An improved carrier phase-shift capacitor voltage balancing method for DC fault ride-through

Abstract

The present invention discloses an improved carrier phase-shift capacitor voltage balancing method suitable for DC fault riding. On the basis of traditional carrier phase-shift modulation, the distribution of signals between half-bridge and full-bridge sub-modules is adjusted based on a virtual half-bridge perspective, and carrier phase-shifting is smoothly applied to realize DC fault riding of a hybrid MMC with a small number of sub-modules. Further on this basis, based on the idea of ​​reconstructing the mapping relationship between sub-modules and pulse signals, the positive half-cycle and negative half-cycle of the modulation wave are distinguished according to the voltage output requirement of the system, and the established distribution relationship between the pulse and the sub-module is adjusted based on the sorting method to realize the balance of the sub-module capacitor voltage during DC fault riding. The present invention does not change the stability of the controller and the system without introducing an additional balance compensation signal. It smoothly realizes the capacitor voltage balance of the hybrid MMC with a small number of sub-modules during DC fault riding by only changing the mapping relationship based on the sorting.

Description

Technical Field

[0001] The present invention belongs to the field of modular multi-level converters, and in particular relates to an improved carrier phase-shift capacitor voltage balancing method suitable for DC fault ride-through. Background Art

[0002] Modular multilevel converters (MMCs) have become increasingly popular in recent years due to their modular structure, low switching frequency, and strong fault handling capabilities. These advantages include flexible direct current transmission (HVDC), medium and low voltage DC distribution networks, and renewable energy integration. In some scenarios, such as renewable energy integration, the number of MMC submodules is relatively small, unlike high-voltage, large-capacity MMCs, which often have hundreds of submodules per bridge arm. Therefore, the typical nearest level control (NLM) modulation is no longer applicable. Instead, phase-shifted carrier PWM (PSC-PWM) can achieve a higher equivalent switching frequency at a lower switching frequency, further maximizing converter performance.

[0003] Submodule capacitor voltage balancing control is one of the key technologies that enables MMC to maintain stable operation. Traditional carrier phase-shift modulation, due to its self-balancing characteristics, can maintain submodule capacitor voltage stability during steady-state operation. However, during a DC fault, the system undergoes complex and rapid transient processes, and the active current limiting control implemented to achieve DC fault ride-through changes the switching state of the full-bridge submodules, which affects the submodule capacitor voltage. The system is very likely to lose balance or even deteriorate due to capacitor voltage fluctuations, causing it to cease normal operation. Therefore, traditional carrier phase-shift modulation cannot meet the requirements of DC fault ride-through and capacitor voltage balancing control during this period.

[0004] To solve the above problems, based on traditional carrier phase shifting, we deeply explore the self-balancing characteristics of traditional carrier phase shifting and propose an improved carrier phase shifting capacitor voltage balancing method suitable for DC fault ride-through. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide an improved carrier phase-shift capacitor voltage balancing method suitable for DC fault ride-through, which not only enables the hybrid MMC to smoothly implement the DC fault ride-through function, but also does not introduce additional balancing signals and complex control algorithms. By reconstructing the pulse mapping relationship, the sub-module capacitor voltage balance during the DC fault period is achieved, maintaining the stable operation of the system.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] An improved carrier phase-shift capacitor voltage balancing method suitable for DC fault ride-through includes the following steps:

[0008] S1. When the system is working normally, the traditional carrier phase shift modulation method is used. The modulated wave is compared with the carrier wave to obtain the switching pulse signal, which is sent to the sub-module of each bridge arm in turn. The carrier amplitude and phase of the upper and lower bridge arms are the same, and the modulated wave is in opposite phase.

[0009] S2. After a DC fault occurs, the DC bus voltage drops to zero. The modulated wave is now a sine wave with equal positive and negative half-cycles around the X-axis. The positive half-cycle corresponds to the positive voltage output requirement, and the negative half-cycle corresponds to the negative voltage output requirement. Based on the different positive and negative voltage output requirements, the positive and negative half-cycles are modulated separately.

