Method for controlling operation of cells in MMC

Abstract

The present application relates to a method for controlling the operation of a cell in an MMC. Specifically, the present application relates to a method for controlling the operation of a cell (12) arranged in a leg of a voltage source converter (10), the leg being associated with a current phase in the voltage source converter (10). The cells (12) may be half-bridge cells or full-bridge cells. The method may comprise: injecting (S1) a predefined circulating current into the arm in accordance with an operating mode of the voltage source converter and an average current in the arm during a fundamental frequency period in the event that a predetermined type of fault is detected in the cell (12); detecting (S2) a time period during which the arm current is during a negative arm current direction (Iva-) corresponding to a direction in which the arm current flows via a diode (D1, D2) of a switch (SW1, SW2) connected in parallel to the bypass switch (BPS); and controlling (S3) the bypass switch (BPS) such that it closes during said time period. A control unit configured to perform the above method is also provided, as well as a voltage source converter comprising the cell and the control unit.

Description

Technical Field

[0001] The present application is directed to the field of power converters, in particular to a voltage source converter (VSC). More specifically, it relates to a method for reducing (preferably, minimizing) the stress of a cell bypass component in a VSC. The present application also relates to a control unit for executing the method and a voltage source converter comprising a cell and a control unit. Background Art

[0002] High Voltage Direct Current (HVDC) power transmission systems are used to efficiently transmit electricity. HVDC systems rely on HVDC converters to convert power from High Voltage Alternating Current (HVAC) to HVDC, or vice versa. One such HVDC converter is called a Voltage Source Converter (VSC), which uses transistors to perform the conversion.

[0003] Due to erroneous behavior of the HVDC VSC system, it may be necessary to mechanically bypass one or more cells of the HVDC VSC system. This can be done by closing a mechanical bypass switch. However, during this operation, there is a risk that one or more switches of the HVDC VSC system do not operate as expected, which may cause the bypass switch to be subjected to an abnormally large short-circuit current of megaamperes (MA). For this reason, the bypass switch must be designed to withstand MA short-circuit currents. Solutions available on the market, such as vacuum circuit breakers, are typically designed to withstand short-circuit currents of only small kA, so bypass switches for HVDC VSC systems must be specially designed, which leads to increased costs.

[0004] In view of the foregoing, it would be desirable to provide a solution that reduces (preferably, minimizes) bypass component stress in a more cost-effective manner. Summary of the invention

[0005] The object of the present invention is to provide an improved solution that alleviates the above-mentioned disadvantages of current solutions. Furthermore, a first object of the present invention is to provide a method for controlling the operation of a cell arranged in an arm of a voltage source converter, the arm being associated with a current phase in the voltage source converter. The first object of the present invention is solved by claim 1. A second object of the present invention is to provide a control unit for performing such a method. The second object is solved by claim 14. A third object of the present invention is to provide a voltage source converter comprising a plurality of cells and the above-mentioned control unit. The third object is solved by claim 15.

[0006] The present invention is based on the inventive concept that, in the event of a cell failure in a voltage source converter where a cell needs to be bypassed, the cell operation can be controlled in a specific manner to limit or prevent a short circuit current of megaamperes (MA) from being conducted through a cell bypass component (e.g., a bypass switch). The stress to which the cell bypass component is subjected can then be reduced, thereby increasing the lifespan. In addition, cell bypass components sized for MA short circuit currents are expensive. Since the inventive concept involves limiting the short circuit current, it allows the cell bypass component to be sized for smaller short circuit currents, thereby reducing costs.

[0007] The monomer may include one or more energy storage elements (e.g., capacitors) and one or more switches for controlling the operation of the monomer. The switch may include a transistor and an anti-parallel diode, the transistor being used to conduct current when it is turned on, and the anti-parallel diode being used to conduct current when the transistor is turned off. The switch state is affected by turning the transistor of the switch on / off. Switch on / off should be understood as the transistor of the switch being turned on / off. The turned-off switch may still conduct current through the anti-parallel diode.

[0008] In the event of a cell failure in a voltage source converter, it may be necessary to bypass one or more cells. The cell may include a bypass switch that enables the cell to be bypassed. The bypassing of the cell may mean that the energy storage element of the cell is bypassed by the bypass switch. One or more switches of the cell may be turned off before the bypass switch is closed. In the case where the cell includes a switch branch (the switch branch includes a pair of switches connected in series, wherein the bypass switch is connected in parallel with one of the switches), it may be desirable to place the cell in a soft bypass mode before closing the bypass switch. Soft bypass mode means that the parallel switch is turned on and the other switch is turned off. As long as the parallel switch can be turned on, the bypass switch will not be subjected to any high short-circuit current generated by the discharge of the cell capacitor.

[0009] However, a cell failure may cause the parallel switch to fail to operate as expected, which may prevent the parallel switch from conducting and thus prevent the cell from being placed in soft bypass mode.

[0010] The present invention based on the inventive concepts discussed herein can reduce cell bypass component stress, particularly in situations where the cell cannot be placed in a soft bypass mode before the bypass switch is closed.

