Fault-tolerant operation method under sub-module fault of modular multilevel converter

Abstract

The invention provides a fault-tolerant operation method for a modular multilevel converter under a sub-module fault, and belongs to the technical field of power electronic control. The method comprises the following steps: when a first sub-module fault occurs in the modular multi-level converter, bypassing a sub-module with the fault, bypassing a plurality of normal sub-modules in a bridge arm without the sub-module fault in the same phase or a bridge arm with a smaller number of sub-module faults, and enabling the number of bypassed sub-modules in two complementary bridge arms to be equal. And if a new sub-module in the two complementary bridge arms breaks down in the subsequent operation period, bypassing the corresponding sub-module, and meanwhile, releasing the bypassed normal sub-module in the bridge arm with more newly-broken sub-modules or bypassing the normal sub-module in the complementary bridge arm so as to ensure that the number of the normally-switched sub-modules in the upper and lower bridge arms is equal to that of the normally-switched sub-modules in the upper and lower bridge arms. Symmetrical operation of the upper bridge arm and the lower bridge arm of the converter can be ensured, and capacitor voltage balance between the upper bridge arm and the lower bridge arm during fault-tolerant operation is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a fault-tolerant operation method under sub-module failure of a modular multi-level converter. Background Art

[0002] With the rapid development of large-scale new energy grid connection, large-scale data centers and long-distance DC transmission, medium and high voltage large-capacity converters have been widely used in many fields. At present, low-voltage power semiconductor devices are connected in series and parallel or low-voltage sub-modules are connected in series and parallel to meet the application requirements of high voltage level and large-capacity configuration. In medium and high voltage occasions, the converter design scheme using a large number of low-voltage power semiconductors in series and parallel is difficult to maintain the consistency of the device, which is easy to cause problems such as uneven electrical stress and thermal stress between semiconductors. On the other hand, when low-voltage power semiconductors are directly connected in series and parallel, local faults of the structure are difficult to eliminate, and it is not easy to expand and repair. Therefore, in practical application engineering, a series of low-voltage sub-modules are usually used as the smallest structural unit of series and parallel connection, such as half-bridge sub-modules, full-bridge sub-modules, etc. Low-voltage sub-modules are easy to put in and remove, have strong scalability and high flexibility, can be effectively used in medium and high voltage large-capacity converters, and are currently the preferred circuit structure for medium and high voltage large-capacity occasions. The valve string composed of low-voltage sub-modules in series is usually divided into upper and lower bridge arms, and together with bridge arm inductors and other components, it forms a symmetrical modular multi-level circuit. The upper and lower bridge arms jointly support a stable DC side voltage, and the midpoint of the bridge arm generates a specified AC output. However, in actual application scenarios, due to a series of factors such as device life, environmental temperature and humidity, and power failure, there is always a possibility of failure of sub-modules during normal operation. If the faulty sub-module is only bypassed, the symmetrical operation between the bridge arms cannot be guaranteed, affecting the stable output of the converter. In addition, when the upper and lower bridge arms operate asymmetrically, the voltage and current stresses borne by the sub-modules of the upper and lower bridge arms will be unbalanced, and the heating of the sub-modules will be more unbalanced. The sub-modules that operate normally in the bridge arm with more sub-module failures may accelerate aging due to excessive electrical and thermal stress, or even be damaged due to overvoltage and overcurrent, thereby further increasing the risk of sub-module failure in the bridge arm. Designing a sub-module fault-tolerant operation method for a specific modular multi-level converter topology can improve its operation reliability. The fault-tolerant operation method suitable for AC / DC conversion occasions and various modulation strategies has wider adaptability and application value. Summary of the invention

[0003] In order to solve the problems in the prior art, the present invention proposes a fault-tolerant operation method of a modular multi-level converter under sub-module failure.