[0010] S3. In the positive half-cycle of the modulation wave, the left equivalent half-bridges of HBSM and FBSM both output positive voltage. The positive half-cycle is compared with the carrier to obtain a switching pulse signal, which is used to drive the submodule to output positive voltage. Similarly, the negative half-cycle is compared with the carrier to obtain a switching pulse signal, which is used to drive the submodule to output negative voltage.

[0011] S4. Considering the capacitor voltage balance control requirements, change the mapping relationship between the generated pulse signal and the actual sub-module, sort the sub-module capacitor voltage values ​​based on the carrier frequency, and divide the switch pulse signal by the carrier frequency to complete the sampling and holding function.

[0012] Furthermore, based on traditional carrier phase-shift modulation, the signal distribution between half-bridge and full-bridge submodules is adjusted, successfully implementing carrier phase shifting to achieve DC fault ride-through in a hybrid MMC with a small number of submodules. Furthermore, based on the concept of reconstructing the mapping relationship between submodules and pulse signals, the positive and negative half-cycles of the modulation wave are distinguished according to the system's voltage output requirements. Based on a sorting method, the established distribution relationship between pulses and submodules is adjusted to achieve balanced submodule capacitor voltages during DC fault ride-through.

[0013] Furthermore, on the basis of traditional carrier phase-shift modulation, the signal distribution between the half-bridge and full-bridge sub-modules is adjusted based on the virtual half-bridge idea. The positive and negative half-cycles of the modulation wave are distinguished by utilizing the characteristic that the DC bias of the modulation wave is zero during DC fault ride-through. The positive voltage is output by the half-bridge sub-module and the virtual half-bridge on the left side of the full-bridge sub-module, and the negative voltage is output only by the virtual half-bridge on the right side of the full-bridge sub-module. The carrier phase-shift is successfully applied to realize the DC fault ride-through of the hybrid MMC with few sub-modules.

[0014] Furthermore, during the DC fault ride-through period, in the positive half-cycle of the modulation wave, based on the sub-module capacitor voltage results obtained by the sorting method, the sub-module with higher voltage is selected to be put into operation to meet the positive voltage output requirement; in the negative half-cycle of the modulation wave, based on the sub-module capacitor voltage results obtained by the sorting method, the full-bridge sub-module is selected to be put into operation to meet the negative voltage output requirement of the system.

[0015] Beneficial effects of the present invention:

[0016] 1. The improved carrier phase-shift capacitor voltage balancing method for DC fault ride-through proposed in the present invention does not introduce additional corrections such as balance compensation signals, which will not affect the output power quality and system stability. It also does not introduce additional circulating current injection or other methods to affect the sub-module capacitor voltage ripple.

[0017] 2. The improved carrier phase-shifted capacitor voltage balancing method for DC fault ride-through proposed in this invention derives a pulse signal by simply separating the positive and negative half cycles of the modulation wave. The mapping relationship between the pulse and the actual submodule is then reconstructed based on a sorting method, achieving capacitor voltage balancing control of each submodule without increasing the switching frequency or switching losses.

[0018] 3. The improved carrier phase-shift capacitor voltage balancing method suitable for DC fault ride-through proposed in the present invention adopts HB+FB few-submodule hybrid MMC. Based on carrier phase-shift modulation, it ensures that the system smoothly implements active current limiting control to complete DC fault ride-through, while maintaining the sub-module capacitor voltage balance, thereby ensuring the stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0020] Figure 1 1 is a schematic diagram of a three-phase hybrid MMC topology structure according to an embodiment of the present invention;

[0021] Figure 2 Schematic diagram of the working state of the HBSM (half-bridge submodule) according to an embodiment of the present invention;

[0022] Figure 3 1 is a schematic diagram of the working state of the FBSM (full bridge submodule) according to an embodiment of the present invention;

[0023] Figure 4 : This is a waveform diagram of the modulation wave of the upper and lower bridge arms of phase A of an embodiment of the present invention;

[0024] Figure 5is a schematic diagram of four groups of carriers according to an embodiment of the present invention;

[0025] Figure 6 Schematic diagram of pulse trigger signals of four submodules of a single bridge arm according to an embodiment of the present invention;

[0026] Figure 7 1 is a schematic diagram of changes in active power transmitted by a converter station according to an embodiment of the present invention;

[0027] Figure 8 2 is a schematic diagram of AC current output from a converter station according to an embodiment of the present invention;