[0011] According to a first aspect of the invention, there is provided a method for controlling the operation of a cell arranged in an arm of a voltage source converter, the arm being associated with a phase of an arm current in the voltage source converter.

[0012] Monomers whose operation can be controlled by this method are monomers including:

[0013] A first monomer connection terminal;

[0014] A second monomer connection terminal;

[0015] A first switch branch, comprising a first switch and a second switch connected in series, wherein one of the first and second cell connection terminals is arranged between the first switch and the second switch, and the first and second switches each comprise a transistor and an anti-parallel diode;

[0016] a bypass switch connected between the first and second cell connection terminals and connected so that the first switch or the second switch is connected in parallel with the bypass switch;

[0017] Wherein, the single cell is adapted so that when the transistors of both the first and second switches are turned off, the single cell conducts the arm current between the first and second single cell connection terminals via the diode of the switch connected in parallel with the bypass switch for the negative arm current direction of the arm current, and conducts the arm current via the diode of the other switch in the first and second switches for the positive arm current direction of the arm current.

[0018] Methods for controlling the monomers include:

[0019] - in case a predetermined fault type is detected in a cell, injecting a predefined circulating current into said arm depending on the operation mode of the voltage source converter and the average current in said arm during one fundamental frequency cycle;

[0020] - a period of time when the detection arm current is in a negative arm current direction; and

[0021] - controlling the bypass switch so that it is closed during said time period.

[0022] By this method, the stress in the cell bypass component can be reduced (preferably, minimized). This increases the life of the cell bypass component. In particular, in the event of a cell failure, the use of this method reduces the stress in the cell bypass component, which would prevent the parallel switch from turning on. The method allows the cell bypass component (particularly the bypass switch) to be designed to withstand a smaller short-circuit current. Therefore, the bypass switch can be made less complex and smaller, thereby reducing costs.

[0023] The first cell connection terminal may be disposed between the first switch and the second switch. The second cell connection terminal may be disposed between the first switch and the second switch.

[0024] The switch connected in parallel with the bypass switch may be the first switch or the second switch.

[0025] In one embodiment, the bypass switch is a mechanical bypass switch.The bypass switch can be sized to withstand short circuit currents of 100A to 1 kA, 1 kA to 10 kA, 10 kA to 100 kA, 100 kA to 1 MA, 1 MA to 10 MA, or more.

[0026] The negative arm current direction may be defined as the direction from the first monomer connection terminal to the second monomer connection terminal. The positive arm current direction may be defined as the direction from the second monomer connection terminal to the first monomer connection terminal. However, it may also be the opposite. The arm current flowing in the negative arm current direction may have a negative value, thereby providing a "negative arm current". The arm current flowing in the positive arm current direction may have a positive value, thereby providing a "positive arm current". Preferably, the monomer may be adapted so that when the arm current is negative, the negative arm current flows via the diode of the parallel switch when both switches are turned off.

[0027] A cell failure can be detected based on an analysis of the arm current. For example, the arm current may deviate from the expected behavior. From this, it can be inferred that a cell failure has occurred. If the average current in the arm deviates from the reference average current in the arm during a fundamental frequency cycle, it can be inferred that a cell failure has occurred. Alternatively or in combination, a cell failure can be detected by one or more sensors arranged to sense one or more characteristics of a cell. A cell failure can be detected by monitoring the behavior of any component in the cell and detecting any anomalies.

[0028] The fundamental frequency may be the lowest frequency of a periodic waveform. In the case of a superposition of sine waves, the fundamental frequency may be the lowest frequency sine wave in the sum of harmonically related frequencies, or the frequency of the difference between adjacent frequencies. Thus, one fundamental frequency period may be the period for such a frequency. Analysis of the arm current may include averaging the arm current over its fundamental frequency period. Additionally, or alternatively, the fundamental frequency may be the connected AC frequency, for example, 50 Hz or 60 Hz.

[0029] Injecting a predefined circulating current may mean injecting a current to affect an existing circulating current in a cell (e.g., including an arm current). The injection of the circulating current may be achieved by modifying a circulating current reference, and a control unit for controlling the circulating current may control the circulating current to follow the reference value. The circulating current may be injected by applying a delta voltage added to a voltage reference of the corresponding phase.

[0030] The injection of the circulating current may depend on the average current in the arm during a fundamental frequency cycle and the operating mode of the voltage converter. The condition depending on the average current in the arm during a fundamental frequency cycle may mean that the average current in the arm during a fundamental frequency cycle deviates from the reference average current in the arm during a fundamental frequency cycle and deviates from a threshold average current value. The condition depending on the operating mode of the voltage converter may mean whether the voltage source converter is in rectifier mode (meaning that power is transferred from AC to DC). When the voltage source converter is in rectifier mode, since the average charging current is below zero, it may not be necessary to inject the circulating current.

[0031] According to one embodiment, if the converter is operated in inverter mode, optionally at a power level of 0.2PU voltage or higher, the injecting of the predefined circulating current into the arm is performed. Optionally, the power level may be within an interval of 0.2PU voltage to 0.3PU voltage, 0.3PU voltage to 0.4PU voltage, or 0.4PU voltage to 0.5PU voltage, or greater. In this way, even in the case where the bypass switch is not closed when a cell fault is detected, the cell voltage increase when the cell experiences a fault can be limited without proper control.