[0004] The technical solution of the present invention is as follows:

[0005] The present invention discloses a fault-tolerant operation method of a modular multilevel converter under submodule failure, wherein the modular multilevel converter comprises one or more phase units, each phase unit comprises two upper and lower complementary bridge arms, each bridge arm comprises N submodules connected in series, N being a positive integer greater than or equal to 2; the fault-tolerant operation method comprises:

[0006] 1) Continuously perform fault detection on the submodules in each phase unit during the control cycle of the modular multilevel converter, and if a new submodule fails, bypass the failed submodule;

[0007] When a submodule failure occurs in the modular multilevel converter for the first time, the submodule with the failure is bypassed, and a number of normal submodules are bypassed in a bridge arm where no submodule failure occurs or in a bridge arm where the number of submodule failures is less, so that the number of bypassed submodules in two complementary bridge arms is equal;

[0008] 2) Check whether the number of bypassed submodules in the two complementary bridge arms is equal, if so, proceed to step 1); if not, proceed to step 3);

[0009] 3) Check whether there are any bypassed normal sub-modules in the bridge arm with a larger number of newly failed sub-modules in the two complementary bridge arms. If yes, release the bypassed normal sub-modules to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to step 1); if no, bypass the normal sub-modules in the complementary bridge arm to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to step 1).

[0010] Furthermore, if the number of faulty sub-modules in any bridge arm of the modular multi-level converter exceeds a threshold, the modular multi-level converter is shut down.

[0011] Compared with the prior art, the present invention has the following beneficial effects:

[0012] Compared with other fault-tolerant operation methods, the present invention relates to a fault-tolerant operation method for a modular multilevel converter when one or more submodules fail during operation, that is, when the modular multilevel converter has a first submodule failure, the submodule with the failure is bypassed, and a number of normal submodules are bypassed in a bridge arm in the same phase where no submodule failure occurs or in a bridge arm with a smaller number of submodule failures, so that the number of bypassed submodules in two complementary bridge arms is equal. During the subsequent operation, if a normally switched submodule of one of the two complementary bridge arms fails, the submodule is bypassed, and the bypassed normal submodule in the bridge arm is released or the normal submodule in its complementary bridge arm is bypassed, so as to ensure that the number of normally switched submodules in the upper and lower bridge arms is the same, and at the same time ensure that the upper and lower bridge arms operate symmetrically. During subsequent operation, if a certain number of submodules in both complementary bridge arms have new faults, the corresponding submodules are bypassed, and at the same time, the bypassed normal submodules in the bridge arm with a larger number of newly faulty submodules are released, or the normal submodules in the complementary bridge arm are bypassed to ensure that the number of normally switched submodules in the upper and lower bridge arms is equal, ensuring symmetrical operation of the upper and lower bridge arms.

[0013] It can be seen that the present invention, under the premise of ensuring the effective removal of the faulty submodule, simultaneously achieves the symmetrical operation of the upper and lower complementary bridge arms, so as to avoid the uneven electrical and thermal stress of the upper and lower bridge arm submodule switch devices caused by asymmetry, and ensure the consistency and reliability of the submodule operation state. The present invention can ensure the fault removal of the submodule and the symmetrical operation of the complementary bridge arm without considering the other parts of the converter structure, and has better versatility. It overcomes the problems of complex control links, strong pertinence, and weak adaptability when the existing modular multi-level converter fault-tolerant operation method considering the influence of bridge arm asymmetry is optimized for the asymmetric operation condition of the bridge arm. Compared with the existing fault-tolerant operation method optimized for the asymmetric operation condition of the bridge arm, the present invention avoids the asymmetric operation condition of the complementary bridge arm, and can be applied to any converter containing the modular multi-level circuit. Therefore, the present invention has a simple and practical strategy implementation principle and steps, overcomes the problem that the faulty submodule is only bypassed in the prior art and the symmetrical operation of the upper and lower bridge arms cannot be guaranteed, and further eliminates the potential fault problems of electrical and thermal stress that may be generated later, and has wide adaptability and application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a topology diagram of a modular multi-level resonant converter.

[0015] Figure 2 This is the working mode diagram of the half-bridge submodule.

[0016] Figure 3 The program flow chart of the fault-tolerant operation method under sub-module failure of the modular multi-level converter is shown in FIG.

[0017] Figure 4 The waveform of the submodule capacitor voltage under the fault-tolerant operation method of bypassing only the faulty submodule is shown in FIG.

[0018] Figure 5 The waveforms of the bridge arm voltage and bridge arm current under the fault-tolerant operation method of bypassing only the faulty submodule are shown in FIG.

[0019] Figure 6 The figure is a bridge arm capacitor voltage waveform diagram when the fault-tolerant operation method proposed in the present invention is adopted.