[0028] Figure 9 is a schematic diagram of the DC bus voltage according to an embodiment of the present invention;

[0029] Figure 10 1 is a schematic diagram of DC bus current according to an embodiment of the present invention;

[0030] Figure 11 Schematic diagram of the average capacitor voltage of all submodules in an embodiment of the present invention;

[0031] Figure 12 3. It is a schematic diagram of specific values ​​of capacitor voltage of the upper bridge arm submodule of phase A in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] Since HBSM does not have DC fault ride-through capability, and FBSM is complex to control and has a higher cost than HBSM, the system's operating conditions and voltage output requirements are comprehensively considered.

[0034] like Figure 1 As shown, the simulation model of this embodiment adopts a three-phase six-bridge structure, and each bridge arm is a hybrid MMC structure of two HBSMs and two FBSMs. Figure 2 、 Figure 3 The working states of HBSM and FBSM are shown respectively. When HBSM is in the positive input state, T1 is turned on and T2 is turned off. At this time, the output voltage of the submodule is U SM Equal to the capacitor voltage U C In bypass state, device T1 is turned off and T2 is turned on. At this time, the submodule capacitor C is bypassed and the submodule output voltage U SM =0. That is, HBSM can output 0, +U by controlling the on-off state of the switching device.C Two levels.

[0035] When the FBSM is in the positive input state, T1 and T4 are turned on, and T2 and T3 are turned off. At this time, the submodule output voltage U SM Equal to the capacitor voltage U C ; In the negative input state, devices T1 and T4 are turned off, T2 and T3 are turned on, and the submodule output voltage U SM Equal to -U C In bypass state, devices T1, T3 or T2, T4 are turned on, the submodule capacitor C is bypassed, and the submodule output voltage U SM =0. That is, FBSM can output 0, +U by controlling the on-off state of the switching device. C 、-U C Therefore, the negative input state of the FBSM can be used to widen the voltage output range of the hybrid MMC bridge arm, thereby reducing the DC output voltage of the converter station and achieving control of the DC fault current.

[0036] Because FBSMs require two additional insulated gate bipolar transistor (IGBT) control signals compared to HBSMs, they are traditionally more complex. The two systems are not well connected, making it difficult to identify common control features to optimize valve-side control strategies. This paper develops a control method for capacitor voltage balancing based on the perspective of a virtual half-bridge and hybrid modulation technology.

[0037] The system adopts the traditional carrier phase shift modulation strategy when it is working normally. The four submodules in the bridge arm correspond to the same modulation wave, and the four groups of carriers are phase shifted 360° / N in sequence. The modulation waves of the lower bridge arm and the upper bridge arm are in opposite phases, and the four groups of carriers are the same in sequence. Figure 5 Comparing the modulation wave with the four groups of carrier waves, we get four groups of switching pulse signals, as shown in Figure 6 As shown, the four sub-modules in the bridge arm are given in turn, that is, the traditional carrier phase shift modulation is realized. Because of its self-balancing characteristics, the sub-module capacitor voltage balance effect is better in steady state. When a DC fault occurs, because the DC bus voltage drops to zero rapidly, it is equivalent to the DC bias of the modulation wave dropping to zero, so the modulation wave is now a sine wave with equal positive and negative half-cycles around the X-axis. The positive half-cycle corresponds to the positive voltage output demand, and the negative half-cycle corresponds to the negative voltage output demand. Therefore, based on the different positive and negative voltage output requirements, it is necessary to modulate the positive and negative half-cycles separately, that is, to classify them according to the positive and negative half-cycles of the modulation wave. As shown Figure 4As shown, in the positive half-cycle of the modulation wave (taking the upper bridge arm of phase A as an example, the lower bridge arm is similar), the left equivalent half-bridges of the two HBSMs and two FBSMs can both output positive voltage. Therefore, after comparing the positive half-cycle with the two groups of carrier waves, two groups of switching pulse signals are obtained to drive the submodule to output positive voltage. Similarly, comparing the negative half-cycle with the two groups of carrier waves also results in two groups of switching pulse signals to drive the submodule to output negative voltage. That is, according to the positive and negative half-cycles of the modulation wave, two types of switching pulse signals will be generated, each with two groups of switching pulse signals:

[0038] (1) In the positive half cycle of the modulation wave, the problem that needs to be solved is how to give two sets of switching pulse signals to two of the four sub-modules, so that the control system can complete fault ride-through and achieve capacitor voltage balance control.