[0032] According to one embodiment, the injecting of the predefined circulating current into the arm is performed before the bypass switch is closed. Thus, the cell charging can be limited before the bypass switch is closed. This can further reduce the short-circuit current caused by the discharge current from the energy storage element.

[0033] According to one embodiment, the negative arm current direction is the direction of the negative arm current, and the positive arm current direction is the orientation of the positive arm current. The switch connected in parallel with the bypass switch can conduct the arm current in the negative arm current direction via the anti-parallel diode in the off state. Depending on the cell design, this may be a suitable configuration.

[0034] According to one embodiment, the method comprises:

[0035] - providing arm current measurements measured by current sensors; and

[0036] -Determining the arm current direction of the arm current from the arm current measurements.

[0037] By measuring the arm current with the aid of a current sensor, it is possible to determine which of the multiple cells in the arm has experienced a fault based on the sensor position. The method may include providing arm current measurements measured by a plurality of current sensors, each current sensor being arranged in association with a corresponding cell. The arm current direction may be determined based on the arm current polarity. If the arm current polarity is negative, the arm current direction may be determined as a negative arm current direction. If the arm current polarity is positive, the arm current direction may be determined as a positive arm current direction. By specifically measuring the arm current and the arm current direction, the present method may achieve more sensitive and accurate performance. Reliance on inferring the arm current and the arm current direction from related measurements (e.g., power output) may be abandoned.

[0038] According to one embodiment, controlling the bypass switch so that it is closed within the time period includes:

[0039] - sending a control signal to the bypass switch to close the bypass switch; and

[0040] - The control signal is timed so that it compensates for the transmission time of the control signal and the closing time of the bypass switch.

[0041] Thereby, it can be ensured that the bypass switch is not closed when the parallel switch in the off state is turned on in the second direction (which may cause stress to the single bypass component). The closing time of the bypass switch may depend on the type of bypass switch and its variation. The transmission time of the control signal may depend on the distance from the source of the control signal to the target of the control signal. When the target receives the control signal, the bypass switch may be actuated to close.

[0042] According to one embodiment, timing the control signal comprises:

[0043] - sending a control signal to the bypass switch during a time period when the arm current direction of the arm current is in a positive arm current direction;

[0044] - Controlling the bypass switch such that it is closed when the arm current direction of the arm current is in a negative arm current direction.

[0045] Thereby, the time window in which the bypass switch can be closed without generating avoidable stresses in the cell bypass components is increased. Thus, the safety margin for correct operation is increased.

[0046] According to one embodiment, the method comprises:

[0047] - based on the waveform of the arm current, estimating a first zero-crossing time point corresponding to a time point when the arm current direction changes from a positive arm current direction to a negative arm current direction and a second zero-crossing time point corresponding to a time point when the arm current direction changes from a negative arm current direction to a positive arm current direction; and

[0048] - Sending a control signal to close the bypass switch so that the bypass switch is closed no earlier than the first zero-crossing time point and no later than the second zero-crossing time point.

[0049] According to one embodiment, the predetermined fault type is a loss of control of the first switch, the second switch and / or a fault in a local cell control board.

[0050] The predetermined fault type can be detected by one or more sensors arranged to sense corresponding characteristics of the single body that can indicate a single body failure. The predetermined fault type can be detected by arm current analysis. For example, the control loss may include a communication loss between the switch and the control unit, or the switch is faulty, or the control unit is faulty. The predetermined fault type may be any single body failure. The predetermined fault type may be a fault of the first switch branch, for example, a fault of the first switch or the second switch, or a fault of both of them. If the single body includes a second switch branch, the predetermined fault type may be a fault of the second switch branch, for example, a fault of the third switch or the fourth switch, or a fault of both of them. The predetermined fault type may be a fault of any control unit that controls the operation of the single body. The predetermined fault type may be a connection fault between two or more elements. The predetermined fault type may be any one of these faults or any combination of these faults.

[0051] According to one embodiment, the cell is a half-bridge cell or a full-bridge cell. The method can be applied to control the operation of a half-bridge cell or a full-bridge cell in a voltage source converter. The method can be applied to control the operation of both a half-bridge cell and a full-bridge cell of a voltage source converter.

[0052] According to one embodiment, the method is applied to at least two monomers arranged in respective arms of a voltage source converter. The method may be applied to one or more monomers arranged in two or three arms of a voltage source converter. The method may be applied to any monomer in either or both arms of a first arm and a second arm in a leg of a voltage source converter. The method may be applied to any monomer in either or both arms of a respective first arm and a second arm in a respective leg of a voltage source converter.

[0053] According to one embodiment, the method is applied to at least two cells arranged in the same arm of a voltage source converter. The method can be applied to two, three, four, five, six, seven, eight, nine, ten, or more cells in the same arm of a voltage source converter.

