Hybrid modular multilevel converter assisted by thyristor commutation circuit

Abstract

The present invention discloses a hybrid modular multilevel converter (MMC) assisted by a thyristor commutation circuit. Based on the hybrid MMC, a thyristor branch (formed by several series-connected thyristors) is connected in parallel to the series half-bridge sub-module in each arm. 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. A fast mechanical switch and a load transfer switch are sequentially connected in series on the DC side and then connected to the DC line. After a DC short-circuit fault occurs, the present invention can quickly and dynamically adjust the fault current path, enabling both the half-bridge sub-module and the full-bridge sub-module to participate in clearing the DC fault current. The present invention can significantly improve the fault current clearing speed of the hybrid MMC and reduce the demand for full-bridge sub-modules. Therefore, the cost and loss of the present invention will also be lower than those of the conventional hybrid MMC. In addition, since the half-bridge sub-module also participates in DC fault clearing and energy absorption, the overvoltage problem of the full-bridge sub-module is effectively alleviated.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible DC power transmission and distribution, and particularly relates to a hybrid modular multilevel converter assisted by a thyristor commutation circuit. Background Art

[0002] Compared with the conventional high-voltage DC power transmission system based on thyristor phase-commutated converters, the flexible DC power transmission system has a more flexible operation mode and is an important means for transmitting new energy power. In order to develop long-distance flexible DC power transmission technology, it is necessary to achieve rapid DC fault clearing. However, at present, the high-voltage DC circuit breaker technology is not yet mature, the cost is relatively high, and the breaking capacity is still limited. Therefore, it is still difficult to be widely promoted and applied on a large scale.

[0003] The modular multilevel converter (MMC) using full-bridge sub-modules is the most typical improved MMC topology with the ability to clear DC faults. However, its cost and loss are relatively too high compared with the half-bridge MMC. In order to utilize the advantages of the full-bridge sub-modules while avoiding the problems of high cost and high loss caused by using all full-bridge sub-modules, the hybrid MMC with doped use of half-bridge and full-bridge sub-modules has received more attention. Compared with the full-bridge MMC, the hybrid MMC has significantly reduced cost and loss, but still retains the DC fault clearing ability of the latter and the characteristic that the modulation ratio can be greater than 1.

[0004] The DC fault clearing ability of the hybrid MMC is severely limited by the number of full-bridge sub-modules. The hybrid MMC requires a high proportion of full-bridge sub-modules (generally 50% and above in order to ensure the DC fault current clearing ability), resulting in high cost and power loss. And because the sub-module capacitors need to absorb a large amount of energy during fault clearing, when the DC transmission line is long, there may also be a serious overvoltage problem of the full-bridge sub-modules.

[0005] It can be seen that most of the existing fault clearing schemes relying on converters can only "clear" DC faults, but cannot achieve "rapid" clearing. And among the existing converters with strong DC fault clearing ability, their cost and loss are both high, making them difficult to be practical. 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 ability of the hybrid MMC.

[0007] The technical solution for achieving the object of the present invention is as follows: A hybrid modular multilevel converter assisted by a thyristor commutation circuit, comprising a three-phase circuit, each phase circuit including upper and lower two arms. The upper arm is successively composed of N 1 half-bridge sub-modules, N 2 full-bridge sub-modules and an arm reactor connected in series from the high-voltage end to the low-voltage end; the lower arm is successively composed of an arm reactor, N 2 full-bridge sub-modules and N 1 half-bridge sub-modules connected in series from the high-voltage end to the low-voltage end. N 1 and N 2 are both natural numbers greater than 1. In each arm, a thyristor branch is connected in parallel at both ends of the series-connected half-bridge sub-modules. The thyristor branch includes a plurality of series-connected thyristors. 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. The DC side is connected to the DC line in series with a fast mechanical switch and a load transfer switch in sequence;

[0008] During normal operation, the fast mechanical switch and the load transfer switch on the DC side remain conducting, all thyristor branches remain off, and the output voltage of each phase arm is controlled by the insertion and removal of the sub-modules;

[0009] If a fault occurs on the DC side, the full-bridge sub-modules and the half-bridge sub-modules are blocked to limit the fault current. The load transfer switch is used to transfer the fault current from the half-bridge sub-module to the thyristor to reconstruct the fault current path, so that the half-bridge sub-module and the full-bridge sub-module can provide a counter electromotive force 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 sub-modules and half-bridge sub-modules are blocked. After the 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, and at the same time, all the half-bridge sub-modules in parallel with these two thyristor branches are bypassed, and the DC side load transfer switch is opened;