[0039] Considering the need for capacitor voltage balance control, the system implements selective activation based on the concept of reconstructing the mapping relationship between pulse signals and actual submodules. First, the capacitor voltage values ​​of the four submodules are sorted. Second, a judgment is made based on the ranking results and the positive or negative direction of the bridge arm current. The judgment logic is: the bridge arm current causes the two submodules with lower voltages to be activated during charging and the two submodules with higher voltages to be activated during discharging. Third, to avoid excessive switching frequency changes due to the comparison result changing too frequently, which would increase meaningless losses, the submodule capacitor voltage values ​​and switching pulse signals are sampled at the carrier frequency to implement a sample-and-hold function. In other words, the carrier frequency is used as a balance control cycle. Finally, within each balance control cycle, two submodules are selected from the four submodules and given pulse signals to be activated, completing capacitor voltage balance control.

[0040] (2) In the negative half-cycle of the modulation wave, since only the FBSM has the ability to output negative voltage, only two FBSMs are put into operation, and the judgment is made based on the voltage sorting results of the two FBSM capacitors and the positive and negative direction of the bridge arm current. It should be noted that since the right equivalent half-bridge of the FBSM, that is, the part that can output negative voltage, is actually equivalent to the HBSM of the reverse connection circuit, the signal outputting positive voltage to the normal HBSM is equivalent to outputting negative voltage on it, so its charging and discharging characteristics are opposite to those of the HBSM. Therefore, for the positive bridge arm current, the HBSM with higher voltage should be put into operation first (at this time, the output is negative level, which is actually equivalent to discharging), and for the negative bridge arm current, the sub-module with lower voltage should be put into operation first (output is negative level, which is actually equivalent to charging). The judgment logic is the same as before, and it is still "low level is charged first when charging, and high level is discharged first when discharging".

[0041] Although only two FBSM sub-modules participate in sorting, reconstructing mapping, and inputting into the circuit to achieve negative voltage output in the negative half-cycle of the modulation wave, two FBSMs and two HBSMs simultaneously participate in sorting, reconstructing mapping, and inputting into the circuit to achieve positive voltage output in the positive half-cycle of the modulation wave. Therefore, the capacitor voltages of all sub-modules participate in the balancing control link, and all sub-module capacitor voltages can be balanced.

[0042] The proposed solution of the present invention was verified using MATLAB / Simulink simulation software. The simulation results are as follows: Figure 7 、 Figure 8 、 Figure 9 、 Figure 10 、 Figure 11 、 Figure 12 As shown. The system is set to operate normally until the DC side inter-pole short circuit fault occurs at 0.3s. In order to verify the effectiveness of the proposed balance control method, the fault time is set to last forever. During normal operation, FBSM is put into operation in the form of HBSM, and the traditional carrier phase shift modulation enables the system to work smoothly. After the DC side inter-pole short circuit fault occurs at 0.3s, Figure 7 The active power waveform shows that the DC side output power drops rapidly to 0 after transient fluctuations. Figure 9 It can be seen that the DC voltage also drops to 0 rapidly. During the DC fault period, the reactive power transmission is set to zero. In order to maintain the balance of the bridge arm power and prevent the submodule capacitor from overvoltage, the AC current controller will reduce the active current command, thereby reducing the active power absorbed by the MMC from the AC system. Figure 8 It can be seen that the fundamental frequency component of the bridge arm current rapidly decays to zero. At the same time, the DC component of the bridge arm current gradually decreases to 0, and there is almost no obvious overcurrent in the bridge arm current before and after the fault. FBSM mainly realizes the output negative voltage function and realizes fault ride-through operation. Figure 11 From the DC bus current waveform, it can be seen that the DC bus current is controlled to 0 during the DC fault period. Figure 11 It can be seen from the waveform of the mean submodule capacitor voltage that the submodule capacitor voltage can be kept stable during the entire DC inter-pole short-circuit fault process. Since the submodule capacitor needs to absorb the fault energy, the capacitor voltage rises in the early stage of the DC fault processing process, deviating from the rated value by 10%, which is within the tolerance range of the device. Since the capacitor voltage balance control based on the reconstructed pulse and submodule mapping of the sorting method is adopted during the DC fault ride-through, not only the mean submodule capacitor voltage, but also the capacitor voltage of each submodule can be maintained near the rated value, that is, the balanced control of all submodule capacitor voltages is achieved. Figure 12 The specific values ​​of the capacitor voltage of the bridge arm submodule (taking the upper bridge arm of phase A as an example) show that the two HBSMs, the two FBSMs, and the FBSM and HBSM can all achieve good capacitor voltage balance, maintaining good capacitance consistency.

