Hybrid modular multilevel converter based on assistance of thyristor commutation circuit

Abstract

The invention discloses a hybrid modular multilevel converter (MMC) based on assistance of a thyristor commutation circuit, which is characterized in that on the basis of the hybrid MMC, a series half-bridge sub-module in each bridge arm is connected in parallel with a thyristor branch (consisting of a plurality of thyristors connected in series), one end of the thyristor branch is connected to an adjacent direct current side, and the other end of the thyristor branch is connected to an adjacent direct current side. And the other end is connected with the junction of the half-bridge sub-module and the full-bridge sub-module, and is connected with a direct-current line after being sequentially connected in series with a quick mechanical switch and a load transfer switch on the direct-current side. After a direct-current short-circuit fault occurs, a fault current path can be rapidly and dynamically adjusted, so that the half-bridge sub-module and the full-bridge sub-module can participate in removing the direct-current fault current. According to the method, the fault current clearing speed of the hybrid MMC can be remarkably improved, and the requirement for a full-bridge sub-module is reduced. Therefore, the cost and the loss of the hybrid MMC are lower than those of a conventional hybrid MMC. In addition, the half-bridge sub-module also participates in direct-current fault removal and energy absorption, so that the overvoltage problem of the full-bridge sub-module is also effectively relieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current power transmission and distribution, and in particular relates to a hybrid modular multi-level converter assisted by a thyristor commutation circuit. Background Art

[0002] Compared with conventional HVDC transmission systems based on thyristor-commutated converters, flexible DC transmission systems are more flexible in operation and are an important means of transmitting renewable energy power. In order to develop long-distance flexible DC transmission technology, fast DC fault clearing must be achieved. However, the current HVDC circuit breaker technology is not mature enough, the cost is high, and the breaking capacity is still limited, so it is still difficult to promote and apply on a large scale.

[0003] The modular multilevel converter (MMC) with full-bridge submodules is the most typical improved MMC topology with DC fault clearing capability. However, its cost and loss are too high compared to half-bridge MMC. In order to take advantage of the full-bridge submodules while avoiding the high cost and high loss problems caused by the use of full-bridge submodules, hybrid MMCs that mix half-bridge and full-bridge submodules have received more attention. Compared with full-bridge MMCs, hybrid MMCs have significantly lower costs and losses, but still retain the latter's DC fault clearing capability and the characteristics of a modulation ratio greater than 1.

[0004] The DC fault clearing capability of hybrid MMC is severely limited by the number of full-bridge submodules. Hybrid MMC requires a high proportion of full-bridge submodules (generally 50% and above to ensure DC fault current clearing capability), resulting in high cost and power loss. Also, because the submodule capacitors need to absorb a lot of energy during fault clearing, serious full-bridge submodule overvoltage problems may occur when the DC transmission line is long.

[0005] It can be seen that most of the existing fault clearing solutions relying on converters are only "capable" of clearing DC faults, but cannot be "quickly" cleared. In addition, the converters with strong DC fault clearing capabilities in the existing solutions have high costs and losses, making them difficult to be used in practice. Summary of the invention

[0006] In order to solve the above problems in the prior art, the present invention proposes a hybrid modular multilevel converter assisted by a thyristor commutation circuit, which changes the fault current path inside the converter through the thyristor commutation circuit to enhance the DC fault current clearing capability of the hybrid MMC.

[0007] The technical solution to achieve the purpose of the present invention is: a hybrid modular multi-level converter based on thyristor commutation circuit assistance, including a three-phase circuit, each phase circuit includes two upper and lower bridge arms, the upper bridge arm is composed of N from the high voltage end to the low voltage end 1 Half-bridge submodules, N 2 The lower bridge arm is composed of a bridge arm reactor, N 2 Full-bridge submodules and N 1 Half-bridge sub-modules are connected in series, N 1 and N 2 are all natural numbers greater than 1; in each bridge arm, a thyristor branch is connected in parallel at both ends of the half-bridge sub-modules connected in series, the thyristor branch includes a plurality of thyristors connected in series, one end of the thyristor branch is connected to the adjacent DC side, and the other end is connected to the junction of the half-bridge sub-module and the full-bridge sub-module, and the DC side is connected to the DC line after being connected in series with a fast mechanical switch and a load transfer switch in sequence;

[0008] In normal operation, the fast mechanical switch and load transfer switch on the DC side remain turned on, all thyristor branches remain turned off, and the output voltage of each phase arm is controlled by switching the submodules on and off.

