Hybrid cascade DC fault disturbed coordination system based on turn-off current source converter

Abstract

The invention relates to the technical field of high-voltage direct-current power transmission, discloses a hybrid cascade direct-current fault disturbed coordination system based on a turn-off current source converter, and belongs to the technical field of high-voltage direct-current power transmission. A receiving-end high-end power grid commutation converter is replaced by a turn-off current source converter, and an active energy consumption mechanism of a lightning arrester is combined, so that control cooperation is realized; a main branch of the turn-off current source converter is formed by connecting a thyristor and a full-control device in series, and an auxiliary branch is connected with a metal oxide lightning arrester in parallel. When a fault occurs, the control unit actively turns off the main branch IGBT, the fault current is switched to the auxiliary branch, the lightning arrester is utilized to establish a reverse commutation voltage and rapidly discharge energy, and the direct current and voltage fluctuation of the low-end MMC is inhibited. The method has the advantages that the commutation failure risk is eliminated, the MMC locking probability is reduced, and the energy loss is reduced, the hybrid direct-current power drop during the fault period is reduced, and the method has remarkable engineering application value.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current transmission, and particularly to a hybrid cascaded DC fault disturbed coordination system based on a turn-off current source converter. Background Art

[0002] High-voltage direct current (HVDC) transmission technology has become an important part of modern power systems due to its advantages of long-distance and large-capacity power transmission. The traditional HVDC system (LCC-HVDC) based on line-commutated converters (LCC) uses semi-controlled thyristor devices, which have the characteristics of low cost and mature technology. However, its commutation process depends on the short-circuit current of the AC grid, and there are inherent defects such as the risk of commutation failure and large reactive power demand.

[0003] To overcome the limitations of LCC, voltage source converter (VSC) technology based on fully-controlled devices has gradually developed. The VSC-HVDC system represented by the modular multilevel converter (MMC) realizes high-voltage and large-capacity power transmission through sub-module cascading, avoids the problem of commutation failure, and has advantages such as independent active / reactive power control and low harmonic level. However, the high construction cost and large operating loss of MMC-HVDC limit its large-scale application.

[0004] To combine the advantages of LCC and VSC, hybrid DC transmission systems have become a research hotspot. Typical projects include:

[0005] Skagerrak HVDC project (ABB): On the basis of the original three-pole LCC-HVDC, the Skagerrak HVDC Interconnections Pole 4 project was newly built, with VSC-HVDC as the fourth pole, becoming the world's first hybrid DC transmission project that combines LCC-HVDC and VSC-HVDC into a bipolar HVDC. VSC-HVDC and LCC-HVDC are fed into the same AC bus, which can give full play to the supporting role of VSC-HVDC for the AC bus and reduce the commutation failure risk of LCC-HVDC.

[0006] Kunliu Long DC project (China Southern Power Grid): The sending-end converter station uses LCC to transmit the clean electric energy of Wudongde Hydropower Station. The receiving-end converter stations in Liuzhou, Guangxi and Longmen, Guangdong both use VSC, and an LCC-VSC type hybrid DC has actually been formed. Moreover, due to the receiving-end being respectively located in Liuzhou, Guangxi and Longmen, Guangdong, a multi-terminal receiving-end structure is formed, and its control characteristics are more complex.

[0007] Furthermore, the hybrid cascaded DC system (the sending - end LCC + the receiving - end LCC in series with the MMC) has attracted attention due to its economy and flexibility. For example, in the Baihetan - Jiangsu UHV project, at the receiving - end, the high - end LCC is in series with the low - end MMC. The reactive power regulation ability of the MMC is used to suppress the commutation failure of the LCC, and at the same time, the difficulty of developing DC circuit breakers is reduced. However, this topology still faces the following problems during receiving - end AC faults:

[0008] 1) Commutation failure causes DC over - current: The commutation failure of the receiving - end LCC leads to a sudden drop in DC voltage. The sending - end constant - current control response lags, and the MMC may block due to its limited over - current capacity (only 2 times the rated current).

[0009] 2) Contradiction between energy discharge requirements and losses: Existing solutions (such as controllable arresters) can quickly discharge fault energy, but long - term operation will cause the arrester to overheat and damage; while the access of a current - limiting resistor brings additional power losses.

[0010] Limitations of existing solutions:

[0011] Solution 1: In case of a fault, a controllable arrester is put into operation to quickly discharge DC energy and suppress DC over - voltage; although the arrester can quickly discharge fault energy, it cannot solve the problem of MMC over - current blocking, and this device cannot meet the fast energy response requirements of long - distance HVDC systems.

[0012] Solution 2: When a commutation failure fault of the LCC occurs, the FCL connects a current - limiting resistor to control the DC current of the MMC within 2 times the rated current to prevent the MMC from blocking; although the current - limiting resistor is connected during a fault, long - term operation in the current - limiter will cause significant power losses. In addition, the heat generated by the long - term operation of the current - limiting resistor has a negative impact on the long - term stability of the fault current limiter.