[0020] Figure 7 The waveform diagram of the bridge arm voltage and bridge arm current when the fault-tolerant operation method proposed in the present invention is adopted. DETAILED DESCRIPTION

[0021] The present invention is further described and illustrated below in conjunction with specific embodiments. The embodiments are merely exemplary of the present disclosure and do not define the scope of limitation. The technical features of each embodiment of the present invention may be combined accordingly without conflicting with each other.

[0022] The present invention proposes a fault-tolerant operation method for a modular multilevel converter under a submodule failure, so as to enhance the operational reliability of the modular multilevel converter under a submodule failure. The scheme makes full use of the feature that the submodule can be flexibly switched, and controls the submodules of the corresponding bridge arm according to a given switching decision, so as to ensure that the number of submodules normally switched in the upper and lower bridge arms is the same, so as to realize symmetrical operation of the upper and lower bridge arms. Compared with the fault-tolerant strategies under various modular topologies, the fault-tolerant operation method is suitable for AC / DC conversion occasions and various modulation strategies, and improves the utilization rate of the hot standby submodule, and has a wide range of application value.

[0023] First, the modular multilevel converter includes one or more phase units, each phase unit includes two upper and lower complementary bridge arms, each bridge arm includes N submodules connected in series, and N is a positive integer greater than or equal to 2. The fault-tolerant operation method of the modular multilevel converter under submodule failure of the present invention includes:

[0024] 1) Continuously perform fault detection on the submodules in each phase unit during the control cycle of the modular multilevel converter, and if a new submodule fails, bypass the failed submodule;

[0025] When a submodule failure occurs in the modular multilevel converter for the first time, the submodule with the failure is bypassed, and a number of normal submodules are bypassed in a bridge arm where no submodule failure occurs or in a bridge arm where the number of submodule failures is less, so that the number of bypassed submodules in two complementary bridge arms is equal;

[0026] 2) Check whether the number of bypassed submodules in the two complementary bridge arms is equal, if so, proceed to step 1); if not, proceed to step 3);

[0027] 3) Check whether there are any bypassed normal sub-modules in the bridge arm with a larger number of newly failed sub-modules in the two complementary bridge arms. If yes, release the bypassed normal sub-modules to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to step 1); if no, bypass the normal sub-modules in the complementary bridge arm to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to step 1).

[0028] Meanwhile, if the number of faulty submodules in any bridge arm of the modular multilevel converter exceeds a threshold, the modular multilevel converter is shut down.

[0029] In a specific embodiment of the present invention, the sub-module fault tolerance operation of the modular multi-level resonant converter is taken as a specific implementation example. Figure 1 As shown, a classic single-phase modular multi-level resonant converter consists of a modular multi-level circuit, an LLC resonant circuit, a medium-high frequency isolation transformer and an output rectifier circuit. The modular multi-level circuit can be composed of one phase, two phases or three phases, each phase is divided into two upper and lower bridge arms, each bridge arm is composed of N half-bridge sub-modules and a bridge arm inductor connected in series in sequence, and the upper and lower bridge arm inductors are wound on the same magnetic core in a coupled winding manner, which can ensure that the upper and lower bridge arm inductances are equal, and the equivalent inductance of the bridge arm inductance on the AC side is zero, realizing the decoupling design of the bridge arm inductance and the LLC resonant inductor. The AC output end of the modular multi-level circuit can output an AC waveform with variable amplitude, and the AC waveform includes a sine wave, a square wave, a quasi-square wave, a trapezoidal wave and a triangular wave. The LLC resonant circuit is composed of a resonant capacitor, a transformer leakage inductance and an excitation inductance. The transformer secondary side is connected to a full-bridge rectifier circuit, which can reduce the voltage and current stress of a single switching device. The output rectifier circuit can be a full-bridge rectifier circuit, a half-bridge rectifier circuit, a voltage doubler rectifier circuit, or a circuit based on a modular multi-level structure.

