Control method for eliminating current oscillation of modular multilevel converter
By controlling the switching of the power submodules before and after the zero-crossing moment of the bridge arm current in the modular multilevel converter, the problem of current oscillation in the modular multilevel converter is solved, the power submodules are protected, and damage to diodes and power switching devices is avoided.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CSG EHV POWER TRANSMISSION
- Filing Date
- 2025-12-17
- Publication Date
- 2026-04-10
AI Technical Summary
In modular multilevel converters, the power submodule generates current oscillations during the commutation process between diodes and power switching devices, which can damage the diodes and power switching devices.
By obtaining the zero-crossing moment of the arm current of the modular multilevel converter, setting the first and second moments, and controlling the switching operation of the power submodules before and after these moments, the switching operation is avoided during the conduction of power switching devices and the reverse recovery of diodes.
It effectively eliminates current oscillations in the power submodules of modular multilevel converters, protects the power submodules, and prevents damage to diodes and power switching devices.
Smart Images
Figure CN121840748A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present disclosure relates to the technical field of converter control, in particular to a control method for eliminating current oscillation of a modular multilevel converter. BACKGROUND
[0002] With the large-scale development of domestic new energy, flexible direct current transmission technology has also been rapidly iterated. At present, most flexible direct current transmission projects adopt modular multilevel converters (MMC). MMC can generate output voltage closer to a sine wave, reduce voltage fluctuation, thereby significantly reducing harmonic content, enhancing the stability of power system operation, and improving the level of power quality.
[0003] During the operation of the power sub-modules of the MMC, commutation of the diodes and the power switching devices occurs. During the commutation of the diodes and the power switching devices, current oscillation occurs. Excessive current oscillation can cause a large voltage across the diodes, thereby causing damage to the diodes and the power switching devices.
[0004] Therefore, how to reduce the current oscillation of the modular multilevel converter becomes a technical problem that needs to be solved by those skilled in the art. SUMMARY
[0005] In order to solve the above technical problems or at least partially solve the above technical problems, the present disclosure provides a control method for eliminating current oscillation of a modular multilevel converter, which can eliminate the current oscillation generated on each power sub-module in the modular multilevel converter, thereby achieving protection of each power sub-module.
[0006] The present disclosure provides a control method for eliminating current oscillation of a modular multilevel converter, the control method comprising: obtaining a zero-crossing time of a bridge arm current of the modular multilevel converter, and setting a first time and a second time based on the zero-crossing time; wherein the first time is before the zero-crossing time, and the second time is after the zero-crossing time; controlling the modular multilevel converter to put in a first number of power sub-modules before the first time; controlling the modular multilevel converter to put in a second number of power sub-modules at the zero-crossing time; wherein the first number is the number of power sub-modules put in the modular multilevel converter between the first time and the zero-crossing time; and the second number is the number of power sub-modules put in the modular multilevel converter between the zero-crossing time and the second time.
[0007] Optionally, after the zero-crossing time of the bridge arm current of the modular multilevel converter is acquired, and the first time and the second time are set based on the zero-crossing time, the method comprises: calculating, according to the power sub-module quantity calculation formula, a first quantity of power sub-modules of the modular multilevel converter put into use between the first time and the zero-crossing time; calculating, according to the power sub-module quantity calculation formula, a second quantity of power sub-modules of the modular multilevel converter put into use between the zero-crossing time and the second time; the power sub-module quantity calculation formula is:
[0008] wherein, n u is the quantity of power sub-modules of the upper bridge arm of the modular multilevel converter put into use, n l is the quantity of power sub-modules of the lower bridge arm of the modular multilevel converter put into use, U c is the average value of the capacitor voltage of the power sub-module, U dc is the DC bus voltage of the modular multilevel converter, and msin(ωt) is the modulation function of the modular multilevel converter.
[0009] Optionally, calculating, according to the power sub-module quantity calculation formula, the first quantity of power sub-modules of the modular multilevel converter put into use between the first time and the zero-crossing time comprises: calculating, according to the power sub-module quantity calculation formula, a first sub-quantity of power sub-modules of the upper bridge arm of the modular multilevel converter put into use between the first time and the zero-crossing time, and a second sub-quantity of power sub-modules of the lower bridge arm of the modular multilevel converter put into use between the first time and the zero-crossing time; wherein, the sum of the first sub-quantity and the second sub-quantity is the first quantity.
[0010] Optionally, after calculating, according to the power sub-module quantity calculation formula, the first sub-quantity of power sub-modules of the upper bridge arm of the modular multilevel converter put into use between the first time and the zero-crossing time, and the second sub-quantity of power sub-modules of the lower bridge arm of the modular multilevel converter put into use between the first time and the zero-crossing time, the method further comprises: in the case that the current direction of the upper bridge arm is the first direction and the current direction of the lower bridge arm is the second direction at the first time, controlling, based on the first sub-quantity, the upper bridge arm to switch power sub-modules before the first time, and controlling, based on the second sub-quantity, the lower bridge arm to switch power sub-modules before the first time.