[0043] In summary, an improved carrier phase-shift capacitor voltage balancing method suitable for DC fault ride-through includes the following steps:

[0044] S1. When the system is working normally, the traditional carrier phase shift modulation method is used. The modulated wave is compared with the carrier wave to obtain the switching pulse signal, which is sent to the sub-module of each bridge arm in turn. The carrier amplitude and phase of the upper and lower bridge arms are the same, and the modulated wave is in opposite phase.

[0045] S2. After a DC fault occurs, the DC bus voltage drops to zero. The modulated wave is now a sine wave with equal positive and negative half-cycles around the X-axis. The positive half-cycle corresponds to the positive voltage output requirement, and the negative half-cycle corresponds to the negative voltage output requirement. Based on the different positive and negative voltage output requirements, the positive and negative half-cycles are modulated separately.

[0046] S3. In the positive half-cycle of the modulation wave, the left equivalent half-bridges of HBSM and FBSM both output positive voltage. The positive half-cycle is compared with the carrier to obtain a switching pulse signal, which is used to drive the submodule to output positive voltage. Similarly, the negative half-cycle is compared with the carrier to obtain a switching pulse signal, which is used to drive the submodule to output negative voltage.

[0047] S4. Considering the capacitor voltage balance control requirements, the mapping relationship between the generated pulse signal and the actual sub-module is changed. The sub-module capacitor voltage values ​​are sorted based on the carrier frequency, and the switch pulse signal is divided by the carrier frequency to complete the sampling and holding function. That is, the carrier frequency is used as a balance control cycle. After the sub-module capacitor voltage and the switch pulse signal are sampled and held, the following situations will occur:

[0048] In the positive half-cycle of the modulation wave, the sub-module with lower voltage is put into operation during charging, and the sub-module with higher voltage is put into operation during discharging, based on the sorting results of the sub-module capacitor voltages and the positive and negative directions of the bridge arm currents.

[0049] In the negative half-cycle of the modulation wave, since only the FBSM has the capability of negative voltage output, the judgment is made only based on the FBSM capacitor voltage sorting results and the positive or negative direction of the bridge arm current.

[0050] It should be noted that the right-hand equivalent half-bridge of the FBSM, i.e., the part capable of outputting negative voltage, is essentially equivalent to an HBSM connected in reverse. Therefore, a signal outputting positive voltage by the normal HBSM is equivalent to outputting negative voltage by this HBSM. Therefore, its charge and discharge characteristics are opposite to those of the HBSM. Therefore, for positive-direction bridge arm current, the higher-voltage HBSM should be used (which outputs a negative level, effectively discharging), while for negative-direction bridge arm current, the lower-voltage sub-module should be used first (which outputs a negative level, effectively charging).

[0051] Although only the FBSM sub-module participates in sorting, reconstructing mapping, and inputting the circuit to achieve negative voltage output in the negative half-cycle of the modulation wave, since the FBSM and HBSM simultaneously participate in sorting, reconstructing mapping, and inputting the circuit to achieve positive voltage output in the positive half-cycle of the modulation wave, the capacitor voltages of all sub-modules participate in the balancing control link, and all sub-module capacitor voltages can be balanced.