[0030] like Figure 2 As shown in the figure, the modular multi-level circuit adopts half-bridge sub-modules. Each half-bridge sub-module consists of two switching tubes, their anti-parallel diodes and DC capacitors. The switching tubes can be power MOSFETs or IGBTs. During normal operation, the half-bridge structure sub-modules in the bridge arm participate in switching according to the normal operation mechanism of the topology. A mechanical switch is connected between the input and output ends of the half-bridge sub-module. When a sub-module fails, the mechanical switch of the sub-module can be closed to bypass the sub-module. When the maximum limit of the number of faulty sub-modules in the bridge arm is not exceeded, the converter continues to operate in a fault-tolerant manner. The working mode of the sub-module is as follows Figure 2 As shown in Figure 2, when the submodule is in the input state, Figure 2 As shown in the leftmost figure, the mechanical switch S3 is disconnected, the switch tube S2 is turned off, and the switch tube S1 is turned on, and the capacitor C is energized; when the submodule is in the cut-off state, such as Figure 2 As shown in the middle figure, the mechanical switch S3 is disconnected, the switch tube S2 is turned on, and the switch tube S1 is turned off, and the capacitor C is cut out of the circuit; when the submodule is in the bypass state, the bypass may be caused by a fault or a compensated bypass, such as Figure 2 As shown in the rightmost figure, the mechanical switch S3 is closed, the switch tube S2 is turned off, and the switch tube S1 is turned off, and the capacitor C is cut out of the circuit.

[0031] like Figure 3 As shown, the present invention proposes a fault-tolerant operation method for a modular multilevel converter under submodule failure. When a submodule failure occurs in the modular multilevel converter, the number of submodules normally switched on and off in the upper and lower bridge arms is ensured to be the same, so that the upper and lower bridge arms can operate symmetrically. Figure 3 As shown in the figure, the specific steps to implement the operation strategy are:

[0032] The submodules of each bridge arm are numbered in sequence, denoted as i, where i is the bridge arm submodule number, i=1,2,3,…,N; the numbers of the bypassed normal submodules of each bridge arm are grouped into a number set, denoted as A; the numbers of the faulty submodules of each bridge arm are grouped into a number set, denoted as B; the numbers of the submodules in normal switching of each bridge arm are grouped into a number set, denoted as C. The numbers in the three number sets A, B, and C are different, and the sum of the number of numbers in the three number sets A, B, and C is always N.

[0033] The first step is to continuously perform fault detection on the submodules in each phase unit during the control cycle of the modular multilevel converter. When a submodule fault occurs in the modular multilevel converter for the first time, the faulty submodule is bypassed, and a number of normal submodules are bypassed in a bridge arm where no submodule fault occurs or in a bridge arm where the number of submodule faults is less, so that the number of bypassed submodules in two complementary bridge arms is equal.

[0034] Specifically, when a submodule failure occurs in the modular multilevel converter for the first time, if the number of submodules with failure is 1, then one normal submodule is bypassed in the bridge arm where no submodule failure occurs in the same phase, so that the number of bypassed submodules in the two complementary bridge arms is equal; if the number of submodules with failure is greater than or equal to 2 and occurs in the same bridge arm, then the same number of normal submodules are bypassed in the bridge arm where no submodule failure occurs in the same phase, so that the number of bypassed submodules in the two complementary bridge arms is equal; if the number of submodules with failure is greater than or equal to 2 and occurs in two bridge arms, then a number of normal submodules are bypassed in the bridge arm where the number of submodule failures is less in the same phase, and the number of normal submodules is the difference between the number of submodules with failure in the two bridge arms, so that the number of bypassed submodules in the two complementary bridge arms is equal;

[0035] After the modular multilevel converter completes a round of fault-tolerant operation, the submodules of each bridge arm are always detected. If a new bridge arm submodule fails, the faulty submodule of the bridge arm is bypassed and the second step is entered.

[0036] In the second step, after a round of fault-tolerant operation strategy is completed, since multiple submodules may fail at the same time and may occur in two bridge arms, if the new faulty bridge arm submodule happens to be a normal submodule that is bypassed after the fault-tolerant strategy is run, then the number of bypassed submodules between the upper and lower bridge arms is the same, and there is no need to release additional normal submodules. In other cases, check whether the number of bypassed submodules in the two complementary bridge arms is equal. If not, proceed to the third step; if so, it means that the above situation occurs, and no additional protection operation is required at this time, and go directly back to the first step.