[0011] Optionally, calculating, according to the power sub-module quantity calculation formula, the second quantity of power sub-modules of the modular multilevel converter put into use between the zero-crossing time and the second time comprises: According to the power submodule quantity calculation formula, a third quantity of power submodules of the upper bridge arm of the modular multilevel converter and a fourth quantity of power submodules of the lower bridge arm of the modular multilevel converter are calculated from the zero-crossing moment to the second moment. The sum of the third quantity and the fourth quantity is the second quantity.
[0012] Optionally, after the third quantity of power submodules of the upper bridge arm of the modular multilevel converter and the fourth quantity of power submodules of the lower bridge arm of the modular multilevel converter are calculated from the zero-crossing moment to the second moment according to the power submodule quantity calculation formula, the method further comprises: In the case that the current direction of the upper bridge arm is the second direction and the current direction of the lower bridge arm is the first direction at the second moment, the power submodules of the upper bridge arm are controlled to be switched at the zero-crossing moment based on the third quantity, and the power submodules of the lower bridge arm are controlled to be switched at the zero-crossing moment based on the fourth quantity.
[0013] Optionally, the first voltage value of each power submodule from the first moment to the zero-crossing moment and the second voltage value of each power submodule from the zero-crossing moment to the second moment are calculated according to the capacitor voltage prediction formula of the power submodule. Based on the fact that there is a first voltage value that does not meet the preset condition in the plurality of first voltage values corresponding to the at least one power submodule, at least one power submodule is controlled to be removed and the same number of power submodules are controlled to be put in before the first moment; Based on the fact that there is a second voltage value that does not meet the preset condition in the plurality of second voltage values corresponding to the at least one power submodule, at least one power submodule is controlled to be removed and the same number of power submodules are controlled to be put in at the zero-crossing moment.
[0014] Optionally, the capacitor voltage prediction formula is:
[0015] Wherein, C is the capacitance value of the power submodule, i arm is the current value flowing through the upper bridge arm or the lower bridge arm, u c is the capacitor voltage value at any moment.
[0016] Optionally, the zero-crossing moment of the bridge arm current of the modular multilevel converter is obtained, and the first moment and the second moment are set based on the zero-crossing moment, comprising: The bridge arm alternating current component and the bridge arm direct current component of the modular multilevel converter are obtained, and the zero-crossing moment of the bridge arm current is calculated according to the bridge arm current calculation formula. The first time and the second time are determined based on a reverse recovery time of a diode of the power sub-module and a zero-crossing time.
[0017] Optionally, the bridge arm current calculation formula is:
[0018] wherein, i u is a current of an upper bridge arm of the modular multilevel converter, i l is a current of a lower bridge arm of the modular multilevel converter, i sa sin(ωt) is an AC component of the bridge arm; I dc is a DC component of the bridge arm.
[0019] The present disclosure provides a control method for eliminating current oscillation of a modular multilevel converter. The control method comprises: obtaining a zero-crossing time of a bridge arm current of the modular multilevel converter, and setting a first time and a second time based on the zero-crossing time; wherein the first time is before the zero-crossing time, and the second time is after the zero-crossing time; controlling the modular multilevel converter to put into a first number of power sub-modules before the first time; controlling the modular multilevel converter to put into a second number of power sub-modules at the zero-crossing time; wherein the first number is a number of power sub-modules put into the modular multilevel converter between the first time and the zero-crossing time; and the second number is a number of power sub-modules put into the modular multilevel converter between the zero-crossing time and the second time. Since the switching operation of the power sub-modules will cause current oscillation of the diode in the process of turning on of the power switching device and reverse recovery of the diode, the present disclosure determines the time of turning on of the power switching device and reverse recovery of the diode, i.e. the first time and the second time, according to the zero-crossing time of the bridge arm current of the modular multilevel converter. The present disclosure completes the switching operation of the power sub-modules before the first time and at the zero-crossing time, so as to avoid the switching operation of the power sub-modules in the process of turning on of the power switching device and reverse recovery of the diode. Therefore, the present disclosure can avoid current oscillation on the diode, so as to eliminate current oscillation generated on each power sub-module of the modular multilevel converter, and realize protection of each power sub-module. BRIEF DESCRIPTION OF DRAWINGS
[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.
[0021] Figure 1A structural schematic diagram of a modular multilevel converter provided by an embodiment of the present disclosure.
[0022] Figure 2 A structural schematic diagram of a power sub-module provided by an embodiment of the present disclosure.
[0023] Figure 3 A flowchart of a control method for eliminating current oscillation of a modular multilevel converter provided by an embodiment of the present disclosure. DETAILED DESCRIPTION
[0024] Features and exemplary embodiments of various aspects of the present application will be described below in detail. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to one of ordinary skill in the art that the present application can be practiced without some of these specific details. The description of the embodiments is merely intended to provide a better understanding of the present application by showing examples of the present application.
[0025] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The embodiments will be described in detail below with reference to the accompanying drawings.
[0026] It should be noted that, in this document, the relationship terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between the entities or operations. Moreover, the terms “include”, “contain” or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device. Without more limitations, the elements defined by the statement “include” do not exclude the presence of other identical elements in the process, method, article or device including the elements.