[0009] If a fault occurs on the DC side, the full-bridge submodule and the half-bridge submodule are locked to limit the fault current, and the load transfer switch is used to transfer the fault current from the half-bridge submodule to the thyristor to reconstruct the fault current path, so that the half-bridge submodule and the full-bridge submodule can provide a reverse potential in the fault current path, thereby clearing the DC fault current.

[0010] Preferably, when a fault occurs on the DC side, the specific processing process is as follows:

[0011] After detecting a short-circuit fault on the DC side, all full-bridge submodules and half-bridge submodules are locked, and after the bridge arm current becomes negative, the upper thyristor branch with the lowest phase voltage and the lower thyristor branch with the highest phase voltage are triggered. At the same time, all the half-bridge submodules connected in parallel with these two thyristor branches are bypassed, and the DC side load transfer switch is turned on;

[0012] The fast mechanical switch is quickly disconnected under zero current conditions. After the insulation voltage of the fast mechanical switch is higher than the voltage of the half-bridge sub-module in a single bridge arm, all half-bridge sub-modules are locked again. During a period of time thereafter, the thyristor branches alternately exchange current, the phase with a lower phase voltage in the upper thyristor branch is turned on, and the phase with a higher phase voltage in the lower thyristor branch is turned on. The half-bridge sub-module and the full-bridge sub-module provide a total back-EMF of twice the rated DC voltage in the fault current path, which quickly clears the DC fault current.

[0013] Preferably, the proportion of the full-bridge sub-module to all sub-modules is designed according to the three-phase AC voltage, that is, the sum of the full-bridge sub-module voltages in an upper bridge arm and a lower bridge arm should be set to about the average value of the DC voltage formed after the three-phase AC voltage is rectified by the diode full-bridge without control.

[0014] Preferably, the voltage stress of each thyristor branch is the sum of the voltages of the half-bridge sub-modules in the parallel-connected bridge arms.

[0015] Preferably, the maximum current stress of the fast mechanical switch is a DC fault current.

[0016] Preferably, each load transfer switch is composed of two IGBTs connected in reverse series, and the maximum current stress thereof is a DC fault current.

[0017] Compared with the prior art, the present invention has the following significant advantages:

[0018] The present invention proposes to use thyristor commutation to clear the DC fault current of the MMC, which reduces the hybrid MMC's demand for the proportion of full-bridge sub-modules, thereby reducing costs and power losses, while enhancing the hybrid MMC's DC fault clearing capability. The solution proposed by the present invention effectively alleviates the full-bridge sub-module overvoltage problem during fault clearing.

[0019] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0021] Figure 1 This is a conventional hybrid MMC topology diagram.

[0022] Figure 2 This is the topological diagram proposed by the present invention.

[0023] Figure 3 This is a flow chart of the DC fault clearing of the topology proposed in the present invention.

[0024] Figure 4 The hybrid MMC equivalent circuit and fault current path of the present invention (in this case u a >u b >u c ),in Figure 4 (a) is the path diagram of state 1. Figure 4(b) is the path diagram of state 2. Figure 4 (c) is a schematic diagram of the path of state 3

[0025] Figure 5 It is a schematic diagram of the simulation model of the flexible DC transmission system at both ends of the pseudo bipolar connection.

[0026] Figure 6 are the simulation results of the DC fault clearing process of the conventional scheme and the proposed scheme. DETAILED DESCRIPTION

[0027] It is easy to understand that according to the technical solution of the present invention, without changing the essential spirit of the present invention, a person skilled in the art can imagine various embodiments of the present invention. Therefore, the following specific embodiments and drawings are only exemplary descriptions of the technical solution of the present invention, and should not be regarded as the whole of the present invention or as a limitation or limitation of the technical solution of the present invention. On the contrary, the purpose of providing these embodiments is to enable those skilled in the art to understand the present invention more thoroughly. The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the innovative concept of the present invention.