[0012] The fast mechanical switch is quickly disconnected under 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 arm, all the half-bridge sub-modules are blocked again; within a period of time thereafter, the thyristor branches commutate alternately, the phase with the lower phase voltage in the upper thyristor branch conducts, and the phase with the higher phase voltage in the lower thyristor branch conducts; the half-bridge sub-module and the full-bridge sub-module provide a counter electromotive force with a total magnitude of twice the rated DC voltage in the fault current path, and this counter electromotive force quickly clears the DC fault current.

[0013] Preferably, the proportion of full-bridge sub-modules among all sub-modules is designed according to the three-phase AC voltage, that is, the sum of the voltages of the full-bridge sub-modules in one upper bridge arm and one lower bridge arm should be set to be around the average value of the DC voltage formed after the three-phase AC voltage passes through an uncontrolled diode full-bridge rectifier.

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

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

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

[0017] Compared with the prior art, the remarkable advantages of the present invention are as follows:

[0018] The present invention proposes to use thyristor commutation to clear the DC fault current of the MMC, which reduces the requirement for the proportion of full-bridge sub-modules in the hybrid MMC, thereby reducing costs and power losses. At the same time, the DC fault clearing ability of the hybrid MMC is enhanced, and the overvoltage problem of the full-bridge sub-modules during fault clearing is effectively alleviated by the solution proposed in the present invention.

[0019] Other features and advantages of the present invention will be described in the following specification, and some of them will become obvious from the specification or be understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the structures specifically pointed out in the written specification, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference numerals represent the same components.

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

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

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

[0024] Figure 4 is the equivalent circuit of the hybrid MMC and the fault current path of the present invention (at this time u a >u b >u c ), where Figure 4 in (a) is the path schematic diagram of state 1, Figure 4In which, (b) is the path schematic diagram of State 2, Figure 4 In which, (c) is the path schematic diagram of State 3

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

[0026] Figure 6 It is the simulation result of the DC fault clearing process of the conventional scheme and the proposed scheme. Detailed implementation manners

[0027] It is easy to understand that according to the technical solution of the present invention, without changing the essence of the present invention, those of ordinary skill in the art can imagine various implementation manners of the present invention. Therefore, the following detailed implementation manners and drawings are only exemplary descriptions of the technical solution of the present invention, and should not be regarded as all of the present invention or as a limitation or restriction on 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 will be specifically described below in conjunction with the drawings, where the drawings form a part of this application and are used together with the embodiments of the present invention to explain the innovative concept of the present invention.

[0028] A hybrid modular multilevel converter assisted by a thyristor commutation circuit, on the basis of a conventional MMC, a thyristor branch (multiple thyristors in series) is connected in parallel to each arm. As Figure 1 shown, the conventional hybrid MMC is a three-phase six-arm structure, each phase includes upper and lower two arms. For the upper arm, from the high-voltage end to the low-voltage end, it is successively composed of N 1 half-bridge sub-modules, N 2 full-bridge sub-modules and an arm reactor in series; for the lower arm, from the high-voltage end to the low-voltage end, it is successively composed of an arm reactor, N 2 full-bridge sub-modules and N 1 half-bridge sub-modules in series, N 1 and N 2 are all natural numbers greater than 1; among them, the half-bridge sub-module (HBSM) is composed of an energy storage capacitor, two IGBTs and two anti-parallel freewheeling diodes, and the full-bridge sub-module (FBSM) is composed of a capacitor, four IGBTs and four anti-parallel freewheeling diodes. As Figure 2 shown, 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. The DC sides connected to the upper and lower two arms are successively connected in series with fast mechanical switches (S 1 and S 2 ) and load transfer switches (LCS 1 and LCS 2 ) and then connected to the DC line.