[0027] It should be understood that, when describing the structure of a component, when one layer, one region is referred to as being located “on” or “above” another layer, another region, it can mean being directly located on the other layer, another region, or containing other layers or regions between it and the other layer, another region. And if the component is flipped, the one layer, one region will be located “under” or “below” the other layer, another region.
[0028] It should be understood that the term "and / or" used herein is only to describe an association relationship of associated objects, which means that there can be three relationships, for example, A and / or B, which can represent three cases of A existing alone, A and B existing simultaneously, and B existing alone. In addition, the character " / " herein generally represents an "or" relationship between the front and rear associated objects.
[0029] In the embodiments of the present application, the term "electrically connected" can mean that two components are directly electrically connected, or that two components are electrically connected via one or more other components.
[0030] In the embodiments of the present application, the first node, the second node and the third node are only defined for the convenience of describing the circuit structure, and the first node, the second node and the third node are not an actual circuit unit.
[0031] Various modifications and changes can be made to the present application without departing from the spirit or scope of the present application, which will be apparent to one skilled in the art. Therefore, the present application is intended to cover modifications and variations of the present application that come within the scope of the corresponding claims (claimed technical solutions) and their equivalents. It should be noted that the embodiments provided by the present application can be combined with each other without contradiction.
[0032] With the large-scale development of domestic new energy, flexible direct current transmission technology has also been rapidly iterated. At present, most flexible direct current transmission projects adopt modular multilevel converters (MMC). MMC can generate output voltage closer to a sine wave, reduce voltage fluctuation, thereby significantly reducing harmonic content, enhancing the stability of power system operation, and improving the level of power quality.
[0033] The power sub-module of the MMC will undergo commutation of the diode and the power switching device during the working process. During the commutation of the diode and the power switching device, current oscillation will occur. Excessive current oscillation will cause a larger voltage across the diode, thereby causing damage to the diode and the power switching device.
[0034] Therefore, how to reduce the current oscillation of the modular multilevel converter becomes a technical problem that needs to be solved by those skilled in the art.
[0035] The control method for eliminating current oscillation of a modular multilevel converter provided by the present disclosure can determine the time when the power switching device is turned on and the time when the diode is reversed recovered, i.e., the first time and the second time, according to the zero-crossing time of the bridge arm current of the modular multilevel converter, because the switching operation of the power sub-module during the process of turning on the power switching device and the reverse recovery of the diode can cause current oscillation of the diode. The switching operation of the power sub-module can be completed before the first time and the zero-crossing time, so that the switching operation of the power sub-module during the process of turning on the power switching device and the reverse recovery of the diode can be avoided, and thus the current oscillation of the diode can be avoided, so as to eliminate the current oscillation generated on each power sub-module in the modular multilevel converter and protect each power sub-module.
[0036] The embodiments will be described in detail below with reference to the accompanying drawings.
[0037] Figure 1 A structural schematic diagram of a modular multilevel converter provided by the present disclosure is shown in FIG. 1. Figure 1 The modular multilevel converter includes three bridge arms 11.
[0038] Each of the upper bridge arm and the lower bridge arm of each bridge arm 11 includes a plurality of power sub-modules 111 connected in series. The first end of the bridge arm 11 is connected to the first end of the direct current port 12, the second end of the bridge arm 11 is connected to the second end of the direct current port 12, and the bridge arm midpoint A of each bridge arm 11 is connected to the alternating current port 13.
[0039] The external control module controls the turn-on and turn-off of the power switching device in each power sub-module 111, so as to put the power sub-module 111 into the bridge arm 11 or remove the power sub-module 111 from the bridge arm 11. The total number of the power sub-modules 111 put into the upper bridge arm and the lower bridge arm is fixed, so that the same number of power sub-modules 111 need to be put into the lower bridge arm when the power sub-modules 111 are removed from the upper bridge arm. The same number of power sub-modules 111 need to be removed from the lower bridge arm when the power sub-modules 111 are put into the upper bridge arm.
[0040] Figure 2 A structural schematic diagram of a power sub-module provided by the present disclosure is shown in FIG. 2. Figure 2 The power sub-module includes a power sub-module capacitor C1, a first power switching device Q1, a second power switching device Q2, a first diode D1, and a second diode D2.
[0041] The first end of the first power switch device Q1 is connected with the first end of the power sub-module capacitor C1, the second end of the first power switch device Q1 is connected with the first end of the second power switch device Q2, and the second end of the second power switch device Q2 is connected with the second end of the power sub-module capacitor C1. The negative end of the first diode D1 is connected with the first end of the first power switch device Q1, and the positive end of the first diode D1 is connected with the second end of the first power switch device Q1. The negative end of the second diode D2 is connected with the first end of the second power switch device Q2, and the positive end of the second diode D2 is connected with the second end of the second power switch device Q2. The first end of the second power switch device Q2 also serves as the first connection end 14, and the first connection end 14 is used for connection with an adjacent power sub-module or connection with the first end of the direct current port 12. The second end of the second power switch device Q2 also serves as the second connection end 15, and the second connection end 15 is used for connection with an adjacent power sub-module or connection with the second end of the direct current port 12.