[0028] A hybrid modular multi-level converter based on thyristor commutation circuit assistance, based on the conventional MMC, with a thyristor branch (multiple thyristors connected in series) connected in parallel to each bridge arm. Figure 1 As shown, the conventional hybrid MMC is a three-phase six-bridge-arm structure, each phase includes two upper and lower bridge arms. For the upper bridge arm, it is composed of N 1 Half-bridge submodules, N 2 The full-bridge submodules and a bridge arm reactor are connected in series; for the lower bridge arm, it is composed of a bridge arm reactor, N 2 Full-bridge submodules and N 1 Half-bridge sub-modules are connected in series, N 1 and N 2 are all natural numbers greater than 1; the half-bridge submodule (HBSM) consists of a storage capacitor, two IGBTs and two anti-parallel freewheeling diodes, and the full-bridge submodule (FBSM) consists of a capacitor, four IGBTs and four anti-parallel freewheeling diodes. Figure 2 As shown in FIG. 1 , one end of the thyristor branch is connected to the adjacent DC side, and the other end is connected to the junction of the half-bridge submodule and the full-bridge submodule. The DC sides connected to the upper and lower bridge arms are connected in series with fast mechanical switches (S 1 and S 2 ) and load transfer switch (LCS 1 and LCS 2 ) and then connected to the DC line.

[0029] In normal operation, the proposed scheme is consistent with the operation mode of conventional hybrid MMC. The fast mechanical switch and load transfer switch on the DC side remain turned on, all thyristor branches remain turned off, and the output voltage of each phase bridge arm is controlled by switching on and off the submodules. After a short circuit fault is detected on the DC side, all half-bridge submodules and full-bridge submodules are locked first. After the bridge arm current becomes negative, state 1) is reached. Figure 4 As shown in (a), the upper thyristor branch with the lowest phase voltage (the upper thyristor branch refers to the thyristor branch connected in parallel with the series half-bridge sub-module in the upper bridge arm) and the lower thyristor branch with the highest phase voltage (the lower thyristor branch refers to the thyristor branch connected in parallel with the series half-bridge sub-module in the lower bridge arm) are triggered, and all the half-bridge sub-modules connected in parallel with these two thyristor branches are bypassed, and the DC side load transfer switch is turned on (state 2), as shown in FIG. Figure 4 As shown in (b), the fast mechanical switch can then be quickly disconnected at zero current. After the insulation voltage of the fast mechanical switch is higher than the voltage of the half-bridge sub-module in a single bridge arm, all half-bridge sub-modules are also locked (state 3), as shown in Figure 4 (c) As shown. In the following period of time, the thyristor branches alternately switch current, which is similar to the active inverter mode: the phase with lower phase voltage in the three upper thyristor branches is turned on, and the phase with higher phase voltage in the three lower thyristor branches is turned on. After that, the half-bridge sub-module and the full-bridge sub-module can provide a total back-EMF of twice the rated DC voltage in the fault current path. This back-EMF can quickly clear the DC fault current, such as Figure 4 (c) As shown. After the fault current is cleared, the thyristor is no longer triggered and the hybrid MMC enters the normal operation mode. The fast mechanical switch and load transfer switch on the DC side of the MMC remain turned on, all thyristor branches remain turned off, and each phase bridge arm controls its output voltage by switching on and off the submodule.

[0030] Preferably, the proportion of the full-bridge submodules is designed as follows: the full-bridge submodules play a role in limiting the rise of fault current (rather than clearing the DC fault current). The proportion of the full-bridge submodules to all submodules is designed according to the three-phase AC voltage, that is, the sum of the full-bridge submodule voltages in an upper bridge arm and a lower bridge arm should be set to about the average value of the DC voltage formed after the three-phase AC voltage passes through the diode full-bridge uncontrolled rectification.

