A method for controlling cell behavior in MMC
By controlling cell operation in HVDC VSC systems with a method that injects a circulating current and times the bypass switch closure, the stress on cell bypass components is minimized, reducing costs and enhancing their lifespan.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2024-06-04
- Publication Date
- 2026-04-03
AI Technical Summary
HVDC VSC systems require specialized bypass switches to handle mega-ampere short-circuit currents, leading to increased costs due to the stress on cell bypass components, which current solutions fail to address effectively.
A method to control cell operation in HVDC VSC systems by injecting a predetermined circulating current and timing the closure of the bypass switch to minimize stress on cell bypass components, allowing them to be sized for smaller short-circuit currents.
Reduces stress on cell bypass components, extending their lifespan and reducing costs by enabling the use of less complex and smaller bypass switches.
Smart Images
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Abstract
Description
Technical Field
[0001] Technical Field The present application relates to the field of power converters, specifically to the field of voltage source converters (VSCs). More particularly, it relates to a method for reducing and preferably minimizing the stress on cell bypass components within a VSC.Furthermore, the present application relates to a control unit for implementing the method, as well as a voltage source converter incorporating the cells and the control unit.
Background Art
[0002] Background High voltage direct current (HVDC) power transmission systems are used to transmit power in an efficient manner. 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 known as a voltage source converter (VSC) that uses transistors to perform the conversion.
[0003] Due to the incorrect behavior of an 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, one or more switches of the HVDC VSC system may not operate as intended, and the bypass switch may be exposed to an abnormally large mega ampere (MA) short-circuit current. For this reason, the bypass switch must be designed to withstand an MA short-circuit current. Commercially available solutions such as vacuum circuit breakers are typically designed to withstand only small kiloampere (kA) short-circuit currents, so a special bypass switch for the HVDC VSC system must be designed, leading to increased costs.
[0004] In light of the above, it is desirable to provide a solution for reducing, and preferably minimizing, the stress on the bypass component in a more cost-effective manner. [Overview of the Initiative] [Means for solving the problem]
[0005] overview An object of the present invention is to provide an improved solution that mitigates the aforementioned shortcomings of current solutions. Furthermore, a first object of the present invention is to provide a method for controlling the operation of cells arranged on an arm of a voltage-type converter, the arm being associated with a current phase in the voltage-type 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-type converter comprising a plurality of cells and the control unit described above. The third object is solved by claim 15.
[0006] This invention is based on the idea that, in the event of a cell failure in a voltage converter where cell bypass is necessary, the operation of a cell can be controlled in a specific way to limit or prevent mega-ampere (MA) short-circuit currents from being induced through cell bypass components such as bypass switches. This can improve the lifespan of the cell bypass components by reducing the stress they experience. Furthermore, cell bypass components sized to accommodate MA short-circuit currents are expensive. Because the idea of this invention involves limiting the short-circuit current, it reduces costs by allowing cell bypass components to be sized to accommodate smaller short-circuit currents.
[0007] A cell may comprise one or more energy storage elements (such as capacitors) and one or more switches for controlling the operation of the cell. The switches may comprise a transistor that conducts current when turned on and an antiparallel diode that conducts current when the transistor is turned off. Turning the transistor of the switch on or off affects the switch state. The on / off state of the switch should be understood as the on / off state of the transistor of the switch. A switched-off switch can still conduct current through the antiparallel diode.
[0008] In a voltage converter, if a cell failure occurs, it may be necessary to bypass one or more cells. A cell may be equipped with a bypass switch that allows the cell to be bypassed. Bypassing a cell may mean that the cell's energy storage element is bypassed by the bypass switch. One or more switches in the cell may be turned off before the bypass switch is closed. In the case of a cell with a switching branch including a pair of switches connected in series, where the bypass switch is connected in parallel with one of the switches, it may be desirable to put the cell into soft bypass mode before closing the bypass switch. Soft bypass mode means that the parallel switch is on and the other switch is off. As long as the parallel switch can be turned on, the bypass switch will not suffer a large short-circuit current from the cell capacitor discharge.
[0009] However, a cell failure may cause the parallel switch to malfunction, which could prevent the parallel switch from turning on and therefore prevent the cell from entering soft bypass mode.
[0010] The present invention, based on the concept of the invention discussed herein, can reduce stress on cell bypass components, particularly when it is not possible to put the cell into soft bypass mode before closing the bypass switch.
[0011] According to a first aspect of the present invention, a method is provided for controlling the operation of a cell located on an arm of a voltage converter, the arm being associated with a phase of arm current in the voltage converter.