[0042] The power sub-module has four working states. The first working state is that the current flows through the first diode D1, the power sub-module capacitor C1 is charged, and the power sub-module is in the put-in state. The second working state is that the current flows through the second power switch device Q2, the voltage of the power sub-module capacitor C1 remains unchanged, and the power sub-module is in the cut-off state. The third working state is that the current flows through the second diode D2, the voltage of the power sub-module capacitor C1 remains unchanged, and the power sub-module is in the cut-off state. The fourth working state is that the current flows through the first power switch device Q1, the power sub-module capacitor C1 is discharged, and the power sub-module is in the put-in state.
[0043] In the case where the working state of the power sub-module is converted from the first working state to the second working state, the second power switch device Q2 is turned on, and the first diode D1 is in the process of reverse recovery, at this time, it is the cut-off operation of the power sub-module, and current oscillation is easy to occur. In the case where the working state of the power sub-module is converted from the second working state to the first working state, the first diode D1 is turned on, and the second power switch device Q2 is turned off, at this time, it is the put-in operation of the power sub-module, and current oscillation will not occur. Therefore, in the case where the bridge arm current is in the forward direction, the cut-off operation of the power sub-module will produce current oscillation, and the put-in operation will not produce current oscillation. The forward direction is the direction in which the current flows from the bridge arm to the first connection end 14 of the power sub-module.
[0044] In the case that the working state of the power sub-module is converted from the third working state to the fourth working state, the first power switching device Q1 is turned on, and the second diode D2 is in the process of reverse recovery, at this time, the power sub-module is in the operation of putting in, and current oscillation is easy to occur. In the case that the working state of the power sub-module is converted from the fourth working state to the third working state, the second diode D2 is turned on, and the first power switching device Q1 is turned off, at this time, the power sub-module is in the operation of cutting off, and current oscillation does not occur. Therefore, in the case that the bridge arm current is reversed, the operation of putting in of the power sub-module will cause current oscillation, and the operation of cutting off will not cause current oscillation. The reverse is the direction of the current flowing out of the first connection end 14 of the power sub-module to the bridge arm.
[0045] However, at the zero-crossing moment of the bridge arm current of the modular multilevel converter, no matter how the four working states of the power sub-module are converted, the first diode D1 and the second diode D2 will not be in reverse recovery, and thus current oscillation will not occur.
[0046] Figure 3 A flowchart of a control method for eliminating current oscillation of a modular multilevel converter provided by the embodiment of the present disclosure is shown in FIG. 1. Figure 3 As shown in FIG. 1, the control method comprises S110-S130.
[0047] S110, the zero-crossing moment of the bridge arm current of the modular multilevel converter is acquired, and the first moment and the second moment are set based on the zero-crossing moment.
[0048] The first moment is before the zero-crossing moment, and the second moment is after the zero-crossing moment.
[0049] For example, the zero-crossing moment of the bridge arm current of the modular multilevel converter is acquired, and the moment when the diode in the power sub-module will be in reverse recovery is determined based on the zero-crossing moment, so that the first moment and the second moment can be set.
[0050] S120, the first number of power sub-modules of the modular multilevel converter is put in before the first moment.
[0051] The first number is the number of power sub-modules of the modular multilevel converter put in between the first moment and the zero-crossing moment.
[0052] For example, since the diode will be in reverse recovery between the first moment and the zero-crossing moment, if the power switching device is controlled to be turned on to realize the putting in or cutting off of the power sub-module in this period of time, current oscillation will occur in the power sub-module. Therefore, the first number of power sub-modules to be put in on the bridge arm is estimated before the first moment, so that the modular multilevel converter can put in the first number of power sub-modules before the first moment.
[0053] S130, at the zero-crossing moment, control the modular multilevel converter to engage the second number of power sub-modules.
[0054] The second quantity refers to the number of power sub-modules put into operation in the modular multilevel converter between the zero-crossing time and the second time.
[0055] For example, since the diode undergoes reverse recovery between the zero-crossing moment and the second moment, controlling the power switching device to turn on to enable or disable the power submodule during this period will cause current oscillation within the power submodule. Therefore, before the zero-crossing moment, the first number of power submodules to be enabled on the bridge arm is estimated, thereby enabling the modular multilevel converter to enable the second number of power submodules at the zero-crossing moment.
[0056] This disclosure avoids switching the power submodules during the power switching device conduction and diode reverse recovery process by completing the switching operation before the first moment and at the zero crossing moment. Therefore, this disclosure can avoid current oscillation on the diodes, thereby eliminating the current oscillation generated on each power submodule in the modular multilevel converter and achieving protection for each power submodule.
[0057] In some embodiments, after obtaining the zero-crossing time of the arm current of the modular multilevel converter and setting a first time and a second time based on the zero-crossing time, the method includes: Based on the formula for calculating the number of power submodules, calculate the first number of power submodules put into operation in the modular multilevel converter from the first moment to the zero crossing moment.
[0058] Based on the formula for calculating the number of power submodules, calculate the second number of power submodules put into operation by the modular multilevel converter between the zero-crossing time and the second time.