[0054] According to a second aspect of the present invention, a control unit is provided. The control unit includes a device for performing a method according to the first aspect or any embodiment thereof. The control unit may be configured to perform a method according to the first aspect or any embodiment thereof. The control unit may include one or more gate control units for controlling a first switch, a second switch, an additional switch in a second switch branch, and / or a bypass switch. The control unit may include a memory element for storing measured values. The control unit may include one or more sensors for measuring one or more corresponding characteristics of a monomer and / or an arm current. The control unit may include a bypass control unit and / or a bypass trigger unit. The control unit may include a delay unit for introducing a predetermined delay when triggering the bypass switch BPS.

[0055] According to a third aspect of the present invention, there is provided a voltage source converter, such as a modular multilevel converter (MMC). The voltage source converter comprises a plurality of cells and a control unit according to the second aspect or any embodiment thereof.

[0056] According to a fourth aspect of the present invention, a computer program is provided, wherein the computer program comprises instructions, and when the computer executes the program, the computer executes the method according to the first aspect or any embodiment thereof.

[0057] A computer may refer to any device that can accept and process information for some result based on a program, software, or sequence of instructions on how to process the information. The information may be in the form of digitized data. As non-limiting examples, a computer may be a server, a static computing device, a portable computing device.

[0058] According to a fifth aspect, a computer readable medium is provided. The computer readable medium comprises instructions which, when executed by a computer, cause the computer to perform a method according to the first aspect or any embodiment thereof. The computer readable medium may be any suitable medium comprising instructions. The computer readable medium may be a computer readable storage medium.

[0059] The invention is defined by the accompanying independent claim, embodiments are set forth in the accompanying dependent claims, the following description, and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The present invention will be described in more detail below with reference to the accompanying drawings, in which:

[0061] Figure 1 A voltage source converter according to an embodiment of the present invention is shown;

[0062] Figures 2 to 7 The corresponding monomers according to the embodiments of the present invention are shown;

[0063] Figure 8is a schematic diagram of a voltage source converter according to an embodiment of the present invention;

[0064] Fig. 9 is a schematic diagram of an aspect of the present invention according to an embodiment of the present invention;

[0065] Fig.10 is a graph indicating the current flow over time of a cell whose operation is controlled in a manner in accordance with one embodiment of the present invention;

[0066] Fig.11 is a schematic diagram of a method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0067] Hereinafter, the principles and spirit of the present disclosure will be described with reference to illustrative embodiments. It should be understood that all these embodiments are given only to enable those skilled in the art to better understand and further practice the present disclosure, rather than to limit the scope of the present disclosure. For example, a feature shown or described as part of one embodiment can be used together with another embodiment to produce yet another embodiment. For the sake of clarity, all features of the actual implementation are not described in this specification. Of course, it should be understood that in the development of any such actual embodiment, many implementation-specific decisions should be made to achieve the specific goals of the developer, for example, to comply with system-related and business-related constraints, which will vary from implementation to implementation. In addition, it should be understood that such development work may be complex and time-consuming, but it will be a routine task for those of ordinary skill in the art who benefit from the present disclosure.

[0068] The disclosed subject matter will now be described with reference to the accompanying drawings. Various structures, systems, and devices are schematically depicted in the accompanying drawings for explanation purposes only, so as to avoid obscuring the description by details known to those skilled in the art. However, the accompanying drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted as having meanings consistent with the understanding of these words and phrases by those skilled in the relevant art. The consistent use of terms and phrases herein does not imply any special definition of a term or phrase, i.e., a definition different from the common and customary meaning understood by those skilled in the art. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than the meaning understood by those skilled in the art, such a special definition will be clearly stated in the specification in a definition manner that directly and clearly provides a special definition of the term or phrase.

[0069] The present subject matter provides a solution for reducing cell bypass component stress and preferably minimizing cell bypass component stress when bypassing a cell when a cell failure is detected. The inventive concept is partially implemented as a method for controlling the operation of one or more cells in an arm of a voltage source converter. The inventive concept is also implemented as a control unit for performing the method. The inventive concept is also implemented as a voltage source converter including a cell and a control unit.

[0070] Figure 1 A voltage source converter 10 is shown, which includes cells 12 arranged in three legs. Each leg may include two arms connected in series and providing a midpoint. The midpoint may refer to a location between two elements, it does not strictly refer to a location exactly in the middle of two elements. The midpoint of each leg may form the AC output of the voltage source converter 10. Since there are three legs, the converter can be connected to a three-phase AC system. As shown in FIG. Figure 8 As shown, in each arm are a plurality of cells arranged in cascade. The operation of the cells C1, C2, ..., CN in the voltage source converter can be controlled by the above method. An arm current can be conducted through each arm, and the arm current is associated with the corresponding phase of the current.

[0071] The voltage source converter can be implemented as a modular multilevel converter (MMC), such as Figure 1 As shown. However, the above method is not specifically limited to controlling the operation of a cell of any particular type of voltage source converter, although the method may bring specific advantages to a particular type of voltage source converter. The phase legs may be wye or delta connected, where the junction between two phase legs is connected to the corresponding phase of the three-phase AC system.

[0072] The voltage source converter may be, for example, a power converter for HVDC power transmission, a FACTS system, or a static frequency converter system.

