A MMC diode clamped submodule topology

Abstract

The present invention provides an MMC diode clamp submodule topology, which relates to the technical field of modular multilevel converters. The present invention improves the connection mode between the internal switch device and the capacitor of the DCSM by adding a small amount of power devices, and proposes an MMC improved diode clamp submodule topology structure, which realizes the independent placement capability of the two capacitors in the IDCSM. The improved IDCSM can realize DC fault ride-through and negative level output. The IDCSM can output c ,0,u c , 2u c Four levels increase the number of output levels and significantly reduce the system hardware cost. In addition, when a DC side short circuit fault occurs in the system, the back electromotive force provided by the module capacitor of IDCSM can quickly block the DC fault current and quickly achieve DC fault ride-through.

Description

Technical Field

[0001] The invention relates to the technical field of modular multi-level converters, and in particular to an MMC diode clamping submodule topology. Background Art

[0002] Since the introduction of modular multilevel converters (MMC), the highly modular topology, superior harmonic characteristics and high energy conversion efficiency have promoted the rapid development of flexible DC transmission technology. However, even if all the IGBTs inside the MMC composed of half-bridge sub-modules (HBSM) are locked, the fault current will still flow directly through the anti-parallel diode and cannot complete the DC fault ride-through. Therefore, how to make MMC-HVDC effectively ride through the DC fault is an urgent problem to be solved.

[0003] There are three main ways to solve the DC side fault problem: the first is the AC circuit breaker method, which has a slow response speed and a long system recovery time; the second is the DC circuit breaker method, which has a high cost and imperfect technology; the third is the converter self-clearing method, which has a strong fault current blocking capability and a short system recovery time, and is more suitable for handling DC side faults in the system. Therefore, the existing literature proposes a variety of MMC sub-module topologies with DC fault self-clearing capabilities. Yang Xiaofeng, Xue Yao, Zheng Qionglin, Lin Zhiqin, Mou Yajie, Chen Bowei, Igarashi Masaki. A new modular multi-level converter using reverse blocking sub-modules proposes a new reverse blocking half bridge sub-module (RB-HBSM) topology based on HBSM. The RB-IGBT replaces the traditional IGBT device on the lower tube, so that the sub-module has a bidirectional blocking capability of the fault current. The fault clearing method of replacing the introduction of the sub-module capacitor into the loop by turning off the IGBT leads to higher voltage resistance requirements for the device and increased system cost. Dong Yufei, Yang Heya, Li Wuhua, He Xiangning. Modulation method for optimizing loss distribution of full-bridge sub-module in MMC. The full-bridge sub-module (FBSM) topology structure described above realizes negative level output by controlling the on-off of IGBT. Its control flexibility realizes better DC fault ride-through and fault-tolerant control, but the number of power devices in FBSM is large, which is not conducive to the lightweight design of MMC. Xue Yinglin, Xu Zheng. Mechanism of DC fault ride-through of C-MMC and improved topology scheme [J]. Proceedings of the CSEE. A clamp double sub-module (CDSM) topology structure is proposed. Compared with FBSM, its dual-capacitor topology structure reduces the number of devices and hardware cost required for unit level, but the different connection modes of the two internal capacitors in the process of charging, discharging and fault ride-through are coupled, which increases the complexity of control and voltage balancing. Li Xiaoqian, Liu Wenhua, Song Qiang, Rao Hong, Xu Shukai. An enhanced MMC topology with DC fault ride-through capability A diode-clamp sub-module (DCSM) topology is proposed. This topology can quickly ride through DC side faults, but the connection method of its power devices results in the two internal capacitors not being able to be independently deployed, reducing the number of output levels and increasing the complexity of system control. At the same time, the above-mentioned sub-module topology that can achieve DC fault ride-through basically does not have negative level output capability. The negative level output capability of the MMC sub-module topology can realize the system's power flow reversal and fault-tolerant control, increasing the flexibility of system control.Based on the above overview, in order to achieve self-clearing of DC side faults and negative level output capabilities, many power devices are added to the new MMC sub-module topology, which significantly increases the cost and loss of the converter. Therefore, reducing the number of power devices used is of great significance to the design of the new sub-module topology of MMC. Summary of the invention