[0059] The formula for calculating the number of power submodules is:
[0060] Where, n u The number of power submodules, n, deployed in the upper arm of a modular multilevel converter. l The number of power submodules deployed in the lower arm of a modular multilevel converter, U c U is the average value of the capacitor voltage of the power submodule. dc Let ωt be the DC bus voltage of the modular multilevel converter, and msin(ωt) be the modulation function of the modular multilevel converter.
[0061] For example, the Round[] is an integer function. Before the first time, the average value of the capacitor voltage of the power sub-module, the DC bus voltage of the modular multilevel converter, and the modulation function of the modular multilevel converter are obtained, and are substituted into the power sub-module quantity calculation formula. Then, the first time is substituted into the modulation function, and the number of power sub-modules of the upper bridge arm and the number of power sub-modules of the lower bridge arm of one bridge arm of the modular multilevel converter, which are put into from the first time to the zero-crossing time, are respectively calculated, so as to determine the first number of power sub-modules of the modular multilevel converter. Before the zero-crossing time, the second time is substituted into the modulation function, and the number of power sub-modules of the upper bridge arm and the number of power sub-modules of the lower bridge arm of one bridge arm of the modular multilevel converter, which are put into from the zero-crossing time to the second time, are respectively calculated, so as to determine the second number of power sub-modules of the modular multilevel converter. Thus, the present disclosure can obtain the first number and the second number corresponding to the first time to the zero-crossing time and the zero-crossing time to the second time, respectively, before the process of turning on the power switching device and the reverse recovery of the diode, and can complete the switching operation of the power sub-module before the first time and at the zero-crossing time, so as to avoid the switching operation of the power sub-module in the process of turning on the power switching device and the reverse recovery of the diode. Therefore, the present disclosure can avoid the current oscillation on the diode, so as to eliminate the current oscillation on each power sub-module in the modular multilevel converter, and realize the protection of each power sub-module.
[0062] In some embodiments, the first number of power sub-modules of the modular multilevel converter put into from the first time to the zero-crossing time is calculated according to the power sub-module quantity calculation formula, including: The first sub-number of power sub-modules of the upper bridge arm of the modular multilevel converter and the second sub-number of power sub-modules of the lower bridge arm of the modular multilevel converter, which are put into from the first time to the zero-crossing time, are calculated according to the power sub-module quantity calculation formula.
[0063] The sum of the first sub-number and the second sub-number is the first number.
[0064] In an example, since the total number of power sub-modules put into the upper bridge arm and the lower bridge arm of the modular multilevel converter is fixed, in the case that the upper bridge arm additionally puts in power sub-modules, the lower bridge arm needs to cut off the same number of power sub-modules; in the case that the upper bridge arm cuts off power sub-modules, the lower bridge arm needs to put in the same number of power sub-modules. Therefore, before the first time, the first number of power sub-modules put into the upper bridge arm of the modular multilevel converter and the second number of power sub-modules put into the lower bridge arm of the modular multilevel converter from the first time to the zero-crossing time are calculated according to the power sub-module number calculation formula, so as to control the switching of the power sub-modules of the upper and lower bridge arms before the first time respectively.
[0065] In some embodiments, after the first number of power sub-modules put into the upper bridge arm of the modular multilevel converter and the second number of power sub-modules put into the lower bridge arm of the modular multilevel converter from the first time to the zero-crossing time are calculated according to the power sub-module number calculation formula, the method further comprises: In the case that the current direction of the upper bridge arm is the first direction and the current direction of the lower bridge arm is the second direction at the first time, the switching of the power sub-modules of the upper bridge arm before the first time is controlled based on the first number, and the switching of the power sub-modules of the lower bridge arm before the first time is controlled based on the second number.
[0066] In an example, continuing to refer to Figure 2 Taking the case that the first direction is forward and the second direction is reverse as an example. The current direction of the upper bridge arm is the first direction, i.e. the direction of the current flowing from the bridge arm to the first connection end 14 of the power sub-module; the current direction of the lower bridge arm is the second direction, i.e. the direction of the current flowing from the first connection end 14 of the power sub-module to the bridge arm. Therefore, in the case that the working state of the power sub-module in the upper bridge arm is converted from the first working state to the second working state, the cutting operation of the power sub-module is prone to current oscillation, and therefore the cutting of the power sub-modules of the upper bridge arm before the first time needs to be controlled according to the first number, so that the total number of power sub-modules put into the upper bridge arm before the first time is the first number. Correspondingly, at this time, the working state of the power sub-module in the lower bridge arm is converted from the third working state to the fourth working state, and the putting operation of the power sub-module is prone to current oscillation, and therefore the putting of the same number of power sub-modules as the cutting of the upper bridge arm into the lower bridge arm at the zero-crossing time needs to be controlled according to the second number, so that the total number of power sub-modules put into the lower bridge arm at the zero-crossing time is the second number.