[0073] The monomer 12 can be a full-bridge monomer or a half-bridge monomer. In the figure, these monomers are exemplarily shown as half-bridge monomers. However, it should be understood that these monomers can be full-bridge monomers. The monomer includes an energy storage element and a switch, which is configured to insert an energy storage element with one of up to two different polarities into the phase leg or bypass the energy storage element. The energy storage element in one or more monomers can be a capacitor. Then, each monomer can have a monomer voltage. This monomer voltage is inserted into the phase leg or bypassed to form a waveform.

[0074] In one variation of the voltage source converter 10, one or more phase legs comprise full-bridge monomers. In another variation, each phase leg comprises a full-bridge monomer. In yet another variation, one or more phase legs comprise half-bridge monomers. In yet another variation, each phase leg comprises a half-bridge monomer. In yet another variation, one or more phase legs comprise a mix of full-bridge monomers and half-bridge monomers. In yet another variation, each phase leg comprises a mix of full-bridge monomers and half-bridge monomers.

[0075] refer to Figure 2 and Figure 3 The monomer 12 whose operation can be controlled by the above method includes a first monomer connection terminal CC1 and a second monomer connection terminal CC2. The monomer 12 also includes a first switch branch and an energy storage element (in Figure 2 and Figure 3 In the example, the energy storage element is exemplarily shown as a capacitor Cc. The switch branch is connected to the energy storage element Cc. The switch branch includes a first switch SW1 and a second switch SW2 connected in series. One of the first and second cell connection terminals CC1, CC2 is arranged between the first switch SW1 and the second switch SW2. Figure 2 and Figure 3 In the example, the first cell connection terminal CC1 is arranged between the first switch SW1 and the second switch SW2. The second cell connection terminal CC2 is arranged between the energy storage element Cc and the first switch SW1. As an alternative, the first cell connection terminal CC1 is arranged between the energy storage element Cc and the second switch SW2, and the second cell connection terminal CC2 is arranged between the first switch SW1 and the second switch SW2 (compare Figures 4 to 5 and Figure 6 to Figure 7 ).

[0076] The first switch SW1 and the second switch SW2 are exemplarily shown to include transistors T1 , T2 and anti-parallel diodes D1 , D2 .

[0077] Either transistor of the first and second switches may be any suitable semiconductor component (or "semiconductor"). As non-limiting examples, suitable semiconductor components may include silicon carbide metal oxide semiconductor field effect transistors (SiCMOSFETs) and other active gate controllable devices, insulated gate bipolar transistors (IGBTs), or dual-mode insulated gate transistors (BiGTs). The type of semiconductor component used does not limit the method; rather, the method may be applicable to controlling the operation of a monomer containing any semiconductor suitable for covering the needs of a power converter. The first switch SW1 may be connected to a low potential end of an energy storage element or a branch of an energy storage element, while the second switch SW2 may be connected to a high potential end of an energy storage element or a branch of an energy storage element. The first switch SW1 may be connected to a high potential end of an energy storage element or a branch of an energy storage element, while the second switch SW2 may be connected to a low potential end of an energy storage element or a branch of an energy storage element.

[0078] The diodes D1, D2 of the switches SW1, SW2 may be connected in anti-parallel to the conduction direction of the respective transistors T1, T2. The diodes may also be components of the transistors (power transistors).

[0079] The transistor allows current to flow in one direction, in this case from the collector to the emitter. The diode, being a semiconductor component, allows current to flow in a direction opposite to that allowed by the transistors T1 and T2. Preferably, the diode is adapted to withstand the same power (or current) as the transistor.

[0080] The transistors T1 and T2 of the switches SW1 and SW2 are in steady-state operation activated by respective master gate control units GU1 and GU2 applying a voltage capable of changing the switch state. A first master gate control GU1 may be connected to the gate of the first transistor T1 in the first switch SW1 and control its operation. A second master gate control GU2 may be connected to the gate of the second transistor T2 in the second switch SW2 and control its operation.

[0081] The cell 12 also includes a bypass switch BPS. Figures 2 to 3 , the bypass switch BPS is exemplarily shown as being connected between the first cell connection terminal CC1 and the second cell connection terminal CC2 to be able to close the connection between the first connection terminal CC1 and the second connection terminal CC2. The bypass switch BPS is also connected so that the first switch SW1 is connected in parallel with the bypass switch BPS, thereby providing a parallel switch SW1. Alternatively, the bypass switch BPS may be connected so that the second switch SW2 is connected in parallel with the bypass switch BPS, thereby providing a parallel switch SW2 (see FIG. Figure 6 to Figure 7 ).

[0082] The bypass switch BPS can be implemented as a mechanical switch. The bypass switch BPS can be controlled to bypass the entire cell in the event of a cell failure. By adding the bypass switch BPS, the converter can continue to be used in the event of a cell failure.

[0083] like Figures 4 to 5 As shown, the cell 12 is adapted so that when the transistors T1 and T2 of both the first and second switches SW1 and SW2 are turned off, the cell 12 conducts the arm current between the first and second cell connection terminals CC1 and CC2 via the first diode D1 of the first switch SW1 connected in parallel with the bypass switch BPS for the negative arm current Iva-, and conducts the arm current between the first and second cell connection terminals CC1 and CC2 via the diode D2 of the second switch SW2 for the positive arm current Iva+.