[0037] The third step is to check whether there is any bypassed normal sub-module in the bridge arm with a larger number of newly failed sub-modules in the two bridge arms. If yes, select the normal sub-module corresponding to the largest number in the number set A of the bridge arm with a larger number of newly failed sub-modules to release it, restore to normal switching, so that the number of bypassed sub-modules in the two complementary bridge arms is equal, and return to the first step; if no, bypass the normal sub-module in the complementary bridge arm of the bridge arm with a larger number of newly failed sub-modules, so that the number of bypassed sub-modules in the two complementary bridge arms is equal, and return to the first step again.

[0038] Specifically, after a round of fault-tolerant operation strategy is completed, if there is a new faulty sub-module, check whether there is a bypassed normal sub-module in the bridge arm where the new faulty sub-module is located. If yes, release one bypassed normal sub-module in the bridge arm to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to the first step; if not, bypass one normal sub-module in the complementary bridge arm of the bridge arm where the new faulty sub-module is located to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to the first step again.

[0039] After a round of fault-tolerant operation strategy is completed, if there are more than or equal to 2 newly faulty submodules and they occur in the same bridge arm, check whether there are bypassed normal submodules in the bridge arm where the newly faulty submodule is located. If yes, release the same number of bypassed normal submodules in the bridge arm to make the number of bypassed submodules in the two complementary bridge arms equal, and return to the first step; of course, if the number of bypassed normal submodules in the bridge arm is less than the number of newly faulty submodules at this time, release all bypassed normal submodules, and bypass a number of normal submodules in the complementary bridge arm of the bridge arm, the number of these normal submodules is the difference between the number of newly faulty submodules and the number of all bypassed normal submodules in the bridge arm; if no, bypass the same number of normal submodules in the complementary bridge arm of the bridge arm where the newly faulty submodule is located to make the number of bypassed submodules in the two complementary bridge arms equal, and return to the first step again.

[0040] After a round of fault-tolerant operation strategy is completed, if the number of failed sub-modules is greater than or equal to 2 and occurs in two bridge arms, check whether there are bypassed normal sub-modules in the bridge arm with more newly failed sub-modules in the two bridge arms. If yes, release a number of bypassed normal sub-modules from the bridge arm with more newly failed sub-modules. The number of normal sub-modules is the number of newly failed sub-modules in the two bridge arms with the difference of normal sub-modules, so that the number of bypassed sub-modules in the two complementary bridge arms is equal, and return to the first step; if the number of bypassed normal sub-modules in the bridge arm is less than the number of newly failed sub-modules in the two bridge arms, the number of bypassed normal sub-modules in the two complementary bridge arms is equal, and return to the first step; if the number of bypassed normal sub-modules in the bridge arm is less than the number of newly failed sub-modules in the two bridge arms, the number of bypassed normal sub-modules in the bridge arm is equal to the number of newly failed sub-modules in the two bridge arms. If there is a difference in the number of sub-modules with new faults, all bypassed normal sub-modules are released, and a number of normal sub-modules are bypassed in the complementary bridge arm of the bridge arm, and the number of these normal sub-modules is the difference between the number of new faulty sub-modules in the two bridge arms and the number of all bypassed normal sub-modules in the bridge arm; if there is no difference, a number of normal sub-modules in the complementary bridge arm of the bridge arm with a larger number of new faulty sub-modules are bypassed, and this number of normal sub-modules is the difference between the number of faulty sub-modules in the two bridge arms, so that the number of bypassed sub-modules in the two complementary bridge arms is equal, and the process returns to the first step.

[0041] The fourth step is to loop the first step to the third step and continue the fault-tolerant operation; if the number of faulty submodules in any bridge arm of the modular multilevel converter exceeds a threshold, the modular multilevel converter is shut down.

[0042] In order to demonstrate the effect of the fault-tolerant operation method of the modular multilevel converter submodule under fault conditions proposed in the present invention, a modular multilevel resonant converter simulation model was built in the PLECS electrical simulation software for verification. The main parameters of the simulation model are as follows: DC input voltage 9kV~18kV, output voltage is constant 375V, maximum output power is 100kW, the number of bridge arm submodules is 18, and the resonant frequency is 12kHz. Table 1 shows the parameter values ​​of the implementation case.