[0067] Taking the first direction as reverse and the second direction as forward as an example, the current direction of the upper bridge arm is the first direction, i.e., the direction in which the current flows out of the bridge arm from the first connection end 14 of the power sub-module; the current direction of the lower bridge arm is the second direction, i.e., the direction in which the current flows into the bridge arm from the first connection end 14 of the power sub-module. If the working state of the power sub-module in the upper bridge arm is converted from the third working state to the fourth working state, the power sub-module putting-in operation is prone to current oscillation, and therefore the first sub-quantity is required to be used to control the power sub-module putting-in of the upper arm before the first time, so that the total number of the power sub-modules put in the upper arm before the first time is the first sub-quantity. Correspondingly, at this time, the working state of the power sub-module in the lower bridge arm is converted from the first working state to the second working state, and the power sub-module cutting-off operation is prone to current oscillation, and therefore the second sub-quantity is required to be used to control the power sub-module cutting-off of the lower arm at the zero-crossing time, so that the total number of the power sub-modules put in the lower arm at the zero-crossing time is the second sub-quantity.
[0068] In some embodiments, the second quantity of the power sub-modules put in by the modular multilevel converter between the zero-crossing time and the second time is calculated according to the power sub-module quantity calculation formula, including: The third sub-quantity of the power sub-modules put in by the upper arm of the modular multilevel converter and the fourth sub-quantity of the power sub-modules put in by the lower arm of the modular multilevel converter between the zero-crossing time and the second time are calculated according to the power sub-module quantity calculation formula.
[0069] The sum of the third sub-quantity and the fourth sub-quantity is the second quantity.
[0070] Exemplarily, since the total number of the power sub-modules put in by the upper and lower arms of the modular multilevel converter is fixed, in the case that the upper arm additionally puts in power sub-modules, the lower arm needs to cut off the same number of power sub-modules; in the case that the upper arm cuts off power sub-modules, the lower arm needs to put in the same number of power sub-modules. Therefore, before the zero-crossing time, the first sub-quantity of the power sub-modules put in by the upper arm of the modular multilevel converter and the second sub-quantity of the power sub-modules put in by the lower arm of the modular multilevel converter between the zero-crossing time and the second time are calculated according to the power sub-module quantity calculation formula, so as to control the putting-in and cutting-off of the power sub-modules of the upper and lower arms at the zero-crossing time, respectively.
[0071] In some embodiments, after the third sub-quantity of the power sub-modules put in by the upper arm of the modular multilevel converter and the fourth sub-quantity of the power sub-modules put in by the lower arm of the modular multilevel converter between the zero-crossing time and the second time are calculated according to the power sub-module quantity calculation formula, the method further includes: In the second moment, the current direction of the upper bridge arm is the second direction, the current direction of the lower bridge arm is the first direction, based on the third sub-quantity, the upper bridge arm is controlled to put in the power sub-modules at the zero-crossing moment, and based on the fourth sub-quantity, the lower bridge arm is controlled to cut off the power sub-modules at the zero-crossing moment.
[0072] For example, continuing to refer to Figure 2 Taking the first direction as the forward direction and the second direction as the reverse direction as an example. The current direction of the upper bridge arm is the second direction, that is, the direction of the current flowing out of the first connection end 14 in the power sub-module to the bridge arm; the current direction of the lower bridge arm is the first direction, that is, the direction of the current flowing into the first connection end 14 in the power sub-module from the bridge arm. Therefore, if the working state of the power sub-module in the upper bridge arm is converted from the third working state to the fourth working state, the power sub-module put-in operation is easy to produce current oscillation, so it is necessary to control the upper arm to put in the power sub-modules according to the first sub-quantity before the first moment, so that the total number of the power sub-modules put in the upper bridge arm before the first moment is the first sub-quantity. Correspondingly, at this time, the working state of the power sub-module in the lower bridge arm is converted from the first working state to the second working state, and the power sub-module cut-off operation is easy to produce current oscillation, so it is necessary to control the lower bridge arm to cut off the same number of power sub-modules as the power sub-modules put in the upper bridge arm according to the second sub-quantity at the zero-crossing moment, so that the total number of the power sub-modules put in the lower bridge arm at the zero-crossing moment is the second sub-quantity.
[0073] Taking the first direction as the reverse direction and the second direction as the forward direction as an example. The current direction of the upper bridge arm is the second direction, that is, the direction of the current flowing into the first connection end 14 in the power sub-module from the bridge arm; the current direction of the lower bridge arm is the first direction, that is, the direction of the current flowing out of the first connection end 14 in the power sub-module to the bridge arm. If the working state of the power sub-module in the upper bridge arm is converted from the first working state to the second working state, the power sub-module cut-off operation is easy to produce current oscillation, so it is necessary to control the upper bridge arm to cut off the power sub-modules according to the first sub-quantity before the first moment, so that the total number of the power sub-modules put in the upper bridge arm before the first moment is the first sub-quantity. Correspondingly, at this time, the working state of the power sub-module in the lower bridge arm is converted from the third working state to the fourth working state, and the power sub-module put-in operation is easy to produce current oscillation, so it is necessary to control the lower bridge arm to put in the same number of power sub-modules as the power sub-modules cut off by the upper bridge arm according to the second sub-quantity at the zero-crossing moment, so that the total number of the power sub-modules put in the lower bridge arm at the zero-crossing moment is the second sub-quantity.