[0012] The cells whose operation can be controlled by this method are: The first cell connection terminal and The second cell connection terminal, It comprises a first switch and a second switch connected in series, with one of the first and second cell connection terminals located between the first switch and the second switch, and each of the first and second switches having a first switching branch comprising a transistor and an antiparallel diode, A bypass switch connected between the first and second cell connection terminals and It is a cell that has the following features: The bypass switch is connected such that either the first switch or the second switch is connected in parallel with the bypass switch. The cell is configured such that, when the transistors of both the first and second switches are turned off, the arm current between the first and second cell connection terminals is guided through the diode of one of the switches connected in parallel with the bypass switch for the negative arm current direction, and through the diode of the other of the first and second switches for the positive arm current direction.
[0013] A method for controlling the aforementioned cell is: -When a predetermined fault type is detected in the cell, a predetermined circulating current is injected into the arm according to the average current of the arm during one cycle of the fundamental frequency and the operating mode of the voltage converter. -Detecting the time period in which the arm current is in the negative arm current direction, - Controlling the bypass switch to close within the aforementioned time period. Includes.
[0014] This method reduces, and preferably minimizes, the stress on the cell bypass component. This extends the lifespan of the cell bypass component. In particular, this method reduces the stress on the cell bypass component, especially when a cell failure occurs that prevents the parallel switch from turning on. This method allows the cell bypass component, and especially the bypass switch, to be designed to withstand smaller short-circuit currents. Therefore, the bypass switch can be made less complex and smaller, reducing costs.
[0015] The first cell connection terminal may be located between the first switch and the second switch. The second cell connection terminal may be located between the first switch and the second switch.
[0016] The switch connected in parallel with the bypass switch may be the first switch or the second switch.
[0017] In one embodiment, the bypass switch is a mechanical bypass switch. The bypass switch can be dimensioned to withstand short-circuit currents of magnitude 100A to 1kA, 1kA to 10kA, 10kA to 100kA, 100kA to 1MA, 1MA to 10MA, or greater.
[0018] The negative arm current direction can be defined as the direction from the first cell connection terminal to the second cell connection terminal. The positive arm current direction can be defined as the direction from the second cell connection terminal to the first cell connection terminal. However, this may also be reversed. An arm current flowing in the negative arm current direction can have a negative value, thereby providing a "negative arm current". An arm current flowing in the positive arm current direction can have a positive value, thereby providing a "positive arm current". Preferably, the cell may be configured such that when the arm current is negative, the negative arm current flows through the diodes of the parallel switches when both switches are turned off.
[0019] Cell failures may be detectable based on an analysis of arm currents. For example, the arm current may deviate from expected behavior, which can lead to the inference that a cell failure has occurred. If the average current within the arm during one cycle of the fundamental frequency deviates from a reference average current within the arm, a cell failure can be inferred. Alternatively, or in combination, cell failures may be detectable by one or more sensors configured to sense one or more characteristics of the cell. Cell failures can be detected by monitoring the behavior of any component within the cell and detecting any anomalies.
[0020] The fundamental frequency can mean the lowest frequency of a periodic waveform. With respect to a superposition of sine waves, the fundamental frequency may be the lowest frequency sine wave in the sum of harmonic-related frequencies, or the frequency of the difference between adjacent frequencies. Therefore, one period of the fundamental frequency may be the period of such frequencies. Analysis of the arm current may include averaging the arm current over the period of the fundamental frequency of the arm current. In addition to this, or alternatively, the fundamental frequency may mean the connected AC frequency, for example, 50 Hz or 60 Hz.
[0021] The injection of a predetermined circulating current can mean that current is injected to affect, for example, an existing circulating current within a cell that includes an arm current. The injection of the circulating current may be achieved by changing a circulating current reference, and a control unit for controlling the circulation can control the circulation to follow a reference value. The circulating current can be injected by applying a delta voltage added to a voltage reference of a corresponding phase.
[0022] The injection of the circulating current may depend on an average current within the arm during one period of a fundamental frequency and an operating mode of a voltage converter. The condition of depending on an average current within the arm during one period of a fundamental frequency can mean that the average current within the arm during one fundamental frequency deviates from a reference average current within the arm during one fundamental frequency and deviates by only a threshold average current value. The condition of depending on an operating mode of a voltage converter can mean whether a voltage-type converter is in a rectifier mode (i.e., transferring power from AC to DC). When the voltage-type converter is in a rectifier mode, since an average charging current is less than 0, the injection of the circulating current may be unnecessary.
[0023] According to one embodiment, the injection of the predetermined circulating current to the arm is performed when the converter is in an inverter mode and optionally operating at a power level of 0.2PU voltage or higher. Optionally, the power level may be in 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 higher. Thereby, it becomes possible to limit an increase in a cell voltage when a cell faces a failure without appropriate control even if a bypass switch is not closed immediately after detection of a cell failure.
[0024] According to one embodiment, the injection of the predetermined circulating current to the arm is performed before the bypass switch is closed. Thereby, cell charging can be limited before the bypass switch is closed. Thereby, a short-circuit current due to a discharge current from an energy storage element can be further reduced.