[0004] The present invention improves the connection mode between the internal switch device and the capacitor of the DCSM by adding a small amount of power devices, and proposes an MMC improved diode-clamp sub-module topology (IDCSM), which realizes the independent placement capability of the two capacitors in the IDCSM. The improved IDCSM can realize DC fault ride-through and negative level output. The present invention provides an MMC diode-clamp sub-module topology, and the specific scheme is as follows:

[0005] An MMC diode clamp type submodule, comprising: a switch tube T 1 , switch tube T 2 , switch tube T 3 , switch tube T 4 , capacitor C 1 , capacitor C 2 、Diode D 1 、Diode D 2 、Diode D 3 and diode D 4 ;

[0006] Switching tube T 1 and diode D 1 In parallel, the switch tube T 2 and diode D 2 Parallel, switch tube T 3 and diode D 3 Parallel, switch tube T 4 and diode D 4 in parallel;

[0007] Switching tube T 1 and diode D 1 One end of the parallel connection is connected to the switch tube T 2 and diode D 2 One end of the parallel connection, the switch tube T 1 and diode D 1 The other end of the parallel connection is connected to capacitor C 1 One end of the capacitor C 1 The other end of the capacitor C 2 One end of the capacitor C 2 The other end is connected to the switch tube T 2 and diode D 2 The other end of the parallel connection;

[0008] Capacitor C 1 The other end is connected to the switch tube T 3 and diode D 3 One end of the parallel connection, the switch tube T 3 and diode D 3 The other end of the parallel connection is connected to the switch tube T 4 and diode D 4 One end of the parallel connection, the switch tube T 4 and diode D 4 The other end of the parallel connection is connected to capacitor C 2 the other end.

[0009] Furthermore, it also includes a switch tube T 5 And switch tube T 6 , switch tube T 5 And switch tube T 6 in parallel;

[0010] Switching tube T 5 And switch tube T 6 One end of the parallel connection is connected to the switch tube T 1 and diode D 1 One end of the parallel connection, the switch tube T 5 And switch tube T 6 The other end of the parallel connection is connected to capacitor C 1 the other end.

[0011] Furthermore, through the switch tube T 3 and diode D 3 Parallel connection to achieve C 1 The separate delivery of 3 Provides the current path required for negative level output for IDCSM.

[0012] Furthermore, IDCSM outputs -u c ,0,u c , 2u c Four levels, increasing the number of output levels.

[0013] Furthermore, through the switch tube T 5 And switch tube T 6 Connect in parallel to realize capacitor C 2 Individual delivery of .

[0014] Furthermore, the MMC improved diode clamp type submodule topology working state includes:

[0015] State 1: C 1 , C 2 Double input, T 1 , T 4Closed, IDCSM output voltage is U c1 +U c2 ;

[0016] State 2: C 1 Single investment, T 1 , T 3 Closed, IDCSM output voltage is U c1 ;

[0017] State 3: C 2 Single investment, T 4 , T 5 , T 6 Closed, IDCSM output voltage is U c2 ;

[0018] State 4: C 2 Negative input, T 2 , T 3 Closed, IDCSM output voltage is -U c2 ;

[0019] State 5: Removal of state 1, C 1 , C 2 are bypassed, T 3 , T 5 , T 6 Closed, IDCSM output voltage is 0;

[0020] State 6: Removal of state 2, C 1 , C 2 are bypassed, T 2 , T 4 Closed, IDCSM output voltage is 0;

[0021] State 7: Locked state, T 1 , T 2 , T 3 , T 4 , T 5 Shutdown, when i sm >0, C 1 , C 2 Connected in series to the fault current loop, the IDCSM output voltage is U c1 +U c2 ; when i sm <0, C 2 Reverse connection to the fault current loop, the IDCSM output voltage is -U c2 .

[0022] Beneficial effects:

[0023] The present invention proposes an improved diode clamp submodule topology structure with DC fault ride-through and negative level output capabilities, which realizes the independent placement capability of two capacitors in IDCSM. IDCSM can output -u c ,0,u c , 2u c Four levels increase the number of output levels and significantly reduce the system hardware cost. In addition, when a DC side short circuit fault occurs in the system, the back electromotive force provided by the module capacitor of IDCSM can quickly block the DC fault current and quickly achieve DC fault ride-through. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 To improve the topology diagram;

[0025] Figure 2 It is the current path diagram under each state of IDCSM;

[0026] Figure 3 This is the simulation result of the negative level output state of IDCSM;

[0027] Figure 4 It is the simulation result of IDCSM DC fault ride-through. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0029] Combination Figures 1 to 4 As shown, the present invention provides an MMC diode clamp type submodule topology, including: a switch tube T 1 , switch tube T 2 , switch tube T 3 , switch tube T 4 , capacitor C 1 , capacitor C 2 、Diode D 1 、Diode D 2 、Diode D 3 、Diode D 4 , switch tube T 5 And switch tube T 6 ;

[0030] Switching tube T 1 and diode D 1 In parallel, the switch tube T 2 and diode D 2Parallel, switch tube T 3 and diode D 3 Parallel, switch tube T 4 and diode D 4 Parallel, switch tube T 5 And switch tube T 6 in parallel

[0031] Switching tube T 1 and diode D 1 One end of the parallel connection is connected to the switch tube T 2 and diode D 2 One end of the parallel connection, the switch tube T 1 and diode D 1 The other end of the parallel connection is connected to capacitor C 1 One end of the capacitor C 1 The other end of the capacitor C 2 One end of the capacitor C 2 The other end is connected to the switch tube T 2 and diode D 2 The other end of the parallel connection;

[0032] Capacitor C 1 The other end is connected to the switch tube T 3 and diode D 3 One end of the parallel connection, the switch tube T 3 and diode D 3 The other end of the parallel connection is connected to the switch tube T 4 and diode D 4 One end of the parallel connection, the switch tube T 4 and diode D 4 The other end of the parallel connection is connected to capacitor C 2 the other end.

[0033] Switching tube T 5 And switch tube T 6 One end of the parallel connection is connected to the switch tube T 1 and diode D 1 One end of the parallel connection, the switch tube T 5 And switch tube T 6 The other end of the parallel connection is connected to capacitor C 1 the other end.

[0034] The MMC improved diode clamp submodule topology working states include:

[0035] State 1: C 1 , C 2 Double input, T 1 , T 4 Closed, IDCSM output voltage is U c1 +U c2 ;

[0036] State 2: C 1 Single investment, T 1 , T 3 Closed, IDCSM output voltage is U c1 ;

[0037] State 3: C 2 Single investment, T 4 , T 5 , T 6 Closed, IDCSM output voltage is U c2 ;

[0038] State 4: C 2 Negative input, T 2 , T 3 Closed, IDCSM output voltage is -U c2 ;

[0039] State 5: Removal of state 1, C 1 , C 2 are bypassed, T 3 , T 5 , T 6 Closed, IDCSM output voltage is 0;

[0040] State 6: Removal of state 2, C 1 , C 2 are bypassed, T 2 , T 4 Closed, IDCSM output voltage is 0;

[0041] State 7: Locked state, T 1 , T 2 , T 3 , T 4 , T 5 Shutdown, when i sm >0, C 1 , C 2 Connected in series to the fault current loop, the IDCSM output voltage is U c1 +U c2 ; when i sm <0, C 2 Reverse connection to the fault current loop, the IDCSM output voltage is -U c2 .

[0042] Figure 1 (a) is the topological structure of DCSM, where the red dotted line and the blue dotted line are the current flow paths of DCSM under normal working conditions. After analysis, it can be seen that although DCSM can achieve DC fault ride-through by putting the module capacitor into the fault circuit, the diode D 3 Limit C 1, C 2 The ability to be deployed independently under normal working conditions, C 1 , C 2 It can only be put in and out together, which reduces the control flexibility and the number of output levels. Therefore, the present invention improves its topological structure. First, the IGBT anti-parallel diode is used to replace the D 3 , which can realize C 1 The new T 3 It provides the current path required for negative level output for IDCSM. 1 With C 2 By connecting reverse blocking (RB) IGBT in parallel, C 2 The individual delivery of Figure 1 (b) shows the IDCSM topology.

[0043] IDCSM Working Principle

[0044] In order to analyze the working principle of IDCSM more intuitively, the IDCSM switch state table shown in Table 1 is listed and the Figure 2 The current path in each state of IDCSM is shown in Figure 1. 1 The voltage across the two ends is U c1 , C 2 The voltage across the two ends is U c2 .

[0045] Table 1 IDCSM switch status table

[0046]

[0047] In order to verify the negative level output capability of the IDCSM topology proposed in this invention, a three-phase IDCSM-MMC rectifier simulation model was built on MATLAB / Simulink. c The IDCSM switches between states 4 and 6. The modulation strategy selects carrier phase shift modulation, the simulation duration is 1s, and the simulation parameters are shown in Table 2:

[0048] Table 2 MMC simulation parameters

[0049]

[0050] Figure 3 (a) and (b) are the AC output voltage and current waveforms of IDCSM-MMC in the negative level output state. The AC voltage and current amplitudes are about 2900V and 110A, which proves that the output level of IDCSM can be between 0 and -U in the negative level working state. CThe port voltages are superimposed on each other to form a five-level step wave. At the same time, the AC voltage and AC current are basically in phase, and the rectifier operates at unity power factor. Figure 3 (c) and (d) are the DC side output voltage and current waveforms, respectively. Since the DC side load is a purely resistive load, the DC side voltage and current waveforms are only different in value, and there is no obvious difference in the waveforms. The DC side voltage and current fluctuate around -6400V and -64A, with a fluctuation of about 0.15%, which proves that the IDCSM-MMC rectifier can output stable DC voltage and current for direct use by the load. Figure 3 (e) is the C of the four submodules in the upper bridge arm of phase a 2 The capacitor voltage waveform, because MMC is in the negative level output state, U C It fluctuates around the rated value of -1600V, with a fluctuation of about 2.5%, which is relatively stable. The above simulation results all prove the effectiveness of the negative level output state of IDCSM.

[0051] IDCSM DC Fault Ride-Through Characteristics Example

[0052] To verify the DC fault ride-through characteristics of IDCSM, simulation models of three-phase IDCSM-MMC, FBSM-MMC and CDSM-MMC were built on MATLAB / Simulink. The simulation parameters are basically the same as those in Table 2. However, under normal operation, the number of upper (lower) bridge arm submodules of IDCSM-MMC and CDSM-MMC becomes 2, while that of FBSM-MMC remains 4. A DC bipolar short circuit fault occurred in the system at 0.6s. After the system detected the fault, it locked all IGBTs in the MMC at 0.602s to complete the DC fault ride-through.

[0053] Figure 4 (a) and (b) are the voltage and current waveforms on the AC side of the IDCSM-MMC. Before 0.6s, the MMC was in normal operation, and the voltage and current waveforms on the AC side were stable sine waves with amplitudes of approximately 2900V and 110A. At 0.6s, a DC bipolar short circuit fault occurred in the system, the submodule capacitors were discharged, and the energy on the AC side was fed into the DC side. The AC side voltage fluctuated significantly, and the AC side current rose rapidly after the fault occurred. At 0.602s, all IGBTs inside the MMC were locked, and the back electromotive force provided by the submodule capacitors blocked the path for the energy to be fed from the AC side to the DC side. The AC side current of the IDCSM-MMC dropped to 0A at around 0.605s. The AC side voltage fluctuated briefly after the MMC was locked, and became the AC side input voltage after the fault was cleared.

[0054] Figure 4(c) and (d) are the voltage and current waveforms of the DC side of IDCSM-MMC. Before 0.2s, the MMC was in normal operation, the DC side current was stable at 64A, and the DC side voltage was stable at 6400V. Figure 4 (e) is the capacitor voltage waveform of the upper bridge arm submodule of phase a, and its capacitor voltage fluctuates around the rated value of 1600V, with a fluctuation of about 1.8%. A DC bipolar short circuit fault occurs at 0.6s, the DC side current rises sharply to about 1500A, and the DC side voltage drops to 0kV. At 0.602s, all IGBTs inside the MMC are locked, and the IDCSM-MMC factor module capacitor is reversely put into the fault circuit. The capacitor is charged to about the rated value, and the DC side current drops rapidly. The DC side current drops to 0A at about 0.605s, completing the DC fault ride-through.

[0055] In summary, the present invention proposes an IDCSM topology structure with DC fault ride-through and negative level output capabilities by improving the connection method between the power devices and capacitors inside the DCSM topology. According to the simulation results, the output voltage and current fluctuation of the rectifier DC side is about 0.15%, and the IDCSM-MMC can operate stably under the negative level output state. When a DC side short circuit fault occurs in the system, the topology completes the DC fault ride-through in about 5ms, and the capacitor voltage fluctuation is about 1.8%, which is relatively balanced. Compared with the unimproved DCSM-MMC, the number of output levels is increased, the MMC hardware cost is reduced, and the system control flexibility is improved.

[0056] The above is a detailed introduction to an MMC diode clamp type submodule topology provided by the present invention. The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea; at the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. An MMC diode clamp type submodule, characterized in that: include: Switch tube T1, switch tube T2, switch tube T3, switch tube T4, capacitor C1, capacitor C2, diode D1, diode D2, diode D3 and diode D4; The switch tube T1 is connected in parallel with the diode D1, the switch tube T2 is connected in parallel with the diode D2, the switch tube T3 is connected in parallel with the diode D3, and the switch tube T4 is connected in parallel with the diode D4; One end of the switch tube T1 and the diode D1 connected in parallel is connected to one end of the switch tube T2 and the diode D2 connected in parallel, the other end of the switch tube T1 and the diode D1 connected in parallel is connected to one end of the capacitor C1, the other end of the capacitor C1 is connected to one end of the capacitor C2, and the other end of the capacitor C2 is connected to the other end of the switch tube T2 and the diode D2 connected in parallel; The other end of the capacitor C1 is connected to one end of the switch tube T3 and the diode D3 in parallel, the other end of the switch tube T3 and the diode D3 in parallel is connected to one end of the switch tube T4 and the diode D4 in parallel, and the other end of the switch tube T4 and the diode D4 in parallel is connected to the other end of the capacitor C2; It also includes a switch tube T5 and a switch tube T6, and the switch tube T5 and the switch tube T6 are connected in reverse parallel; One end of the switch tube T5 and the switch tube T6 connected in reverse parallel is connected to one end of the switch tube T1 and the diode D1 connected in parallel, and the other end of the switch tube T5 and the switch tube T6 connected in parallel is connected to the other end of the capacitor C1; The MMC improved diode clamp submodule topology working states include: State 1: C1 and C2 are both on, T1 and T4 are closed, and the output voltage of IDCSM is U c1 +U c2 ; State 2: C1 is switched on alone, T1 and T3 are closed, and the output voltage of IDCSM is U c1 ; State 3: C2 is switched on alone, T4, T5, and T6 are closed, and the output voltage of IDCSM is U c2 ; State 4: C2 is negatively input, T2 and T3 are closed, and the output voltage of IDCSM is -U c2 ; State 5: State 1 is removed, C1 and C2 are bypassed, T3, T5, and T6 are closed, and the IDCSM output voltage is 0; State 6: Cut off state 2, C1 and C2 are bypassed, T2 and T4 are closed, and the IDCSM output voltage is 0; State 7: Locked state, T1, T2, T3, T4, T5 are turned off, when i sm >0, C1 and C2 are connected in series to the fault current loop, and the output voltage of IDCSM is U c1 +U c2 ; When ism<0, C2 is reversely connected to the fault current loop, and the output voltage of IDCSM is -U c2 .

2. The MMC diode clamp type submodule according to claim 1, characterized in that: The separate launch of C1 is achieved by connecting the switch tube T3 and the diode D3 in parallel, and the current path required for the negative level output is provided for IDCSM through T3.

3. The MMC diode clamp type submodule according to claim 2, characterized in that: IDCSM output -u c ,0,u c , 2u c Four levels, increasing the number of output levels.

4. The MMC diode clamp type submodule according to claim 3, characterized in that: By connecting the switch tube T5 and the switch tube T6 in parallel, the capacitor C2 can be independently switched on.

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