[0074] In some embodiments, the control method further comprises: According to the capacitor voltage prediction formula of the power sub-module, the first voltage value of the power sub-module at each time from the first time to the zero-crossing time is calculated, and the second voltage value of the power sub-module at each time from the zero-crossing time to the second time is calculated.
[0075] Based on the fact that there is a first voltage value that does not meet the preset condition in the plurality of first voltage values corresponding to the at least one power sub-module, at least one power sub-module is controlled to be cut off and the same number of power sub-modules are put into operation before the first time.
[0076] Based on the fact that there is a second voltage value that does not meet the preset condition in the plurality of second voltage values corresponding to the at least one power sub-module, at least one power sub-module is controlled to be cut off and the same number of power sub-modules are put into operation at the zero-crossing time.
[0077] For example, the capacitor voltage prediction formula is:
[0078] wherein C is the capacitance value of the power sub-module, i arm is the current value flowing through the upper bridge arm or the lower bridge arm, u c is the capacitor voltage value at any time.
[0079] According to the capacitor voltage prediction formula, the voltage value of the capacitor of the power sub-module at each time can be calculated, so that it can be predicted whether the capacitor of the power sub-module will be charged to the upper limit or discharged to the lower limit between the first time and the zero-crossing time, and between the zero-crossing time and the second time. Therefore, for the charging process of the capacitor, the voltage value of the capacitor in the power sub-module that does not meet the preset condition is the voltage value equal to the charging voltage threshold. For the discharging process of the capacitor, the voltage value of the capacitor in the power sub-module that does not meet the preset condition is the voltage value equal to the discharging voltage threshold.
[0080] For example, the plurality of power sub-modules charge from the first time to the zero-crossing time, and then the plurality of power sub-modules discharge from the zero-crossing time to the second time. In the case that there is a first voltage value equal to the charging voltage threshold in the plurality of first voltage values corresponding to the at least one power sub-module, the power sub-modules are cut off before the first time, and the same number of power sub-modules are put into operation, so as to ensure the normal operation of the modular multilevel converter from the first time to the zero-crossing time, and to avoid overcharging of the capacitor in the power sub-module. In the case that there is a second voltage value equal to the discharging voltage threshold in the plurality of second voltage values corresponding to the at least one power sub-module, the power sub-modules are cut off at the zero-crossing time, and the same number of power sub-modules are put into operation, so as to ensure the normal operation of the modular multilevel converter from the zero-crossing time to the second time, and to avoid over-discharging of the capacitor in the power sub-module.
[0081] For example, the plurality of power sub-modules discharge from the first time to the zero-crossing time, and then the plurality of power sub-modules charge from the zero-crossing time to the second time. In the case that there is a first voltage value equal to the discharging voltage threshold in the plurality of first voltage values corresponding to the at least one power sub-module, the power sub-modules are cut off before the first time, and the same number of power sub-modules are put into operation, so as to ensure the normal operation of the modular multilevel converter from the first time to the zero-crossing time, and to avoid over-discharging of the capacitor in the power sub-module. In the case that there is a second voltage value equal to the charging voltage threshold in the plurality of second voltage values corresponding to the at least one power sub-module, the power sub-modules are cut off at the zero-crossing time, and the same number of power sub-modules are put into operation, so as to ensure the normal operation of the modular multilevel converter from the zero-crossing time to the second time, and to avoid overcharging of the capacitor in the power sub-module.
[0082] In some embodiments, the zero-crossing time of the bridge arm current of the modular multilevel converter is obtained, and the first time and the second time are set based on the zero-crossing time, including: The bridge arm alternating current and the bridge arm direct current of the modular multilevel converter are obtained, and the zero-crossing time of the bridge arm current is calculated according to a bridge arm current calculation formula.
[0083] The first time and the second time are determined based on the reverse recovery time of the diode of the power sub-module and the zero-crossing time.
[0084] For example, the bridge arm current calculation formula is:
[0085] wherein, i u is the current of one upper bridge arm of the modular multilevel converter, i lfor the current of a lower bridge arm of the modular multilevel converter, i sa sin(ωt) is the bridge arm alternating current component; I dc is the bridge arm direct current component.
[0086] According to the bridge arm current calculation formula, the current of the bridge arm at any time can be calculated, and thus the time of the zero-crossing of the bridge arm current can be calculated according to the condition that the bridge arm current is zero. According to the reverse recovery time of the diode in the power sub-module and the zero-crossing time, a time period during which the switching of the power sub-module causes the current oscillation of the power sub-module can be determined, and the two time nodes of the time period are the first time and the second time.
[0087] The above is only a specific embodiment of the present disclosure, enabling those skilled in the art to understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to these embodiments herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. A control method for eliminating current oscillations in a modular multilevel converter, characterized in that, The control method includes: The zero-crossing time of the arm current of the modular multilevel converter is obtained, and a first time and a second time are set based on the zero-crossing time; wherein, the first time is before the zero-crossing time, and the second time is after the zero-crossing time; Before the first moment, control the modular multilevel converter to engage a first number of power sub-modules; At the zero-crossing moment, the modular multilevel converter is controlled to engage a second number of the power sub-modules; Wherein, the first quantity is the number of power sub-modules put into operation in the modular multilevel converter between the first time and the zero-crossing time; the second quantity is the number of power sub-modules put into operation in the modular multilevel converter between the zero-crossing time and the second time.