[0052] Based on traditional carrier phase-shift modulation, this application adjusts the signal distribution between half-bridge and full-bridge submodules, successfully applying carrier phase shifting to achieve DC fault ride-through in a hybrid MMC with a small number of submodules. Furthermore, based on the idea of ​​reconstructing the mapping relationship between submodules and pulse signals, the positive and negative half-cycles of the modulated wave are distinguished according to the system's voltage output requirements. Based on the sorting method, the established distribution relationship between pulses and submodules is adjusted to achieve submodule capacitor voltage balance during DC fault ride-through.

[0053] Based on the idea of ​​virtual half-bridge, the signal distribution between the half-bridge and full-bridge sub-modules is adjusted. The positive and negative half-cycles of the modulation wave are distinguished by the characteristic that the DC bias of the modulation wave is zero during DC fault ride-through. The positive voltage is output by the half-bridge sub-module and the virtual half-bridge on the left side of the full-bridge sub-module, and the negative voltage is output only by the virtual half-bridge on the right side of the full-bridge sub-module. The carrier phase shifting is successfully applied to realize the DC fault ride-through of the hybrid MMC with few sub-modules.

[0054] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0055] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the invention as claimed.

Claims

1. An improved carrier phase-shift capacitor voltage balancing method suitable for DC fault ride-through, characterized in that: The steps include: S1. When the system is working normally, the traditional carrier phase shift modulation method is used. The modulated wave is compared with the carrier wave to obtain the switching pulse signal, which is sent to the sub-module of each bridge arm in turn. The carrier amplitude and phase of the upper and lower bridge arms are the same, and the modulated wave is in opposite phase. S2. After a DC fault occurs, the DC bus voltage drops to zero. The modulated wave is now a sine wave with equal positive and negative half-cycles around the X-axis. The positive half-cycle corresponds to the positive voltage output requirement, and the negative half-cycle corresponds to the negative voltage output requirement. Based on the different positive and negative voltage output requirements, the positive and negative half-cycles are modulated separately. S3. In the positive half-cycle of the modulation wave, the left equivalent half-bridges of HBSM and FBSM both output positive voltage. The positive half-cycle is compared with the carrier to obtain a switching pulse signal, which is used to drive the submodule to output positive voltage. Similarly, the negative half-cycle is compared with the carrier to obtain a switching pulse signal, which is used to drive the submodule to output negative voltage. During the DC fault ride-through period, during the positive half-cycle of the modulation wave, based on the sub-module capacitor voltage results obtained by the sorting method, the sub-module with the higher voltage is selected for operation to meet the positive voltage output requirement. During the negative half-cycle of the modulation wave, based on the sub-module capacitor voltage results obtained by the sorting method, the full-bridge sub-module is selected for operation to meet the negative voltage output requirement of the system. S4. Considering the capacitor voltage balance control requirements, change the mapping relationship between the generated pulse signal and the actual sub-module, sort the sub-module capacitor voltage values ​​based on the carrier frequency, and divide the switch pulse signal by the carrier frequency to complete the sampling and holding function.

2. The improved carrier phase-shift capacitor voltage balancing method for DC fault ride-through according to claim 1, characterized in that: Based on traditional carrier phase-shift modulation, the signal distribution between the half-bridge and full-bridge sub-modules is adjusted, and carrier phase shifting is successfully applied to realize DC fault ride-through of the hybrid MMC with few sub-modules. Furthermore, based on the idea of ​​reconstructing the mapping relationship between sub-modules and pulse signals, the positive and negative half-cycles of the modulation wave are distinguished according to the voltage output requirements of the system. Based on the sorting method, the established distribution relationship between pulses and sub-modules is adjusted to achieve the balance of sub-module capacitor voltage during DC fault ride-through.

3. The improved carrier phase-shift capacitor voltage balancing method for DC fault ride-through according to claim 1, characterized in that: On the basis of traditional carrier phase-shift modulation, the signal distribution between the half-bridge and full-bridge sub-modules is adjusted based on the virtual half-bridge idea. The positive and negative half-cycles of the modulation wave are distinguished by the characteristic that the DC bias of the modulation wave is zero during DC fault ride-through. The positive voltage is output by the half-bridge sub-module and the virtual half-bridge on the left side of the full-bridge sub-module, and the negative voltage is output only by the virtual half-bridge on the right side of the full-bridge sub-module. The carrier phase-shift is successfully applied to realize DC fault ride-through of the hybrid MMC with few sub-modules.

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