[0084] As an alternative, Figure 6 to Figure 7 As shown, the cell 12 is adapted so that when the transistors T1 and T2 of both the first and second switches SW1 and SW2 are turned off, the cell 12 conducts the arm current between the first and second cell connection terminals CC1 and CC2 for the negative arm current Iva- via the second diode D2 of the second switch SW2 connected in parallel with the bypass switch BPS, and conducts the arm current for the positive arm current Iva+ via the diode D1 of the first switch SW1.

[0085] from Figures 4 to 5 It can be seen that when the positive arm current Iva+ flows through the monomer 12, if the bypass switch BPS is closed, a large discharge current can flow out of the energy storage element Cc, but when the negative arm current Iva- flows through the monomer 12, if the bypass switch BPS is closed, no such large discharge current will flow out of the energy storage element Cc. Figure 6 to Figure 7 The alternative shown is the opposite.

[0086] Therefore, in view of the above-mentioned conditions of cell operation and cell discharge, the above-mentioned method is provided, which is used to control the operation of a cell 12 arranged in an arm of a voltage source converter 10, and the cell 12 is a cell 12 as described above. The method is schematically shown in Fig.11 is shown in and includes:

[0087] In case a predetermined fault type is detected in a cell 12, a predetermined circulating current is injected (S1) into the arm according to the average current in the arm during one fundamental frequency cycle and the operation mode of the voltage source converter;

[0088] Detecting (S2) a time period when the arm current is in a negative arm current direction Iva-; and

[0089] The bypass switch BPS is controlled ( S3 ) such that it is closed during the time period.

[0090] By controlling the bypass switch BPS so that it is closed during the time period, the bypass switch BPS is always closed when the arm current flows between the first connection terminal CC1 and the second connection terminal CC2 via the diodes D1, D2 of the switches SW1, SW2 connected in parallel to the bypass switch BPS. As mentioned above, this diode can be the first diode D1 or the second diode D2, depending on the configuration of the cell 12. This prevents a large discharge current from flowing through the bypass switch BPS at the moment it is closed. This reduces the cell bypass component stress and allows the bypass switch BPS to be sized for smaller currents (it no longer needs to withstand the MA short-circuit current), which subsequently reduces the complexity and cost of the bypass switch BPS.

[0091] The predetermined fault type may include a loss of control of the first switch SW1, the second switch SW2 and / or a fault in a local cell control board. Loss of control may include a loss of communication, i.e., the switch is operable but cannot communicate with it. Loss of control may include an operational fault, i.e., communication with the switch is enabled but the switch does not operate according to the instructions provided.

[0092] The predetermined fault type may be detected by analysis of the arm current. The analysis of the arm current may include determining the average arm current in the arm during a fundamental frequency period or other suitable period, and checking whether the measured average arm current deviates from the expected average arm current.

[0093] The method may further comprise:

[0094] - providing (S4) arm current measurements measured by current sensors; and

[0095] - Determining (S5) an arm current direction of the arm current from the arm current measurement value.

[0096] Thereby, a specific measurement of the arm current and the arm current direction can be provided. This may be more accurate than relying on inference of the arm current and the arm current direction from other measurements.

[0097] Alternatively, or in combination with analysis of the arm currents, the predetermined fault type may be detected by one or more sensors arranged to sense characteristics of the cell.

[0098] If the converter is operated in inverter mode, optionally at a power level of 0.2PU voltage or above, a predefined circulating current can be injected. However, the power levels are non-limiting examples, and other power levels may be suitable. In addition, if the converter is operated in rectifier mode, the injection of the predefined circulating current can be omitted. The injection of the predefined circulating current into the arm can be performed before the bypass switch is closed. Injecting the predefined circulating current can limit or prevent the energy storage element Cc from being charged to an excessively high level. By maintaining the charging level at a satisfactory level even during a single cell failure, the stress on the single cell bypass components can be reduced.

[0099] like Fig. 9 As shown, controlling the circulating current for one phase of a voltage source converter may comprise using a proportional regulator having an arm current reference and the measured circulating current as inputs, which then gives an incremental voltage as output which may be added to a voltage reference of the corresponding phase generated by a system control of the voltage source converter.

[0100] The controlling (S3) the bypass switch BPS so that it is closed during the time period may include:

[0101] - sending (S31) a control signal to the bypass switch BPS to close the bypass switch BPS; and

[0102] - The control signal is timed (S32) such that it compensates for the transmission time of the control signal and the closing time of the bypass switch BPS.

[0103] like Figures 2 to 3 As shown, the bypass switch BPS can be controlled to close by the bypass control unit BPC and the bypass trigger unit BPF. One or more of the delay unit D and the auxiliary gate control unit AUX GU can also be used to control the operation of the cell 12.

[0104] The bypass control unit BPC provides or generates a control signal as an actuation signal of the bypass switch BPS. The control signal issued by the bypass control unit BPC may be a logic signal. The control signal issued by the bypass control unit BPC may be an actuation signal of the bypass switch.