[0025] 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 direction of the positive arm current. A switch connected in parallel with the bypass switch may lead to the off-state arm current via an antiparallel diode for the negative arm current direction. This may be an appropriate configuration depending on the cell design.
[0026] According to one embodiment, this method is - To provide arm current measurement values measured by a current sensor, - Determining the direction of the arm current from the arm current measurement value. Includes.
[0027] By measuring the arm current using a current sensor, it may be possible to determine, depending on the sensor location, which of several cells in the arm is facing failure. This method may include providing arm current measurements taken by multiple current sensors, each positioned in relation to its respective 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 arm current direction, this method can provide more responsive and accurate performance. It avoids relying on inferences of the arm current and arm current direction from related measurements (e.g., power output).
[0028] According to one embodiment, controlling the bypass switch to close within the aforementioned time period is: -Sending a control signal to the bypass switch to close the bypass switch, - Determining the timing of the control signal to compensate for the transmission time of the control signal and the closing time of the bypass switch. Includes.
[0029] This ensures that the bypass switch does not close when an off-state parallel switch is conducting in the second direction, which could stress the cell bypass component. The closing time of the bypass switch may depend on the type of bypass switch and its variations. The transmission time of the control signal may depend on the distance from the control signal source to the control signal target. Once the control signal is received by the target, the bypass switch can be activated and closed.
[0030] According to one embodiment, determining the timing of the control signal is -Sending a control signal to the bypass switch during a time period in which the arm current direction is positive, - Control the bypass switch to close when the direction of the arm current is the negative arm current direction. Includes.
[0031] This increases the time window during which the bypass switch can be closed without causing avoidable stress to the cell bypass component. Therefore, the safety margin for proper operation is increased.
[0032] According to one embodiment, this method is - Based on the waveform of the arm current, estimate a first zero-crossing point corresponding to the point in time when the arm current direction changes from the positive arm current direction to the negative arm current direction, and a second zero-crossing point corresponding to the point in time when the arm current direction changes from the negative arm current direction to the positive arm current direction. - A step of transmitting a control signal to close the bypass switch so that the bypass switch does not close earlier than the first zero crossing point and does not close later than the second zero crossing point. Includes.
[0033] According to one embodiment, the predetermined failure type is a loss of control of the first switch and / or the second switch, and / or a failure in the local cell control board.
[0034] A predetermined fault type can be detected by one or more sensors configured to sense the characteristics of each cell that may indicate a cell failure. A predetermined fault type can also be detected by arm current analysis. For example... 、 Loss of control may include loss of communication between the switch and the control unit, or a switch failure, or a control unit failure. A given failure type may be any cell failure. A given failure type may be a failure of the first switching branch, e.g., the first switch or the second switch, or both. If the cell has a second switching branch, a given failure type may be a failure of the second switching branch, e.g., the third switch or the fourth switch, or both. A given failure type may be a failure of any control unit that controls the operation of the cell. A given failure type may be a failure in the connection between two or more elements. A given failure type may be any of these failures, or may include any combination of these failures.
[0035] According to one embodiment, the cell is either a half-bridge cell or a full-bridge cell. This method can be applied to control the operation of either a half-bridge cell or a full-bridge cell in a voltage converter. This method can be applied to control the operation of both half-bridge cells and full-bridge cells in a voltage converter.
[0036] According to one embodiment, the method is applicable to at least two cells located in each arm of a voltage converter. The method may be applied to one or more cells located in two or three arms of a voltage converter. The method may be applied to any cell in either or both of the first arm and the second arm within a leg of a voltage converter. The method may be applied to any cell in either or both of the first arm and the second arm within each leg of a voltage converter.
[0037] According to one embodiment, the method is applied to at least two cells located on the same arm of a voltage converter. The method may be applied to two, three, four, five, six, seven, eight, nine, ten, or more cells within the same arm of a voltage converter.
[0038] According to a second aspect of the present invention, a control unit is provided. The control unit comprises means for carrying out the method according to the first aspect or any embodiment thereof. The control unit may be configured to carry out the 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, additional switches in a second switching branch, and / or bypass switches. The control unit may include memory elements for storing measured values. The control unit may include one or more sensors for measuring one or more respective characteristics of cell and / or arm currents. The control unit may include a bypass control unit and / or a bypass ignition unit. The control unit may include a delay unit for introducing a predetermined delay to the ignition of the bypass switch BPS.
[0039] According to a third aspect of the present invention, a voltage-type converter such as a modular multilevel converter (MMC) is provided. The voltage-type converter comprises a plurality of cells and a control unit according to the second aspect or any embodiment thereof.
[0040] According to a fourth aspect of the present invention, a computer program is provided. The computer program includes instructions that cause the computer to perform the method according to the first aspect or any embodiment thereof when the program is executed by a computer.