[0031] Preferably, the voltage stress of each thyristor branch is the sum of the voltages of the half-bridge submodules in the parallel bridge arm. The current stress of each thyristor branch is selected according to the magnitude of the fault current. It is worth noting that no current flows through the thyristor during non-fault periods, so the type of thyristor is selected mainly based on the thyristor's ability to withstand transient currents, and the thyristor has a strong ability to withstand short-term overcurrents, which helps to reduce the investment cost of the thyristor.

[0032] Preferably, the maximum current stress of the fast mechanical switch is a DC fault current. Each load transfer switch is composed of two IGBTs connected in reverse series, and its maximum current stress is also a DC fault current.

[0033] The topology proposed in the present invention has broad application prospects in long-distance direct current transmission systems.

[0034] Example 1

[0035] like Figure 5 As shown in the figure, a two-terminal MMC-HVDC (high-voltage direct current transmission system based on modular multilevel converter) simulation model with pseudo bipolar main connection mode was established in MATLAB / Simulink to verify the effectiveness of the proposed sub-module selection algorithm.

[0036] A converter station contains only one MMC, and the fault type to be dealt with is the most serious bipolar short circuit fault. The key parameters of the simulation model are as follows:

[0037]

[0038] There are 228 half-bridge submodules and 128 full-bridge submodules in each bridge arm. The rated DC voltage is 640kV, the rated submodule capacitor voltage is 1.8kV, the rated DC current is 1.875kA, and the proportion of full-bridge submodules is 36%.

[0039] In a long-distance DC transmission system, the inductance of the DC transmission line is large, and the converter will experience serious full-bridge submodule overvoltage. In the simulation, the inductance from the converter to the fault point (including the inductance of the DC reactor and the DC transmission line) is 1015mH.

[0040] Because the converter MMC1 operating in the rectification mode will face a larger DC short-circuit fault current and the difficulty of clearing the fault current will be correspondingly greater, the simulation and experimental results in this article are all from the converter MMC1 operating in the rectification mode.

[0041] At t = 0.5s, a bipolar short circuit fault occurs on the DC side, and the DC fault current begins to rise. At t = 0.502s, the system detects a short circuit fault on the DC side and immediately locks all submodules to enter the DC fault clearing mode, and the DC fault current will be quickly cleared.

[0042] like Figure 6 As shown, the simulation results provide DC voltage and DC current waveforms to demonstrate the DC fault clearing process, show the AC current and bridge arm current, and provide full-bridge sub-module voltage and half-bridge sub-module voltage waveforms to demonstrate the effectiveness of the proposed method in alleviating sub-module overvoltage.

[0043] Figure 6(a) shows the simulation results of a conventional hybrid MMC with a full-bridge submodule ratio of 50%. A DC short circuit fault occurred at t=0.5s, and the DC fault current rose rapidly. 2ms later, the system detected a reverse DC fault, all submodules were locked, the hybrid MMC entered the fault current clearing mode, and the DC output voltage of the converter became negative. Subsequently, the DC fault current began to drop rapidly, and at t=0.513s the DC fault current was completely cleared. The full-bridge submodule voltage began to rise after the submodule was locked, with the maximum value of 2.48kV, and a relatively serious submodule overvoltage occurred. After the half-bridge submodule was locked, the submodule voltage remained basically unchanged, and there was a large difference from the full-bridge submodule voltage.

[0044] Figure 6 (b) shows the simulation results of the proposed scheme. In the simulation model of the proposed scheme, the proportion of full-bridge submodules is 36%. Similarly, the DC current begins to rise after the DC fault occurs. After the submodule is locked, the DC current begins to become negative, and the converter enters the DC fault current clearing mode. At t=0.507s, the DC fault current is completely cleared, and the fault current clearing time is only 45% of that of the conventional hybrid MMC. After the submodule is locked, the full-bridge submodule voltage and the half-bridge submodule voltage both increase. The maximum submodule voltage of the full bridge is 2.11kV, which is within the acceptable range, and no significant submodule overvoltage occurs. The voltage difference between the half-bridge submodule and the full-bridge submodule is also significantly reduced.