[0105] As a non-limiting example, the control signal may also be used to trigger the operation of the auxiliary gate control unit AUX GU. The control signal may be supplied to the auxiliary gate control unit AUX GU, which in turn applies a gate control signal to the gate of the first transistor T1 of the first switch SW1 based on receipt of the control signal. The gate control signal may turn off the first switch SW1.

[0106] The control signal from the bypass control unit BPC may also be branched to the bypass trigger unit BPF via a delay unit D so that the bypass trigger unit BPF applies a control voltage to the bypass switch BPS, wherein the delay unit D introduces a delay when transmitting the control signal to the bypass trigger unit BPF. Therefore, the actuation of the bypass switch BPS may also be delayed relative to the operation performed by the auxiliary gate control unit AUX GU. The control signal is provided for closing the bypass switch BPS in accordance with the detection of a single-cell fault or a predetermined single-cell fault type. The bypass trigger unit BPF is configured to operate the bypass switch BPS based on the control signal. The auxiliary gate control unit AUX GU connected to the gate of the first transistor T1 of the first switch SW1 according to a non-limiting example is in turn configured to apply a gate control signal when a single-cell fault is detected, which gate control signal is capable of turning on the first transistor T1 of the first switch SW1 (at least for as long as the bypass switch remains unclosed).

[0107] Upon detection of a cell failure, the bypass control unit BPC may cause triggering of the bypass switch BPS after a delay such that the bypass switch BPS is closed if the arm current direction is in the desired direction, which means that the parallel switch conducts the arm current via the diode.

[0108] Although these control elements for controlling the bypass switch are explained with respect to the first switch SW1, it should be understood that the unit may be configured such that the auxiliary gate control unit AUX GU is connected to the second switch SW2 instead (in the case where the second switch SW12 is connected in parallel with the bypass switch BPS, as in the example of FIG. Figure 6 to Figure 7 shown).

[0109] Furthermore, it should be understood that the auxiliary gate control unit AUX GU is optional and may be removed from any embodiment.

[0110] The timing of the control signal (S32) may include:

[0111] - During a time period when the arm current direction of the arm current flows in the opposite direction relative to the arm current direction in which the bypass switch BPS is to be closed, a control signal is sent (S321) to the bypass switch BPS.

[0112] For example, the cell 12 may be adapted so that the bypass switch BPS is configured to be closed when the arm current is flowing in the negative arm current direction. The negative arm current direction may be the direction from the second cell connection terminal CC2 to the first cell connection terminal CC1, and the positive arm current direction may be the direction from the first cell connection terminal CC1 to the second cell connection terminal CC2. Then, when the arm current flows in the positive arm current direction, a control signal may be sent.

[0113] Thus, once the arm current changes to the desired arm current direction, the control signal can advantageously reach and activate the bypass switch BPS. This results in a more responsive solution when a cell failure is detected, thereby improving the performance of the voltage source converter.

[0114] The method may further comprise:

[0115] - based on the waveform of the arm current, estimating (S33) a first zero-crossing time point tz1 corresponding to the time point when the arm current direction changes from the positive arm current direction Iva+ to the negative arm current direction Iva- and a second zero-crossing time point tz2 corresponding to the time point when the arm current direction changes from the negative arm current direction Iva- to the positive arm current direction Iva+; and

[0116] - sending ( S34 ) a control signal to close the bypass switch BPS so that the bypass switch is closed no earlier than the first zero-crossing time point tz1 and no later than the second zero-crossing time point tz2 .

[0117] Fig.10 An example waveform of the arm current Iva is shown. The first zero-crossing point P2 is shown here, which corresponds to the first zero-crossing time point tz1 when the polarity of the arm current changes from positive to negative. The second zero-crossing point P4 is also shown here, which corresponds to the second zero-crossing time point tz2 when the polarity of the arm current changes from negative to positive. These points can be estimated by analyzing the rate of change of the waveform of the arm current over one or more time intervals before the time points tz1, tz2. For example, the zero-crossing time point can be approximated based on the time intervals Δt1, Δt2, where the rate of change is substantially constant in these time intervals. Fig.10 In the example, the first time interval is Δt1 approximately starting from time point ts, and the second time interval is Δt2 approximately starting from time te. Thus, the zero-crossing time point can be estimated in a reliable and fast manner. However, it should be understood that the zero-crossing time point can also be approximated by other methods.

[0118] The control unit and the gate control unit are described above with respect to the half-bridge monomer. It should be appreciated that the principles described herein can also be applied in the full-bridge monomer. In this case, there may be a second switch branch including a third and a fourth switch connected in series and a third and a fourth main control unit for controlling the third and the fourth switch, wherein the second switch branch is connected in parallel with the energy storage element or the energy storage branch including a plurality of energy storage elements, and the second monomer connection terminal is arranged between the third and the fourth switch. In this case, there may be a first additional auxiliary gate control unit connected to one of the switches of the second switch branch, and it is configured to apply a gate control signal when a predetermined monomer fault type is detected, so that one of the switches of the second switch branch is turned on at least as long as the bypass switch remains unclosed. There may also be a second additional auxiliary gate control unit connected to another switch in the switch of the second switch branch, which is configured to apply a gate control signal when a fault is detected, so that the other switch of the second switch branch is turned on at least as long as the bypass switch remains unclosed. The operation of the two additional auxiliary gate control units can also be triggered by the actuation signal of the bypass switch.