[0043] Table 1

[0044] Parameters Parameter Value DC input voltage 9kV~18kV DC output voltage 375V Rated output power 100kW Number of single bridge arm submodules N 18 Submodule maximum voltage 782V Bridge arm inductance 500μH Transformer ratio 12：1 Transformer leakage inductance 600μH Transformer magnetizing inductance 150mH Resonant capacitor 293nF Resonant frequency, switching frequency 12kHz

[0045] Figure 4 The variation of the submodule capacitor voltage under the fault-tolerant operation method of bypassing only the faulty submodule is shown. This fault-tolerant operation method is that when a submodule in one of the bridge arms fails, no additional fault-tolerant operation strategy is adopted, and only the faulty submodule is bypassed. Before 40ms, the entire converter is in normal working state; at t=40ms, a fault is detected in the upper bridge arm No. 1 submodule, at which time the faulty submodule is bypassed, and all submodules in the lower bridge arm are working normally; at t=50ms, a fault is detected in the upper bridge arm No. 2 submodule, at which time the faulty submodule is bypassed.

[0046] Figure 5 Shown Figure 4 The changes in the voltage and current of the upper and lower bridge arms corresponding to the scheme. At t = 40ms, a fault was detected in the upper bridge arm submodule No. 1, and the faulty submodule was bypassed. After that, the number of submodules switched in the upper and lower bridge arms was different, the voltage amplitudes of the upper and lower bridge arms were different, the current amplitude of the upper bridge arm was larger, and the upper and lower bridge arms operated asymmetrically. When the upper and lower bridge arms operate asymmetrically, the voltage and current stresses borne by each submodule will be unbalanced, and the heating of the submodules will become more unbalanced. Figure 5 In the scenario shown, more sub-modules have failed in the upper arm. Sub-modules that operate normally in the upper arm may age faster due to excessive electrical and thermal stress, or even be damaged due to overvoltage and overcurrent, further increasing the risk of sub-module failure in the upper arm.

[0047] Figure 6The variation of the submodule capacitor voltage under the fault-tolerant operation method of the modular multilevel converter under submodule failure of the present invention is shown. Before 40ms, the entire converter is in normal working state; at t=40ms, a fault is detected in the upper bridge arm No. 1 submodule, and the faulty submodule is bypassed at this time. All submodules of the lower bridge arm are in normal state, the number of faulty modules of the upper bridge arm is greater than the number of faulty modules of the lower bridge arm, the numbering elements in the numbering set C include 1, 2, ..., N, and the corresponding minimum number is 1, then the No. 1 submodule of the lower bridge arm is selected for bypass; at t = 50ms, it is detected that the No. 2 submodule of the lower bridge arm fails, and the faulty submodule is bypassed at this time, and the normal state submodule of the lower bridge arm has been bypassed, the numbering elements in the numbering set A include 1, and the maximum number is 1, then the No. 1 submodule of the lower bridge arm is selected for release, and the No. 1 submodule participates in normal switching work; at t = 60ms, it is detected that the No. 1 submodule of the lower bridge arm fails, and the faulty submodule is bypassed at this time, the number of faulty modules of the lower bridge arm is greater than the number of faulty modules of the upper bridge arm, the numbering elements in the numbering set C of the upper bridge arm include 2, ..., N, and the corresponding minimum number is 2, then the No. 2 submodule of the upper bridge arm is selected for bypass. After the fault-tolerant operation method of the present invention is implemented to reach a steady state, the capacitor voltages of the upper and lower bridge arm submodules are equal.

[0048] Figure 7 The changes of the voltage and current of the upper and lower bridge arms under the fault-tolerant operation method of the present invention are shown. At t = 40ms, a fault is detected in the upper bridge arm submodule No. 1. At this time, the faulty submodule is bypassed, and the lower bridge arm submodule No. 1 is bypassed. The number of switched submodules in the upper and lower bridge arms is equal, and the upper and lower bridge arms maintain symmetrical operation. It can be observed that after the fault-tolerant operation reaches a steady state, the voltage and current of the upper and lower bridge arms have no obvious changes compared to before the fault, and the modular multi-level resonant converter still maintains good working characteristics after the submodule failure.

