the total time period is about 1 % - 5% of the particular time period.

In an example, the resonant tank comprises an inductor connected in series with a capacitor.

In a second aspect of the present disclosure, there is provided an arrangement of a series resonant converter in accordance with any of the previous claims and a bipolar Direct Current, DC, power grid, wherein: said first and second switch are connected between a first DC terminal of said bipolar DC power grid and a neutral terminal of said bipolar DC power grid; said third and said fourth switch are connected between said neutral terminal of said bipolar DC power grid and a second DC terminal of said bipolar DC power grid.

It is noted that the advantages as explained with reference to the first aspect of the present disclosure, being the series resonant converter, also apply to the second aspect of the present disclosure, being the arrangement of a series resonant converter and the bipolar DC power grid.

In an example, the bipolar DC power grid is arranged to provide at least 600Volts DC between said first and said second terminal of said bipolar DC power grid. This is just an example, the DC power grid may also be arranged to provide less than 600Volts. In a third aspect of the present disclosure, there is provided a method of operating a series resonant converter in accordance with any of the examples as provided above, wherein the method comprises the step of: balancing, by said controller, said bipolar DC power grid by controlling said switches.

It is noted that the advantages as explained with reference to the first aspect of the present disclosure, being the series resonant converter, also apply to the third aspect of the present disclosure, being the method of operating the series resonant converter.

In an example, the method comprises the step of: controlling, by said controller, said switches such that, momentarily, said first switch and said fourth switch are activated, and said second switch and said third switch are deactivated, such that a voltage over said first DC terminal and said second DC terminal of said bipolar DC power grid is momentarily provided to said resonant tank.

In a further example, the method further comprises the step of: controlling, by said controller, said switches using a switching frequency, wherein said switching frequency is lower than a resonant frequency of said resonant tank such that said series resonant converter operates in a capacitive region, or controlling, by said controller, said switches using a switching frequency, wherein said switching frequency is higher than a resonant frequency of said resonant tank such that said series resonant converter operates in an inductive region.

In another example, said controller is arranged for controlling said switches such that power is flowing from said first DC terminal to said second DC terminal, or vice versa, by following any of the following patterns over time:

Group 1 :

- SP1. SP2. SP3;

- SP1 , SP2, SP3, SP4;

- SP1. SP4, SP3;

- SP1 , SP4, SP3, SP2;

Group 2: - SP4, SP3, SP2, SP1 ;

- SP3. SP1. SP2;

- SP4. SP3. SP1 ;

- SP4. SP1. SP2. SP3; wherein said controller is arranged to operate said switching in a capacitive region for group 1 and in an inductive region for group 2.

In yet another example, the controller is arranged for controlling said switches using a switching frequency corresponding to a particular time period, wherein: a time duration of said SP2 is between 1 % - 25% of said particular time period.

In an example, the resonant tank comprises an inductor connected in series with a capacitor.

In a fourth aspect of the present disclosure, there is provided a computer program product comprising a computer readable medium having instructions stored thereon, which instruction, when executed by a controller of any of the examples as provided above, cause said controller to balance a bipolar Direct Current, DC, power grid by controlling said switches of said series resonant converter.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief description of the Drawings

Figure 1 shows a series resonant converter in accordance with the present disclosure;

Figure 2 shows the voltage and current properties during an operation cycle of the series resonant converter;

Figures 3a - 3e show an example of a different states during the operation cycle as shown in figure 2;

Figure 4 shows an example of a method in accordance with the present disclosure;

Figures 5a - 5d shown a further example of different states during an operation cycle; Figure 6 shows an example of the converter states and gate signals in the capacitive region when power is flowing from the positive to the negative pole;

Figure 7 shows an example of the converter states and gate signals in the capacitive region when power is flowing from the negative to the positive pole.

Detailed description

Figure 1 shows a series resonant converter 1 in accordance with the present disclosure.

The series resonant converter comprises four switches 2, 3, 4, 5 all connected in series. A controller is present (not shown) for controlling the switches 2, 3, 4, 5. Controlling the switches means that the controller is arranged to drive the switches by providing control signals on the respective gates of the switches 2, 3, 4, 5. In this particular case, the switches 2, 3, 4, 5 are embodied as so-called NPN Metal Oxide Semiconductor, MOS, Field Effect Transistor, FETs, MOSFETs. The controller is arranged to either activate or disactivate each of the MOSFETs. Activating an MOSFET means that the MOSFET is able to conduct current from its drain to its source terminal, or vice versa. Deactivating an MOSFET means that there is no conductive part between the drain terminal and the source terminal of the MOSFET.

The series resonant converter 1 further comprises a resonant tank 6 for operation. In this particular case, the resonant tank 6 consists of an inductor 7 that is connected in series with a capacitor 8. The resonant frequency of the series resonant converter depends on the value choices of the inductor 7 and the capacitor

8.

The series resonant converter 1 is used for balancing a bipolar Direct Current, DC, power grid. The DC power grid has three terminals, i.e. a first DC terminal

9, a neutral terminal 10 and a second DC terminal 11.

Figure 2 shows the voltage and current properties 21 during an operation cycle of the series resonant converter.