[0041] A computer can mean any device that can accept and process information for some result, based on a program, software, or set of instructions on how the information should be processed. The information may be in the form of digitized data. In non-limiting examples, a computer may be a server, a fixed computing device, or a portable computing device.
[0042] According to the fifth aspect, a computer-readable medium is provided. The computer-readable medium includes instructions that cause the computer to perform the method according to the first aspect or any embodiment thereof when executed by the computer. The computer-readable medium may be any suitable medium for including instructions. The computer-readable medium may be a computer-readable storage medium.
[0043] The present invention is defined by the appended independent claims, and embodiments are described in the appended dependent claims, the following description, and the drawings.
[0044] Brief explanation of the drawing The present invention will be described in further detail below with reference to the accompanying drawings. [Brief explanation of the drawing]
[0045] [Figure 1] This shows a voltage converter according to one embodiment of the present invention. [Figure 2] The images show each cell according to an embodiment of the present invention. [Figure 3] The images show each cell according to an embodiment of the present invention. [Figure 4]The images show each cell according to an embodiment of the present invention. [Figure 5] The images show each cell according to an embodiment of the present invention. [Figure 6] The images show each cell according to an embodiment of the present invention. [Figure 7] The images show each cell according to an embodiment of the present invention. [Figure 8] This is a schematic diagram of a voltage converter according to one embodiment of the present invention. [Figure 9] This is a schematic diagram of an embodiment of the present invention according to one embodiment of the present invention. [Figure 10] This graph shows the current over time for a cell whose operation is controlled according to one embodiment of the present invention. [Figure 11] This is a schematic diagram of a method according to one embodiment of the present invention. [Modes for carrying out the invention]
[0046] Description of the Embodiment The principles and intent of this disclosure will be described below with reference to exemplary embodiments. It should be understood that all these embodiments are provided solely to help those skilled in the art better understand and implement this disclosure, and not to limit its scope. For example, features illustrated or described as part of one embodiment can be used in conjunction with another embodiment to obtain yet another embodiment. For clarity, not all features of actual embodiments are described herein. Naturally, it will be understood that in developing such actual embodiments, numerous embodiment-specific decisions will have to be made to achieve the developer's specific goals, which will differ from embodiment to embodiment, such as compliance with system-related and business-related constraints. Furthermore, while such development efforts may be complex and time-consuming, it will nevertheless be understood as routine work for those skilled in the art who are interested in this disclosure.
[0047] The subject matter of this disclosure is described here with reference to the accompanying drawings. Various structures, systems, and apparatuses are shown schematically in the drawings for illustrative purposes only, so as not to obscure the description with details familiar to those skilled in the art. Nevertheless, the accompanying drawings are included to illustrate exemplary examples of the subject matter disclosed. The terms and phrases used herein should be understood and interpreted as having meanings consistent with the understanding of those terms and phrases by those skilled in the art. No special definition of a term or phrase, i.e., a definition different from the ordinary, customary meaning understood by those skilled in the art, is intended to be implied by the consistent use of the term or phrase herein. 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 is expressly provided herein in a definitive manner that directly and clearly provides the special definition of the term or phrase.
[0048] This subject provides a solution for reducing, and preferably minimizing, the stress on the cell bypass component in a cell bypass when a cell failure is detected. The concept of the present invention is partially realized as a method for controlling the operation of one or more cells in the arm of a voltage converter. Furthermore, the concept of the present invention is realized as a control unit for carrying out this method. Furthermore, the concept of the present invention is realized as a voltage converter comprising cells and a control unit.
[0049] Figure 1 shows a voltage converter 10 having cells 12 arranged in three legs. Each leg may have two arms connected in series to provide a midpoint. The midpoint can mean a position between two elements, and does not strictly refer to the exact midpoint between the two elements. The midpoint of each leg can form the AC output of the voltage converter 10. Because there are three legs, the converter may be connected to a three-phase AC system. In each arm, multiple cells are arranged in a column as shown in Figure 8. The operation of cells C1, C2, ..., CN in the voltage converter can be controlled in the manner described above. Arm currents can be induced through each arm, and these arm currents are associated with each phase of the current.
[0050] The voltage converter may be implemented as a modular multilevel converter (MMC), as shown in Figure 1. However, the method described above is not particularly limited to controlling the operation of the cells of a particular type of voltage converter, although this method can offer certain advantages with respect to a particular type of voltage converter. The phase legs may be Y-connected or delta-connected, and the junction between two phase legs is connected to the corresponding phase of a three-phase AC system.
[0051] The voltage converter may be a power converter used, for example, in HVDC power transmission, FACTS systems, or static frequency converter systems.
[0052] Cell 12 may be a full-bridge cell or a half-bridge cell. In the figure, the cell is exemplified as a half-bridge cell. However, it should be understood that the cell may also be a full-bridge cell. The cell comprises an energy storage element and a switch configured to insert the energy storage element, which has up to two different polarities, into the phase leg, or to bypass the energy storage element. One or more energy storage elements in the cell may be capacitors. Thus, each cell may have a cell voltage. This cell voltage is inserted into or bypassed into the phase leg to form a waveform.
[0053] In one variation of the voltage converter 10, one or more phase legs are provided with full bridge cells. In another variation, each phase leg is provided with a full bridge cell. In yet another variation, one or more phase legs are provided with half bridge cells. In yet another variation, each phase leg is provided with a half bridge cell. In yet another variation, one or more phase legs are provided with a mixture of full bridge cells and half bridge cells. In yet another variation, each phase leg is provided with a mixture of full bridge cells and half bridge cells.
[0054] Referring to Figures 2 and 3, for example, a cell 12 whose operation can be controlled by the method described above is equipped with a first cell connection terminal CC1 and a second cell connection terminal CC2. Cell 12 is equipped with a first switching branch and a capacitor C in Figures 2 and 3. c The switching branch further comprises an energy storage element C, which is exemplified as such. cThe switching branch comprises a first switch SW1 and a second switch SW2 connected in series. One of the first and second cell connection terminals CC1 and CC2 is located between the first switch SW1 and the second switch SW2. In Figures 2 and 3, it is illustrated that the first cell connection terminal CC1 is located between the first switch SW1 and the second switch SW2. The second cell connection terminal CC2 is connected to the energy storage element C c It is positioned between the first switch SW1 and the first cell connection terminal CC1, and the energy storage element C c The second cell connection terminal CC2 is positioned between the first switch SW1 and the second switch SW2 (compare Figures 4 and 5 with Figures 6 and 7).
[0055] The first switch SW1 and the second switch SW2 are exemplified as comprising transistors T1 and T2 and antiparallel diodes D1 and D2.
[0056] Either the first or second switch transistor may be any suitable semiconductor component (or "semiconductor"). As a non-limiting example, suitable semiconductor components may include silicon carbide metal oxide semiconductor field-effect transistors (SiC MOSFETs) and other active-gate controllable devices, insulated-gate bipolar transistors (IGBTs), or bi-mode insulated-gate transistors (BiGTs). The type of semiconductor component used is not limiting to the method; rather, the method may be applicable to controlling the operation of any semiconductor-integrated cell suitable to cover the needs of a power converter. The first switch SW1 may be connected to the low-potential end of an energy storage element or branch of an energy storage element, and the second switch SW2 may be connected to the high-potential end of this energy storage element or branch of an energy storage element. A first switch SW1 may be connected to the high-potential end of the energy storage element or a branch of the energy storage element, and a second switch SW2 may be connected to the low-potential end of the energy storage element or a branch of the energy storage element.
[0057] The diodes D1 and D2 of switches SW1 and SW2 may be connected in antiparallel to the conduction direction of their respective transistors T1 and T2. Furthermore, the diodes may be an integral part of the transistor (power transistor).
[0058] A transistor allows a unidirectional flow of current, in this case from the collector to the emitter. A diode, as a semiconductor component, allows a current flow in the opposite direction to the direction enabled by transistors T1 and T2. Preferably, the diode is configured to withstand the same power (or current) as the transistor.
[0059] The transistors T1 and T2 of switches SW1 and SW2 are in a steady-state operation, activated via corresponding primary gate control units GU1 and GU2, which apply voltages that change the switch state. The first primary gate control unit GU1 is connected to the gate of the first transistor T1 in the first switch SW1 and can control its operation. The second primary gate control unit GU2 is connected to the gate of the second transistor T2 in the second switch SW2 and can control its operation.
[0060] Cell 12 further includes a bypass switch BPS. In Figures 2 and 3, the bypass switch BPS is illustrated as being connected between a first cell connection terminal CC1 and a second cell connection terminal CC2, and capable of closing the connection between the first connection terminal CC1 and the second connection terminal CC2. Furthermore, the bypass switch BPS is connected such that a first switch SW1 is connected in parallel with the bypass switch BPS, resulting in a parallel switch SW1. Alternatively, the bypass switch BPS may be connected such that a second switch SW2 is connected in parallel with the bypass switch BPS, resulting in a parallel switch SW2 (see Figures 6 and 7).
[0061] The bypass switch BPS may be implemented as a mechanical switch. The bypass switch BPS may be controlled to bypass the entire cell when a cell fails. By adding the bypass switch BPS, it may be possible to continue using the converter even when a cell has failed.
[0062] As shown in Figures 4 and 5, cell 12 is configured to guide the arm current between the first and second cell connection terminals CC1 and CC2 when the transistors T1 and T2 of both the first and second switches SW1 and SW2 are turned off, through the first diode D1 of the first switch SW1, which is connected in parallel with the bypass switch BPS, for the negative arm current Iva-, and through the diode D2 of the second switch SW2, for the positive arm current Iva+.
[0063] As an alternative, as shown in Figures 6 and 7, cell 12 is connected in parallel with the bypass switch BPS for the negative arm current Iva- when transistors T1 and T2 of both the first and second switches SW1 and SW2 are turned off. Second switch The positive arm current Iva+ is guided through the second diode D2 of SW2, and through the diode D1 of the first switch SW1.
[0064] As can be seen from Figures 4 and 5, when the bypass switch BPS is closed while a positive arm current Iva+ is flowing through cell 12, a large discharge current can flow from the energy storage element Cc. However, when the bypass switch BPS is closed while a negative arm current Iva- is flowing through cell 12, such a large discharge current cannot flow from the energy storage element Cc. This situation is the opposite for the alternatives shown in Figures 6 and 7.
[0065] Therefore, in light of the above insights into cell operation and cell discharge, the above-described method is provided, which is a method for controlling the operation of a cell 12 located on an arm of a voltage converter 10, the cell 12 being the cell 12 described above. The method is schematically shown in Figure 11, Step S1, when a predetermined fault type is detected in cell 12, injects a predetermined circulating current into the arm according to the average current of the arm during one cycle of the fundamental frequency and the operating mode of the voltage converter, Step S2 detects the time period in which the arm current is negative in the arm current direction Iva-, Step S3 controls the bypass switch BPS to close within the aforementioned time period. Includes.
[0066] By controlling the bypass switch BPS to close within the aforementioned time period, the bypass switch BPS closes only when an arm current is always flowing between the first and second connection terminals CC1 and CC2 through diodes D1 and D2 of switches SW1 and SW2, which are connected in parallel with the bypass switch BPS. As described above, depending on the configuration of cell 12, this diode may be either the first diode D1 or the second diode D2. This prevents a large discharge current from flowing through the bypass switch BPS at the moment it closes. This reduces stress on the cell bypass component, allows the bypass switch BPS to be sized to accommodate smaller currents (no longer needing to withstand the short-circuit current of MA), and thus reduces the complexity and cost of the bypass switch BPS.
[0067] A predetermined failure type may include loss of control of the first switch SW1 and / or the second switch SW2, and / or a failure in the local cell control board. Loss of control may include loss of communication, i.e., the switch is operational but communication with the switch is not possible. Loss of control may include malfunction, i.e., communication with the switch is possible but the switch does not operate according to the command given.
[0068] A predetermined fault type can be detected by analyzing the arm current. The analysis of the arm current may include determining the average arm current of the arm over one cycle of the basic cycle or another appropriate period, and checking whether the measured average arm current deviates from the expected average arm current.
[0069] This method, - Step S4 provides an arm current measurement value measured by a current sensor, - Step S5 to determine the direction of the arm current from the arm current measurement value. It can further include
[0070] This allows for specific measurements of the arm current and its direction. This may be more accurate than relying on inferences of the arm current and its direction through other measurements.
[0071] Instead of, or in combination with, arm current analysis, a predetermined fault type may be detected by one or more sensors configured to sense the characteristics of the cell.
[0072] A predetermined circulating current can be injected when the converter is operating in inverter mode at a power level of 0.2 PU voltage or higher, as specified. However, the power level is a non-limiting example, and other power levels may be appropriate. In other cases, such as when the converter is operating in rectifier mode, the injection of the predetermined circulating current can be omitted. The injection of the predetermined circulating current into the arm may be performed before the bypass switch is closed. By injecting the predetermined circulating current, it is possible to limit or prevent the energy storage element Cc from being charged to an excessively high level. By maintaining a satisfactory charge level even during cell failure, stress on the cell bypass component can be reduced.
[0073] As shown in Figure 9, controlling the circulating current of one phase of a voltage converter may involve using a proportional regulator that takes an arm current reference and the measured circulating current as inputs, which then provides as an output a delta voltage that can be added to the corresponding phase voltage reference generated from the system control unit of the voltage converter.
[0074] Step S3, which controls the bypass switch BPS to close within the aforementioned time period, -Step S31 involves sending a control signal to the bypass switch BPS to close the bypass switch BPS, - Step S32 determines the timing of the control signal to compensate for the transmission time of the control signal and the closing time of the bypass switch BPS. It can include...
[0075] As shown in Figures 2 and 3, the bypass switch BPS may be controlled to close by the bypass control unit BPC and the bypass ignition unit BPF. In addition, one or more delay units D and auxiliary gate control units AUX GU may be used to control the operation of cell 12.
[0076] The bypass control unit BPC supplies or generates a control signal that is the activation signal for 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 the activation signal for the bypass switch.
[0077] As a non-limiting example, the control signal may also be used to trigger the operation of an auxiliary gate control unit (AUX GU). The control signal can be supplied to the AUX GU, which, upon receiving the control signal, applies a gate control signal to the gate of the first transistor T1 of the first switch SW1. The gate control signal can turn off the first switch SW1.
[0078] Furthermore, the control signal from the bypass control unit BPC can be branched to the bypass ignition unit BPF via a delay unit D so that the bypass ignition unit BPF applies a control voltage to the bypass switch BPS, and the delay unit D introduces a delay in the delivery of the control signal to the bypass ignition unit BPF. Thus, the operation of the bypass switch BPS can also be delayed relative to the operation performed by the auxiliary gate control unit AUX GU. The control signal is provided to close the bypass switch BPS in response to the detection of a cell fault or a predetermined type of cell fault. The bypass ignition unit BPF is configured to operate the bypass switch BPS based on the control signal. In a non-limiting example, the auxiliary gate control unit AUX GU connected to the gate of a first transistor T1 of a first switch SW1 is configured to apply a gate control signal that causes the first transistor T1 of the first switch SW1 to conduct when a cell fault is detected, as long as the bypass switch remains open.
[0079] Upon detecting a cell failure, the bypass control unit BPC may trigger the ignition of the bypass switch BPS after a delay, so that the bypass switch BPS closes when the arm current direction is in the desired direction, i.e., when the parallel switch is leading the arm current through the diode.
[0080] These control elements for controlling the bypass switch are described in relation to the first switch SW1, but it should be understood that the cell may be configured such that an auxiliary gate control unit AUX GU is connected to the second switch SW2 instead (when the second switch SW2 is connected in parallel with the bypass switch BPS as shown in Figures 6 and 7).
[0081] Furthermore, it should be understood that the auxiliary gate control unit (AUX GU) is optional and may be omitted from any embodiment.
[0082] The step of determining the timing of the control signalS 32 is - Step S321: Send a control signal to the bypass switch BPS during a period of time when the direction of the arm current is flowing in the opposite direction to the arm current direction that should close the bypass switch BPS. It can include...
[0083] For example, cell 12 may be configured such that the bypass switch BPS closes when the arm current flows in the negative arm current direction. The negative arm current direction may be from the second cell connection terminal CC2 to the first cell connection terminal CC1, and the positive arm current direction may be from the first cell connection terminal CC1 to the second cell connection terminal CC2. In this case, a control signal can be transmitted when the arm current flows in the positive arm current direction.
[0084] This allows the control signal to conveniently reach and activate the bypass switch BPS as soon as the arm current flow changes in the desired direction. This provides a more responsive solution when detecting cell faults and can improve the performance of the voltage converter.
[0085] This method, -Step S33 involves estimating a first zero-crossing time tz1 corresponding to the point in time 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 tz2 corresponding to the point in time when the arm current direction changes from the negative arm current direction Iva- to the positive arm current direction Iva+, based on the waveform of the arm current. - Step S34 transmits a control signal to close the bypass switch BPS so that the bypass switch does not close earlier than the first zero-crossing time tz1 and does not close later than the second zero-crossing time tz2. It can further include
[0086] Figure 10 shows an example waveform of the arm current Iva. Here, the first zero-crossing point P2 is shown, corresponding to the first zero-crossing point tz1 when the arm current changes polarity from positive to negative. Here, the second zero-crossing point P4 is also shown, corresponding to the second zero-crossing point tz2 when the arm current changes polarity from negative to positive. These points can be estimated by analyzing the waveform of the arm current with respect to the rate of change in one or more time intervals prior to the aforementioned times tz1 and tz2. As an example, the zero-crossing point can be approximated based on time intervals Δt1 and Δt2 where the rate of change is substantially constant. In Figure 10, the first time interval is Δt1, starting approximately from time ts, and the second time interval is Δt2, starting approximately from time te. This allows for reliable and rapid estimation of the zero-crossing point. However, it should be understood that the zero-crossing point may be approximated by other methods.
[0087] Control units and gate control units have been described above in relation to half-bridge cells. It should be understood that the principles described herein can also be applied to full-bridge cells. In this case, there may be a second switching branch comprising a third and fourth switch connected in series and a third and fourth primary control unit for controlling the third and fourth switches, the second switching branch being connected in parallel to an energy storage branch comprising an energy storage element or a plurality of energy storage elements, and the second cell connection terminal being provided between the third and fourth switches. In this case, there may be a first further secondary gate control unit connected to one of the switches of the second switching branch and configured to apply a gate control signal that conducts to one of the switches of the second switching branch when a predetermined cell fault type is detected, provided that at least the bypass switch remains open. Furthermore, there may be a second further secondary gate control unit connected to the other switch of the second switching branch and configured to apply a gate control signal that conducts to this other switch of the second switching branch when a fault is detected, provided that at least the bypass switch remains open. The operation of both additional secondary gate control units may be triggered by the activation signal of the bypass switch.
[0088] While the above applies to embodiments of this disclosure, other embodiments and further embodiments can be devised without departing from the inventive concept described herein. However, the scope of the present invention is determined by the claims.
Claims
1. A method for controlling the operation of a cell (12) positioned on an arm of a voltage converter (10), wherein the arm is associated with a current phase in the voltage converter (10), and the cell (12) The first cell connection terminal (CC1), The second cell connection terminal (CC2), It comprises a first switch and a second switch (SW1, SW2) connected in series, with one of the first and second cell connection terminals (CC1) positioned between the first switch and the second switch, and each of the first and second switches (SW1, SW2) comprises a first switching branch having transistors (T1, T2) and antiparallel diodes (D1, D2), A bypass switch (BPS) connected between the first and second cell connection terminals (CC1, CC2), wherein the first switch (SW1) or the second switch (SW2) is connected in parallel with the bypass switch (BPS) Equipped with, The cell (12) is configured such that, when the transistors (T1, T2) of both the first and second switches (SW1, SW2) are turned off, it guides the arm current between the first and second cell connection terminals (CC1, CC2) through the diodes (D1, D2) of one of the switches (SW1, SW2) connected in parallel with the bypass switch (BPS) in the negative arm current direction (Iva-) of the arm current, and through the diodes (D1, D2) of the other of the first and second switches (SW1, SW2) in the positive arm current direction (Iva+) of the arm current. The aforementioned method, - When a predetermined fault type is detected in the cell (12), a predetermined circulating current is injected into the arm (S1), depending on whether the average current of the arm in one cycle of the fundamental frequency deviates from the reference average current in the arm, and whether the operating mode of the voltage converter is inverter mode. -Detecting the time period in which the arm current is in the negative arm current direction (Iva-) (S2), - Controlling the bypass switch (BPS) to close within the aforementioned time period (S3) Methods that include...
2. The method according to claim 1, wherein injecting a predetermined circulating current into the arm (S1) is performed when the converter is operating at a power level of 0.2 PU voltage or higher.
3. The method according to claim 1, wherein the injection of a predetermined circulating current into the arm (S1) is performed before the bypass switch is closed.
4. The method according to claim 1, wherein the direction of the negative arm current is from the second cell connection terminal (CC2) to the first cell connection terminal (CC1).
5. The method according to claim 1, wherein the positive arm current direction is from the first cell connection terminal (CC1) to the second cell connection terminal (CC2).
6. - To provide arm current measurement values measured by the current sensor (S4), - Determine the direction of the arm current from the measured arm current (S5) and The method according to claim 1, including the method described in claim 1.
7. Controlling the bypass switch (BPS) to close within the aforementioned time period (S3) is, - Sending a control signal to the bypass switch (BPS) to close the bypass switch (BPS) (S31), - Determining the timing of the control signal so as to compensate for the transmission time of the control signal and the closing time of the bypass switch (BPS) (S32) The method according to claim 1, including the method described in claim 1.
8. Determining the timing of the control signal (S32) - Transmitting the control signal to the bypass switch (BPS) during a time period in which the direction of the arm current is the positive arm current direction (Iva+) (S321), - Control the bypass switch to close when the direction of the arm current is the negative arm current direction (Iva-) The method according to claim 7, including the method described in claim 7.
9. - Estimate a first zero-crossing time (tz1) corresponding to the time when the direction of the arm current changes from the positive arm current direction (Iva+) to the negative arm current direction (Iva-), and a second zero-crossing time (tz2) corresponding to the time when the direction of the arm current changes from the negative arm current direction (Iva-) to the positive arm current direction (Iva+), based on the waveform of the arm current (S33). - A control signal is transmitted (S34) to close the bypass switch (BPS) so that the bypass switch does not close earlier than the first zero crossing time (tz1) and does not close later than the second zero crossing time (tz2). The method according to claim 7, including the method described in claim 7.
10. The method according to claim 1, wherein the predetermined failure type is loss of control of the first switch (SW1), loss of control of the second switch (SW2), and / or failure of the local cell control board.
11. The method according to claim 1, wherein the cell is a half-bridge cell or a full-bridge cell.
12. The method according to claim 1, wherein the method is applied to at least two cells (12) located on each arm of the voltage converter (10).
13. The method according to claim 1, wherein the method is applied to at least two cells (12) located on the same arm of the voltage converter (10).
14. A control unit comprising means for carrying out the method according to any one of claims 1 to 13.
15. A voltage converter such as a modular multilevel converter (MMC) comprising a plurality of cells (12) and the control unit described in claim 14.
Citation Information
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