2. The control method according to claim 1, characterized in that, After obtaining the zero-crossing time of the arm current of the modular multilevel converter and setting a first time and a second time based on the zero-crossing time, the method includes: Based on the formula for calculating the number of power sub-modules, calculate the first number of power sub-modules put into operation by the modular multilevel converter from the first moment to the zero-crossing moment; Based on the formula for calculating the number of power sub-modules, calculate the second number of power sub-modules put into operation by the modular multilevel converter from the zero crossing time to the second time. The formula for calculating the number of power submodules is: Where, n u The number of power submodules deployed in the upper arm of the modular multilevel converter, n l The number of power submodules deployed in the lower arm of the modular multilevel converter, U c U is the average value of the capacitor voltage of the power submodule. dc Let ωt be the DC bus voltage of the modular multilevel converter, and msin(ωt) be the modulation function of the modular multilevel converter.
3. The control method according to claim 2, characterized in that, The calculation of the first number of power submodules engaged in the modular multilevel converter from the first moment to the zero-crossing moment, according to the formula for calculating the number of power submodules, includes: Based on the formula for calculating the number of power submodules, calculate the first number of power submodules put into the upper arm of the modular multilevel converter and the second number of power submodules put into the lower arm of the modular multilevel converter from the first moment to the zero crossing moment. The sum of the first number of sub-sub ...
4. The control method according to claim 3, characterized in that, After calculating, according to the formula for calculating the number of power submodules, the first number of power submodules switched on the upper arm of the modular multilevel converter and the second number of power submodules switched on the lower arm of the modular multilevel converter between the first time point and the zero-crossing time point, the method further includes: At the first moment, if the current direction of the upper bridge arm is the first direction and the current direction of the lower bridge arm is the second direction, Based on the first number of sub-modules, the upper bridge arm is controlled to disconnect the power sub-module before the first moment. And, based on the second sub-quantity, control the lower bridge arm to engage the power sub-module before the first moment.
5. The control method according to claim 2, characterized in that, The step of calculating the second number of power submodules engaged in the modular multilevel converter from the zero-crossing time to the second time, according to the formula for calculating the number of power submodules, includes: Based on the formula for calculating the number of power sub-modules, calculate the third number of power sub-modules put into the upper arm of the modular multilevel converter and the fourth number of power sub-modules put into the lower arm of the modular multilevel converter between the zero crossing time and the second time. The sum of the third sub-quantity and the fourth sub-quantity is the second quantity.
6. The control method according to claim 5, characterized in that, After calculating, according to the formula for calculating the number of power submodules, the third number of power submodules engaged in the upper arm of the modular multilevel converter and the fourth number of power submodules engaged in the lower arm of the modular multilevel converter between the zero-crossing time and the second time, the method further includes: At the second moment, when the current direction of the upper bridge arm is the second direction and the current direction of the lower bridge arm is the first direction, Based on the third sub-quantity, the upper bridge arm is controlled to switch on the power sub-module at the zero-crossing time. Furthermore, based on the fourth sub-quantity, the lower bridge arm is controlled to switch the power sub-module at the zero-crossing time.
7. The control method according to claim 1, characterized in that, The method further includes: Based on the capacitor voltage prediction formula of the power submodule, calculate the first voltage value of the power submodule at each time from the first time to the zero crossing time, and calculate the second voltage value of the power submodule at each time from the zero crossing time to the second time. Based on the fact that there is a first voltage value that does not meet the preset conditions among the multiple first voltage values corresponding to at least one of the power sub-modules, before the first moment, control the disconnection of at least one of the power sub-modules and the activation of the same number of power sub-modules; Based on the fact that there is a second voltage value that does not meet the preset conditions among the multiple second voltage values corresponding to at least one of the power sub-modules, at the zero crossing time, control is to cut off at least one of the power sub-modules and put in the same number of power sub-modules.
8. The control method according to claim 7, characterized in that, The capacitor voltage prediction formula is as follows: Where C is the capacitance value of the power submodule, i arm u is the current value flowing through the upper bridge arm or the lower bridge arm. c The capacitor voltage value at any given moment.
9. The control method according to claim 1, characterized in that, The step of obtaining the zero-crossing time of the arm current of the modular multilevel converter and setting the first and second times based on the zero-crossing time includes: Obtain the AC and DC components of the bridge arm of the modular multilevel converter, and calculate the zero-crossing time of the bridge arm current according to the bridge arm current calculation formula. The first time and the second time are determined based on the reverse recovery time of the diodes in the power submodule and the zero-crossing time.
10. The control method according to claim 9, characterized in that, The formula for calculating the bridge arm current is as follows: Among them, i u i represents the current of one upper arm of the modular multilevel converter. l i represents the current of one lower arm of the modular multilevel converter. sa sin(ωt) is the AC component of the bridge arm; I dc The DC component of the bridge arm.