The line having reference numeral 22 indicates the voltage over the capacitor in the resonant tank. The line having reference numeral 25 indicates the current through the inductor in the resonant tank. The vertical axis on the left hand side of the figure indicates the voltage 23. The horizontal axis 24 at the bottom side of the figure indicates the time.

A shown in figure 2, the control cycle is divided into multiple sections or Switching Patterns. The first Switching Pattern is provided by to - ti. The second Switching Pattern is provided by t2 - to. The third Switching Pattern is provided by t4 - ts. The fourth Switching Pattern is provided by > ts.

Figures 3a - 3e show an example of a different Switching Patterns during the operation cycle as shown in figure 2.

It is noted that the converter states and gate pulses are shown in steady state.

Figure 3a shows the series resonant converter in a first Switching Pattern, wherein the first switch “S1” and the third switch “S3” are activated. In that case, the voltage between the first DC terminal and the neutral terminal of the bipolar DC power grid is provided to the resonant tank. This is shown in figure 2 in that the current through the inductor starts to rise to a first value and in that the voltage over the capacitor starts to approach this same voltage.

Figure 3b shows the series resonant converter in an in between state, wherein the first switch “S1” is activated but the third switch “S3” is being deactivated. In that case, the current is flowing through the body diode of the third switch “S3”. This state is typically just utilized for a very brief time period, just to prevent simultaneous switching of multiple switches at the same time. This is, for example, shown in figure 2 in that the time period ti - 12 is very short.

Figure 3c shows the series resonant converter in a second Switching Pattern, wherein the first switch “S1” is activated and the fourth switch “S4” is activated. This is, in accordance with the present disclosure, done to ensure that a voltage over said first DC terminal and said second DC terminal of said bipolar DC power grid is momentarily provided to said resonant tank. This is shown in figure 2 by the time period t2 - ts. Here, it is shown that the voltage over the capacitor increases compared to the previous state, i.e. the second state.

Figure 3d shows the series resonant converter in a in between state, wherein none of the switches are activated. In that case, the body diodes of the second switch “S2” and the third switch “S3” allow the current to recirculate to the resonant tank. Finally, figure 3e shows a third Switching Pattern in which the second switch “S2” and the fourth switch “S4” are activated. The fourth Switching Pattern is not shown, but this is the situation in which the second switch “S2” and the third switch “S3” are activated.

Following the above, the principles of the control method may be summarized as follows. The series resonant converter comprises an inductor “L” and a capacitor “C” in its resonant tank. The capacitor is used to store the energy every half cycle from the first DC terminal of the bipolar DC power grid, and to transfer it to the second terminal. This alternate connection of the resonant tank with the input and then to the output leads to the power transfer. The same may be utilized for transferring power from the second DC terminal to the first DC terminal.

It is noted that the present disclosure is elaborated in that the power is flowing from the first DC terminal to the second DC terminal. The present disclosure may however also be applicable for power flowing from the second DC terminal to the first DC terminal.

The maximum voltage that the capacitor is able to reach depends on the pole to neutral voltage and, hence, the maximum power transfer is limited by this particular voltage. The present disclosure provides for a solution in that the resonant tank is momentarily connected between the two terminals, i.e. between the two pole of the bipolar DC power grid. This leads to an increase in the voltage over the capacitor as is shown by the third state of figure 2, and thus also by an increase power transfer accomplished by the series resonant converter.

It is further noted that the method described above may lead to an undesired side effect being that the current through the inductor also increases to an undesired high level. To mitigate this particular issue, the switching frequency of the converter is decreased to bring the converter in the capacitive region. As such, the time period shown in figure 2, and thus the corresponding switching frequency, is lower than the oscillating frequency of the resonant tank.

The above decreases the current through the inductor and also aids in balancing the bipolar DC power grid.

Figure 4 shows an example 301 of a method in accordance with the present disclosure. The first step 302 is directed to balancing, by said controller, said bipolar DC power grid by controlling said switches.

The second step 303 is directed to an example of the first step in that the first step comprises controlling, by said controller, said switches such that, momentarily, said first switch and said fourth switch are activated, and said second switch and said third switch are deactivated, such that a voltage over said first DC terminal and said second DC terminal of said bipolar DC power grid is momentarily provided to said resonant tank.

The third step 304 is directed to a further detailed example of the first step in that the first step comprises controlling, by said controller, said switches using a switching frequency, wherein said switching frequency is lower than a resonant frequency of said resonant tank such that said series resonant converter operates in a capacitive region.

Figures 5a - 5d shown a further example of different states during an operation cycle.

Figure 5a shows an example of state “SP1”, wherein switches S1 and S3 are activated. Figure 5b shows an example of state “SP2”, wherein switches S1 and S4 are activated. Figure 5c shows an example of state “SP3”, wherein switches S2 and S4 are activated. Figure 5d shows an example of state “SP4”. Wherein switches S2 and S3 are activated.

Figure 6 shows an example of the converter states and gate signals in the capacitive region when power is flowing from the positive to the negative pole;

Figure 7 shows an example of the converter states and gate signals in the capacitive region when power is flowing from the negative to the positive pole.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “Comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof