CN104779825B - Cross type sub-module structure of modular multilevel converter (MMC) - Google Patents

Abstract

The invention discloses a cross-type sub-module structure of a modular multi-level converter, which includes four switch tubes, four capacitors, a switch tube and an independent diode; the sub-module structure can output up to 4 levels , in the actual application of modular multilevel converters, the number of sub-modules used can be effectively reduced, the scale of the control circuit and the complexity of control can be reduced; the MMC composed of the sub-modules of the present invention has the ability to self-clear DC faults, It can quickly and effectively handle DC faults and reduce the impact of faults on MMC and the entire AC and DC system; compared with CMMC, the number of IGBT devices is reduced by 10%, and the operating loss is also reduced to a certain extent, which shows that the economy is better than CMMC.

Description

A cross-type sub-module structure of a modular multilevel converter

technical field

The invention belongs to the technical field of power electronic systems, and in particular relates to a cross-type sub-module structure of a modular multilevel converter.

Background technique

Modular multilevel converter (MMC) has the advantages of low harmonic distortion rate of AC output voltage, easy packaging of modular structure, small electrical stress on switching devices, and low switching loss. Since it was proposed in 2002, it has been widely recognized by the industry after more than ten years of development. Half-bridge sub-module (half-bridge sub-module, HBSM) structure is the optional structure of the main sub-module of MMC. Due to the small loss and low voltage of HBSM, almost all MMC high-voltage DC projects are based on the half-bridge type MMC ( MMC using HBSM, HMMC) as its topology. However, when a DC bipolar short-circuit fault occurs in the high-voltage DC system composed of HMMC, the system cannot cut off the short-circuit fault current by blocking the converter itself, which seriously endangers the safety of the system.

At present, the engineering application of DC circuit breakers in high-voltage and high-power occasions cannot be realized, and the DC transmission system composed of HMMC cannot cut off the faulty short-circuit current by blocking the converter station when a bipolar short-circuit fault occurs. After the fault occurs, the fault current can be cut off through the AC circuit breaker, which has a serious impact on the safe operation of the system. In order to avoid DC faults as much as possible, existing MMC projects use cables with low failure rates as transmission lines instead of low-cost overhead lines, which has a great impact on the development of MMC DC transmission systems in long-distance power transmission. big constraints.

In order to effectively solve the above technical problems, many literatures have proposed a variety of sub-module topologies with DC fault self-clearing capabilities, such as clamp double sub-module (CDSM) and full-bridge sub-module (full-bridge sub-module). module, FBSM), etc. From the perspective of equipment cost and operating loss, CDSM has obvious advantages compared with other sub-module topologies, but the MMC (MMC using CDSMs, CMMC) composed of it needs to use 25% more IGBT devices than HMMC. , additional consumption of about 40% of the operating loss. If a sub-module topology that is more economical than CDSM can be researched, it must have very important engineering value and application prospects.

Contents of the invention

Aiming at the above-mentioned technical problems existing in the prior art, the present invention provides a cross-type sub-module structure of a modular multilevel converter. The MMC formed by it not only has the capability of self-clearing DC faults, but also has the , the number of IGBT devices used is less, the operating loss is lower, and it has better economy.

A cross-type sub-module structure of a modular multilevel converter, including: four switching tubes S ₁ -S ₄ , four capacitors C ₁ -C ₄ , one switching tube and one independent diode; wherein:

The positive terminal of capacitor _C1 is connected to the negative terminal of switch tube S2 and constitutes the high _- voltage terminal of the sub-module structure, the negative terminal of capacitor _C1 is connected to the positive terminal _of switch tube S1, and the negative terminal _of switch tube S1 is connected to capacitor C ₂ is connected to the positive terminal of the independent diode, the negative terminal of the capacitor _C2 is connected to the positive terminal of the switching tube S2 and the positive terminal _of the switching tube _; the negative terminal of the switching tube is connected to the positive terminal _of the capacitor C3 and the switching tube S3 _The negative terminal of the independent diode is connected to the negative terminal of the capacitor C3 and the positive terminal _of the switch tube _S4 , the negative terminal of the switch tube S4 is connected to the positive terminal of the capacitor _C4 , and the negative terminal of the capacitor _C4 is connected to the switch _The positive end of the tube S3 is connected to form the low-voltage end of the sub-module structure;

Any of the four switch tubes S ₁ -S ₄ includes two IGBT tubes T ₁ -T ₂ with anti-parallel diodes; wherein, the emitter of the IGBT tube T ₁ constitutes the positive terminal of the switch tube, and the IGBT _The collector of the tube T1 is connected to the emitter of the IGBT tube _T2 , and the collector of the IGBT tube T2 constitutes the negative terminal of the switch tube, and the bases _of the _two IGBT tubes T1 ~ _T2 receive the switch provided by the external equipment. control signal.

The switching tube is composed of an IGBT tube with an anti-parallel diode connected in parallel with a lightning arrester; wherein, the emitter of the IGBT tube constitutes the positive terminal of the switching tube, the collector constitutes the negative terminal of the switching tube, and the base receives the signal provided by the external device. switch control signal.

The lightning arrester can effectively prevent the IGBT from being damaged by the overvoltage.

The sub-module structure of the present invention can output 4 levels at most, and can effectively reduce the number of sub-modules used in the actual application of the modularized multi-level converter, and reduce the scale of the control circuit and the complexity of control.

Compared with the prior art, the beneficial effects of the sub-module structure of the present invention are as follows:

(1) The MMC composed of the sub-modules of the present invention has the self-clearing capability of DC faults, can quickly and effectively handle DC faults, and reduce the impact of faults on the MMC and the entire AC-DC system.

(2) Compared with CMMC, the number of IGBT devices reduced by 10% is reduced by MMC composed of sub-modules of the present invention, and the operating loss is also reduced to a certain extent, which shows that the economy is better than CMMC.

(3) The submodule of the present invention can output four levels at most. Compared with HBSM outputting 1 level and CDSM outputting 2 levels, in the MMC of the same voltage level, the submodule of the present invention uses the least number and can effectively Reduce the complexity of control hardware and control algorithms.

Description of drawings

Fig. 1 is a schematic diagram of the cross-type sub-module structure of the present invention.

Fig. 2(a) is a schematic diagram of the 4-level input state of the cross-type sub-module of the present invention.

Fig. 2(b) is a schematic diagram of the 2-level input state of the cross-type sub-module of the present invention.

Fig. 2(c) is a schematic diagram of another 2-level input state of the cross-type sub-module of the present invention.

Fig. 2(d) is a schematic diagram of the bypass state of the cross-type sub-module of the present invention.

Fig. 3(a) is a schematic diagram of the current flowing from A to B in the locked state of the intersecting sub-module of the present invention.

Fig. 3(b) is a schematic diagram of the current flow direction from B to A in the blocking state of the cross-type sub-module of the present invention.

Figure 4 is a schematic diagram of two CDSM cascades.

detailed description

In order to describe the present invention more specifically, the technical solutions and related principles of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Fig. 1, the cross-type sub-module structure of the modular multilevel converter of the present invention includes 4 switching tubes S ₁ -S ₄ , 4 capacitors C ₁ -C ₄ , 1 switching tube and 1 independent diode D ₁₀ .

The positive end of the capacitor _C1 is connected to the No. ₂ end of the switch tube S2 and constitutes the A end of the cross-type sub-module structure, the negative end of the capacitor _C1 is connected to the _{No. 1} end of the switch tube S1, and the 2nd end of the switch tube S1 The _No. terminal is connected to the positive terminal of the capacitor C2 and the cathode of the diode _D10 , and the negative terminal of the capacitor C2 is connected to the No. ₁ terminal of the switching tube S2 and the _No. 1 terminal of the switching tube. The No. 2 terminal of the switch tube is connected to the positive terminal of the capacitor C3 and the No. ₂ terminal _of the switch tube S3, and the anode of the diode _D10 is connected to the negative terminal of the capacitor C3 and the No. ₁ terminal of the switch tube S4 _. The No. ₂ terminal of the switch tube S4 is connected to the positive terminal of the capacitor _C4 , and the negative terminal of the capacitor _C4 is connected to the No. ₁ terminal of the switch tube S3 to form the B terminal of the cross-type sub-module structure.

The switch tubes S ₁ -S ₄ have the same internal structure, including two IGBT tubes T ₁ and T ₂ and reverse diodes D ₁ and D ₂ connected in parallel thereto. Wherein, the emitter of the IGBT tube T1 is used as the _{No. 1} terminal of the switch tube, the collector of the IGBT tube T1 is connected to the emitter of the IGBT tube T2, and the collector of the IGBT tube T2 is used as the _{No. 2} terminal of the switch tube.

The switching tube is composed of an IGBT tube T9, a diode _D9 and a lightning arrester, and the three are connected in parallel, wherein the diode _D9 and the IGBT tube _T9 are in an _antiparallel relationship. The addition of the arrester can effectively prevent the overvoltage of the IGBT tube T ₉ and prevent it from being damaged by the overvoltage.

Figure 2 shows four operating states under the steady-state operation of the cross-type sub-module structure. Figure 2(a) shows the state of the four-level output of the sub-module when T ₁ T ₂ , T ₇ T ₈ , and T ₉ are turned on, and T ₃ T ₄ , T ₅ T ₆ are turned off; Figure 2(b) Shown is the status of the two-level output of the sub-module when T ₁ T ₂ , T ₅ T ₆ , and T ₉ are turned on, and T ₃ T ₄ , T ₇ T ₈ are turned off; Figure 2(c) shows T ₃ When T ₄ , T ₇ T ₈ , and T ₉ are turned on, and T ₁ T ₂ , T ₅ T ₆ are turned off, the sub-module outputs a two-level state; Figure 2(d) shows T ₃ T ₄ , T ₅ When T ₆ and T ₉ are on, and T ₁ T ₂ , T ₇ T ₈ are off, the sub-module is bypassed. During steady-state operation, T9 is always _on , and there is no overvoltage problem. In practical engineering applications, a bridge arm of the MMC is often formed by cascading hundreds of sub-modules, which increases the requirements for control hardware and complicates the control algorithm. Therefore, in application, the cross-type sub-module of the present invention does not adopt a two-level state, but adopts a four-level input mode, which greatly reduces the number of sub-modules used, and reduces the scale of the control circuit and the complexity of control .

Figure 3 shows two operating states of the cross-type sub-module structure with different current flow downwards in the case of blocking. Figure 3(a) is the case where the current direction is from A to B, and Figure 3(b) is the case where the current direction is from B to A. When the current flows from A to B, the current direction will bear four levels of back EMF; when the current flows from B to A, the current direction will bear two levels of back EMF. Therefore, no matter what direction the current flows in, the current will always charge the capacitor to increase the capacitor voltage, so that a large reverse electromotive force can be established to cut off the DC fault current. It can be seen from Figure 3(b) that when D ₁₀ is turned on, the voltages on C ₂ and C ₃ will be superimposed on T ₉ , making T ₉ temporarily withstand twice the rated voltage. Therefore, it is necessary to A surge arrester is connected in parallel to protect T ₉ . In fact, in engineering applications, the rated voltage of the IGBT under steady-state operation is generally about half of its withstand voltage. Therefore, T ₉ itself has the ability to withstand twice the voltage, so an IGBT device can do it.

Under normal operating conditions, a cross-type sub-module can output four levels, while a CDSM can only output two levels. Therefore, in practical applications, a cross-type sub-module is equivalent to two CDSMs, and the two cascaded CDSMs are regarded as a new sub-module, and the economical comparison between the cross-type sub-module and the CDSM can be easily compared. sex. Figure 4 shows a schematic diagram of two CDSMs cascaded. It can be seen from the figure that the two cascaded CDSMs include 10 IGBT tubes, while the cross-type sub-module only needs 9 IGBT tubes. Therefore, each of them consists of In terms of the number of IGBT tubes used by the MMC, the cross-type sub-module saves 10% compared with the CDSM.

Table 1 shows the conduction state of the power electronic devices under the steady-state operation of the above two types of sub-modules. It can be seen from the table that when the current direction is from A to B, no matter what state the sub-module is in, the cascaded structure of two CDSMs will invest one more diode than the cross-type sub-module; when the current direction is from B to A , no matter what state the sub-module is in, the cascaded structure of two CDSMs will invest one more IGBT tube than the cross-type sub-module. Therefore, it can be concluded that under the same voltage and capacity, CMMC consumes more on-state loss than MMC composed of cross-type sub-modules.

Table 1

Table 2 shows the on-off status of the power electronic devices under the steady-state operation of the above two types of sub-modules. It can be seen from the table that the crossover sub-module and two CDSM cascades are the same between switching, indicating that both have the same switching loss.

Table 2

It can be concluded from Table 1 and Table 2 that compared with CDSM, the cross-type sub-module of the present invention has smaller operating loss, and in addition to using 10% less IGBT tubes, the cross-type sub-module is more economical.

The above description of the embodiments is for those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above-mentioned embodiments, and apply the general principles described here to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art according to the disclosure of the present invention should fall within the protection scope of the present invention.

Claims (1)

1. A cross-type sub-module structure of a modular multilevel converter, characterized in that it includes: four switch tubes S ₁ to S ₄ , four capacitors C ₁ to C ₄ , one switch tube and an independent Diodes; where: The positive terminal of capacitor _C1 is connected to the negative terminal of switch tube S2 and constitutes the high _- voltage terminal of the sub-module structure, the negative terminal of capacitor _C1 is connected to the positive terminal _of switch tube S1, and the negative terminal _of switch tube S1 is connected to capacitor C ₂ is connected to the positive terminal of the independent diode, the negative terminal of the capacitor _C2 is connected to the positive terminal of the switching tube S2 and the positive terminal _of the switching tube _; the negative terminal of the switching tube is connected to the positive terminal _of the capacitor C3 and the switching tube S3 _The negative terminal of the independent diode is connected to the negative terminal of the capacitor C3 and the positive terminal _of the switch tube _S4 , the negative terminal of the switch tube S4 is connected to the positive terminal of the capacitor _C4 , and the negative terminal of the capacitor _C4 is connected to the switch _The positive end of the tube S3 is connected to form the low-voltage end of the sub-module structure; Any of the four switch tubes S ₁ -S ₄ includes two IGBT tubes T ₁ -T ₂ with anti-parallel diodes; wherein, the emitter of the IGBT tube T ₁ constitutes the positive terminal of the switch tube, and the IGBT _The collector of the tube T1 is connected to the emitter of the IGBT tube _T2 , and the collector of the IGBT tube T2 constitutes the negative terminal of the switch tube, and the bases _of the _two IGBT tubes T1 ~ _T2 receive the switch provided by the external equipment. control signal; The switching tube is composed of an IGBT tube with an anti-parallel diode connected in parallel with a lightning arrester; wherein, the emitter of the IGBT tube constitutes the positive terminal of the switching tube, the collector constitutes the negative terminal of the switching tube, and the base receives the signal provided by the external device. switch control signal.