[0049] from Figure 5 and Figure 7 It can be seen from the comparison that the fault-tolerant strategy of only bypassing the faulty submodule will lead to asymmetric operation of the upper and lower bridge arms, and the voltage and current stresses borne by each submodule will be unbalanced, and the heating of the submodule will be more unbalanced, thereby further increasing the risk of submodule failure in the bridge arm with more faulty submodules; while the fault-tolerant operation strategy under submodule failure of the modular multilevel converter described in the present invention can achieve symmetrical operation of the upper and lower complementary bridge arms while ensuring effective removal of the faulty submodule, so as to avoid uneven electrothermal stress of the upper and lower bridge arm submodule switch devices caused by asymmetry, and ensure the consistency and reliability of the submodule operation status.

[0050] It should be noted that, with respect to the specific selection of sub-modules, the present invention only proposes one of the schemes as a describable implementation method, and other selection methods do not affect the use of the fault-tolerant operation strategy. The fault-tolerant operation strategy using other sub-module selection methods still falls within the protection scope of the present invention.

[0051] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. For ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A fault-tolerant operation method for a modular multilevel converter under submodule failure, wherein the modular multilevel converter comprises one or more phase units, each phase unit comprises two upper and lower complementary bridge arms, each bridge arm comprises N submodules connected in series, N being a positive integer greater than or equal to 2; characterized in that: Fault-tolerant operation methods include: 1) Continuously perform fault detection on the submodules in each phase unit during the control cycle of the modular multilevel converter, and if a new submodule fails, bypass the failed submodule; When a submodule failure occurs in the modular multilevel converter for the first time, the submodule with the failure is bypassed, and a number of normal submodules are bypassed in a bridge arm where no submodule failure occurs or in a bridge arm where the number of submodule failures is less, so that the number of bypassed submodules in two complementary bridge arms is equal; 2) Check whether the number of bypassed submodules in the two complementary bridge arms is equal, if so, proceed to step 1); if not, proceed to step 3); 3) Check whether there are any bypassed normal sub-modules in the bridge arm with a larger number of newly failed sub-modules. If yes, release the bypassed normal sub-modules to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to step 1); if no, bypass the normal sub-modules in the complementary bridge arm to make the number of bypassed sub-modules in the two complementary bridge arms equal, and return to step 1).

2. The fault-tolerant operation method of a modular multilevel converter under submodule failure according to claim 1, characterized in that: A switch is connected between the input end and the output end of the submodule. When the submodule needs to be bypassed, the switch connected thereto remains closed; when the submodule is switched normally, the switch connected thereto remains open.

3. The fault-tolerant operation method of a modular multilevel converter under submodule failure according to claim 1, characterized in that: The submodules in each bridge arm are numbered in sequence and recorded as i, i = 1, 2, 3, ..., N; the numbers of the normal submodules bypassed in each bridge arm are grouped into a number set, recorded as A; the numbers of the faulty submodules in each bridge arm are grouped into a number set, recorded as B; the numbers of the submodules in normal switching in each bridge arm are grouped into a number set, recorded as C; the numbers in the three number sets A, B, and C are different, and the sum of the number of numbers in the three number sets A, B, and C is always N.

4. The fault-tolerant operation method of a modular multilevel converter under submodule failure according to claim 3, characterized in that: In step 3), when releasing the bypassed normal submodules, the corresponding normal submodules are released from the number set A of the bridge arm with more newly failed submodules according to the numbers from large to small, so that the numbers of bypassed submodules in the two complementary bridge arms are equal.

5. The fault-tolerant operation method of a modular multilevel converter under submodule failure according to claim 3, characterized in that: In step 3), when bypassing the normal submodules in the complementary bridge arm, the corresponding normal submodules are bypassed from the number set C of the complementary bridge arm according to the numbers from small to large, so that the number of bypassed submodules in the two complementary bridge arms is equal.

6. The fault-tolerant operation method of a modular multilevel converter under submodule failure according to claim 1, characterized in that: If the number of faulty submodules in any bridge arm of the modular multilevel converter exceeds a threshold, the modular multilevel converter is shut down.

7. The fault-tolerant operation method of a modular multilevel converter under submodule failure according to claim 1, characterized in that: In step 3), when releasing the bypassed normal submodules, if the number of bypassed normal submodules in the bridge arm is less than the difference in the number of newly faulty submodules in the two complementary bridge arms, all bypassed normal submodules are released, and a number of normal submodules are bypassed in the complementary bridge arm, where the number of these normal submodules is the difference between the difference in the number of newly faulty submodules in the two bridge arms and the difference in the number of released normal submodules.

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