[0119] While the above is directed to embodiments of the present disclosure, other and further embodiments may be devised without departing from the inventive concepts discussed herein. The scope of the present invention, however, is determined by the claims.

Claims

1. A method for controlling the operation of a cell (12) arranged in an arm of a voltage source converter (10), the arm being associated with a current phase in the voltage source converter (10), the cell (12) comprising: A first monomer connection terminal (CC1); A second monomer connection terminal (CC2); a first switch branch, the first switch branch comprising a first switch and a second switch (SW1, SW2) connected in series, wherein one of the first and second cell connection terminals (CC1) is arranged between the first switch and the second switch, the first and second switches (SW1, SW2) each comprising a transistor (T1, T2) and an anti-parallel diode (D1, D2); a bypass switch (BPS) connected between the first and second cell connection terminals (CC1, CC2) and connected so that the first switch (SW1) or the second switch (SW2) is connected in parallel with the bypass switch (BPS); The cell (12) is adapted so that when the transistors (T1, T2) of both the first and second switches (SW1, SW2) are turned off, the cell (12) conducts the arm current between the first and second cell connection terminals (CC1, CC2) for a negative arm current direction (Iva-) of the arm current via the diode (D1, D2) of the switch (SW1, SW2) connected in parallel with the bypass switch (BPS), and conducts the arm current between the first and second cell connection terminals (CC1, CC2) for a positive arm current direction (Iva+) of the arm current via the diode (D1, D2) of the other switch (SW1, SW2) of the first and second switches (SW1, SW2), The method comprises: - in case a predetermined fault type is detected in the cell (12), injecting (S1) a predefined circulating current into the arm according to the operation mode of the voltage source converter and the average current in the arm during one fundamental frequency cycle; - detecting (S2) a time period when the arm current is in the negative arm current direction (Iva-); and - controlling (S3) said bypass switch (BPS) so that it is closed during said time period.

2. The method according to claim 1, wherein: The injecting (S1) of the predefined circulating current into the arm is performed in case the converter operates in inverter mode, optionally in case the converter operates at a power level of 0.2PU voltage or above.

3. A method according to any one of the preceding claims, wherein: The injecting (S1) of a predefined circulating current into the arm is performed before the bypass switch is closed.

4. A method according to any one of the preceding claims, wherein: The negative arm current direction is from the second monomer connection terminal (CC2) to the first monomer connection terminal (CC1).

5. The method according to any one of claims 1 to 3, wherein: The positive arm current direction is from the first cell connection terminal (CC1) to the second cell connection terminal (CC2).

6. A method according to any one of the preceding claims, comprising: - providing (S4) arm current measurement values ​​measured by current sensors; as well as - determining (S5) an arm current direction of said arm current from said arm current measurement value.

7. A method according to any one of the preceding claims, wherein: The controlling (S3) the bypass switch (BPS) so that it is closed within the time period comprises: - sending (S31) a control signal to the bypass switch (BPS) to close the bypass switch (BPS); and - timing the control signal (S32) so that it compensates for the transmission time of the control signal and the closing time of the bypass switch (BPS).

8. The method according to claim 7, wherein: The timing of the control signal (S32) comprises: - sending (S321) the control signal to the bypass switch (BPS) during a time period when the arm current direction of the arm current is in the positive arm current direction (Iva+); - controlling the bypass switch so that it is closed when the arm current direction of the arm current is in the negative arm current direction (Iva-).

9. The method according to any one of claims 7 to 8, comprising: - based on the waveform of the arm current, estimating (S33) a first zero-crossing time point (tz1) corresponding to a time point when the arm current direction changes from the positive arm current direction (Iva+) to the negative arm current direction and a second zero-crossing time point (tz2) corresponding to a time point when the arm current direction changes from the negative arm current direction (Iva-) to the positive arm current direction (Iva+); as well as - sending (S34) a control signal to close the bypass switch (BPS) so that the bypass switch is closed no earlier than the first zero-crossing time point (tz1) and no later than the second zero-crossing time point (tz2).

10. A method according to any one of the preceding claims, wherein: The predetermined fault type is a loss of control of the first switch (SW1), the second switch (SW2) and / or a fault in a local cell control board.

11. A method according to any one of the preceding claims, wherein: The monomers are half-bridge monomers or full-bridge monomers.

12. A method according to any one of the preceding claims, wherein: The method is applied to at least two cells (12) arranged in respective arms of the voltage source converter (10).

13. A method according to any one of the preceding claims, wherein: The method is applied to at least two cells (12) arranged in the same arm of the voltage source converter (10).

14. A control unit comprising means for performing the method according to any of the preceding claims.

15. A voltage source converter, such as a modular multilevel converter MMC, comprising a plurality of cells (12) and a control unit according to claim 14.

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