[0045] Furthermore, compared with the conventional hybrid MMC, the proposed scheme can significantly improve the fault current clearing speed while reducing the proportion of full-bridge sub-modules, while suppressing the overvoltage of the full-bridge sub-modules and weakening the capacitor voltage imbalance between the full-bridge sub-modules and the half-bridge sub-modules.

[0046] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A hybrid modular multilevel converter based on thyristor commutation circuit assistance, characterized in that: It comprises a three-phase circuit, each phase circuit comprises two upper and lower bridge arms, the upper bridge arm is sequentially composed of N1 half-bridge sub-modules, N2 full-bridge sub-modules and a bridge arm reactor in series from the high voltage end to the low voltage end; the lower bridge arm is sequentially composed of a bridge arm reactor, N2 full-bridge sub-modules and N1 half-bridge sub-module in series from the high voltage end to the low voltage end, and both N1 and N2 are natural numbers greater than 1; in each bridge arm, a thyristor branch is connected in parallel at both ends of the half-bridge sub-modules connected in series, the thyristor branch comprises a plurality of thyristors connected in series, one end of the thyristor branch is connected to the adjacent DC side, and the other end is connected to the junction of the half-bridge sub-module and the full-bridge sub-module, and the DC side is sequentially connected in series with a fast mechanical switch and a load transfer switch and then connected to the DC line; In normal operation, the fast mechanical switch and load transfer switch on the DC side remain turned on, all thyristor branches remain turned off, and the output voltage of each phase arm is controlled by switching the submodules on and off. If a fault occurs on the DC side, the full-bridge submodule and the half-bridge submodule are locked to limit the fault current, and the load transfer switch is used to transfer the fault current from the half-bridge submodule to the thyristor to reconstruct the fault current path, so that the half-bridge submodule and the full-bridge submodule can provide a reverse potential in the fault current path, thereby clearing the DC fault current.

2. The hybrid modular multilevel converter based on thyristor commutation circuit assistance according to claim 1, characterized in that: When a fault occurs on the DC side, the specific processing process is as follows: After detecting a short-circuit fault on the DC side, all full-bridge submodules and half-bridge submodules are locked, and after the bridge arm current becomes negative, the upper thyristor branch with the lowest phase voltage and the lower thyristor branch with the highest phase voltage are triggered. At the same time, all the half-bridge submodules connected in parallel with these two thyristor branches are bypassed, and the DC side load transfer switch is turned on; The fast mechanical switch is quickly disconnected under zero current conditions. After the insulation voltage of the fast mechanical switch is higher than the voltage of the half-bridge sub-module in a single bridge arm, all half-bridge sub-modules are locked again. During a period of time thereafter, the thyristor branches alternately exchange current, the phase with a lower phase voltage in the upper thyristor branch is turned on, and the phase with a higher phase voltage in the lower thyristor branch is turned on. The half-bridge sub-module and the full-bridge sub-module provide a total back-EMF of twice the rated DC voltage in the fault current path, which quickly clears the DC fault current.

3. The hybrid modular multilevel converter based on thyristor commutation circuit assistance according to claim 1, characterized in that: The proportion of the full-bridge submodule to all submodules is designed according to the three-phase AC voltage, that is, the sum of the full-bridge submodule voltages in an upper bridge arm and a lower bridge arm is set to the average value of the DC voltage formed after the three-phase AC voltage is rectified by the diode full-bridge uncontrolled.

4. The hybrid modular multilevel converter based on thyristor commutation circuit assistance according to claim 1, characterized in that: The voltage stress of each thyristor branch is the sum of the voltages of the half-bridge sub-modules in the parallel-connected bridge arms.

5. The hybrid modular multilevel converter based on thyristor commutation circuit assistance according to claim 1, characterized in that: The maximum current stress of the fast mechanical switch is a DC fault current.

6. The hybrid modular multilevel converter based on thyristor commutation circuit assistance according to claim 1, characterized in that: Each load transfer switch consists of two IGBTs connected in reverse series, and its maximum current stress is the DC fault current.

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