[0029] During normal operation, the proposed scheme operates in the same manner as that of a conventional hybrid MMC. The fast mechanical switch and the load transfer switch on the DC side remain conducting, all thyristor branches remain off, and the output voltage of each phase arm is controlled by the insertion and removal of sub-modules. After detecting a short-circuit fault on the DC side, all half-bridge sub-modules and full-bridge sub-modules are blocked first. After the arm current becomes negative, in state 1), as Figure 4 (a) shows, 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-modules in the upper 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-modules in the lower arm) are triggered, and at the same time, all the half-bridge sub-modules connected in parallel with these two thyristor branches are bypassed, and the DC side load transfer switch is opened (state 2), as Figure 4 (b) shows. Subsequently, the fast mechanical switch can be quickly disconnected under zero current. After the insulation voltage of the fast mechanical switch is higher than the voltage of the half-bridge sub-modules in a single arm, all half-bridge sub-modules are also blocked (state 3), as Figure 4 (c) shows. During a period of time thereafter, the thyristor branches commutate alternately, which is similar to the active inverter mode: the phase with the lower phase voltage in the three upper thyristor branches conducts, and the phase with the higher phase voltage in the three lower thyristor branches conducts. Thereafter, the half-bridge sub-modules and full-bridge sub-modules can provide a back electromotive force with a total magnitude of twice the rated DC voltage in the fault current path, and this back electromotive force can quickly clear the DC fault current, as Figure 4 (c) shows. After the fault current is cleared, the triggering of the thyristors is stopped, and the hybrid MMC enters the normal operation mode. The fast mechanical switch and the load transfer switch on the DC side of the MMC remain conducting, all thyristor branches remain off, and the output voltage of each phase arm is controlled by the insertion and removal of sub-modules.

[0030] Preferably, the proportion of full-bridge sub-modules is designed such that the full-bridge sub-modules play a role in restricting the rise of the fault current (rather than clearing the DC fault current). The proportion of full-bridge sub-modules in all sub-modules is designed according to the three-phase AC voltage, that is, the sum of the voltages of the full-bridge sub-modules in one upper arm and one lower arm should be set to be around the average value of the DC voltage formed after the three-phase AC voltage is rectified by an uncontrolled diode full-bridge.

[0031] Preferably, the voltage stress of each thyristor branch is the sum of the voltages of the half-bridge sub-modules in the arm to which it is connected. The current stress of each thyristor branch is selected according to the magnitude of the fault current. It should be noted that during non-fault periods, no current flows through the thyristors. Therefore, the type of thyristor is mainly selected according to the ability of the thyristor to withstand transient current, and the short-time over-current withstand ability of the thyristor is very strong, which helps to reduce the investment cost of the thyristor.

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

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

[0034] Embodiment 1

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

[0036] One converter station only contains 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 sub-modules and 128 full-bridge sub-modules in each arm. The rated DC voltage is 640 kV, the rated sub-module capacitor voltage is 1.8 kV, the rated DC current is 1.875 kA, and the proportion of full-bridge sub-modules is 36%.

[0039] In a long-distance DC power transmission system, the inductance of the DC transmission line is large, and serious over-voltage of full-bridge sub-modules will occur in the converter. The inductance from the converter to the fault point (including the inductance of the DC reactor and the DC transmission line) in the simulation is 1015 mH.

[0040] Since 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 paper all come from the converter MMC1 operating in the rectification mode.

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

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

[0043] Figure 6(a) shows the simulation results of a conventional hybrid MMC with a full-bridge sub-module ratio of 50%. A DC short-circuit fault occurs at t = 0.5 s, and the DC fault current rises rapidly. After 2 ms, the system detects the occurrence of the DC fault, all sub-modules are blocked, and the hybrid MMC enters the fault current clearing mode, and the DC output voltage of the converter becomes negative. Subsequently, the DC fault current begins to decline rapidly, and the DC fault current is completely cleared at t = 0.513 s. The voltage of the full-bridge sub-module starts to rise after the sub-module is blocked, and the maximum value is 2.48 kV, showing a relatively serious overvoltage of the sub-module. The voltage of the half-bridge sub-module remains basically unchanged after being blocked, showing a large difference from the voltage of the full-bridge sub-module.

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

[0045] Furthermore, compared with the conventional hybrid MMC, the proposed scheme can significantly improve the fault current clearing speed, suppress the overvoltage of the full-bridge sub-module, and weaken the capacitor voltage imbalance between the full-bridge sub-module and the half-bridge sub-module on the premise of reducing the ratio of the full-bridge sub-module.

[0046] As described above, it is only the 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 thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by 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 an upper bridge arm and a lower bridge arm, 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-modules in series from the high voltage end to the low voltage end, N1 and N2 are both 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, 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; 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.

2. 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.

3. 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.

4. 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.

5. 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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