[0013] In view of the above problems, there is an urgent need for a hybrid cascaded DC fault coordination strategy that can simultaneously eliminate the risk of commutation failure, quickly suppress over - voltage and over - current, and reduce energy losses. By replacing the high - end LCC at the receiving - end with a current - source converter with turn - off capability (CLCC) and combining the active energy - dissipation mechanism of the arrester, realizing the switching of the fault current path and efficient energy discharge, is the key path to solve the contradictions of the existing technology. Summary of the Invention

[0014] In view of the above problems, the purpose of the present invention is to provide a hybrid cascaded DC fault - disturbed coordination system based on a current - source converter with turn - off capability. On the basis of the traditional hybrid cascaded power transmission system, it is transformed by combining a current - source converter with turn - off capability, and combined with corresponding fault control strategies, it can effectively reduce the over - voltage and over - current problems in the fault state of the traditional hybrid cascaded power transmission system under the condition of receiving - end grid faults. The technical solutions are as follows:

[0015] A hybrid cascaded DC fault disturbed coordination system based on a turn-off current source converter, comprising:

[0016] Sending-end converter: adopting a line-commutated converter for converting AC electric energy into DC electric energy;

[0017] Receiving-end converter: composed of a cascaded connection of a high-end turn-off current source converter and a low-end MMC;

[0018] The high-end turn-off current source converter includes:

[0019] Main branch: composed of a thyristor V 41 and a fully controlled device V 42 connected in series, for conducting current under normal operating conditions;

[0020] Auxiliary branch: composed of a thyristor V 44 connected in parallel with a fully controlled device V 43 and a metal oxide arrester connected in series, for forced commutation and energy dissipation during faults;

[0021] The low-end MMC is connected in series with the high-end turn-off current source converter, for regulating reactive power and suppressing DC voltage fluctuations;

[0022] Control unit: configured to perform the following operations when a fault in the receiving-end AC system is detected:

[0023] Turn off the fully controlled device in the main branch of the high-end turn-off current source converter, switch the fault current to the auxiliary branch; and trigger the arrester in the auxiliary branch to act, establish a commutation voltage through the arrester and dissipate the fault energy.

[0024] The beneficial effects of the present invention are:

[0025] 1) Compared with Scheme 1, the present invention makes full use of the controllability advantage of topology transformation. By transforming the high-end LCC at the receiving end into a turn-off current source converter and using the energy-consuming working mode of the turn-off current source converter under fault conditions, the overvoltage and overcurrent problems can be quickly suppressed;

[0026] 2) Compared with Scheme 2, the present invention takes into account the advantages of fully controlled devices and semi-controlled devices. Under fault conditions, the main branch is actively turned off, the fault current is switched to the auxiliary branch and commutation is completed by providing an auxiliary commutation voltage through the arrester. At the same time, the arrester acts as a power-consuming resistor, avoiding the power loss caused by the long-term connection of the current limiter resistor. Description of the Drawings

[0027] Figure 1 It is a hybrid cascaded DC system based on a turn-off current source converter.

[0028] Figure 2Comparison of active power of the cascaded hybrid DC monopole with and without a turn-off current source converter (new LCC).

[0029] Figure 3 Comparison of active power of LCC in the cascaded hybrid DC with and without a turn-off current source converter (new LCC).

[0030] Figure 4 Comparison of DC voltage of MMC in the cascaded hybrid DC with and without a turn-off current source converter (new LCC).

[0031] Figure 5 Comparison of DC current of MMC in the cascaded hybrid DC with and without a turn-off current source converter (new LCC). Specific embodiments

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] During the commutation failure of the cascaded hybrid DC, the low-end MMC will be overvoltage and overcurrent. Therefore, in engineering, a controllable lightning arrester is used to dissipate energy during commutation failure and avoid the overvoltage and overcurrent problems of the MMC. To solve this problem more effectively, a hybrid cascaded DC fault disturbance coordination strategy based on a turn-off current source converter is proposed, that is, the high-end LCC is replaced by a turn-off current source converter. On the one hand, it can completely eliminate the risk of commutation failure. At the same time, it can also utilize the forced commutation function of the turn-off current source converter to dissipate the energy in the fault condition through the lightning arrester in the auxiliary branch of the turn-off current source converter, avoiding overvoltage and overcurrent of the low-end MMC during the fault. The specific topological structure is as Figure 1 shown.

[0034] The hybrid cascaded DC fault disturbance coordination system based on the turn-off current source converter of the present invention specifically includes:

[0035] Sending-end converter: Adopting a line-commutated converter to convert AC electrical energy into DC electrical energy;

[0036] Receiving-end converter: Composed of a cascaded high-end turn-off current source converter and a low-end MMC;

[0037] As Figure 1 shown in the right frame, the high-end turn-off current source converter includes:

[0038] Main branch: Composed of a thyristor V 41 and a fully controlled device V 42 connected in series, used to conduct current under normal conditions.

[0039] Auxiliary branch: Composed of a thyristor V 44 and a parallel fully controlled device V 43It is composed in series with a metal oxide varistor (MOV) and is used for forced commutation and energy dissipation during a fault.

[0040] The low - end MMC is connected in series with a high - end turn - off current - source converter, which is used to regulate reactive power and suppress DC voltage fluctuations.

[0041] Control unit: Configured to perform the following operations when a fault in the receiving - end AC system is detected:

[0042] Turn off the fully - controlled device V in the main branch of the high - end turn - off current - source converter 42 , switch the fault current to the auxiliary branch; and trigger the varistor in the auxiliary branch to act, establish a commutation voltage through the varistor and dissipate the fault energy.

[0043] Thyristor V 41 is shunted with a series branch composed of a resistor R d1 and a capacitor C d1 ; the fully - controlled device V 42 is shunted with a series branch composed of a resistor R d2 and a capacitor C d2 , and the resistor R d2 is also shunted with a diode; the fully - controlled device V 43 is shunted with a series branch composed of a resistor R d3 and a capacitor C d3 , and the resistor R d3 is also shunted with a diode; thyristor V 44 is shunted with a series branch composed of a resistor R d4 and a capacitor C d4 .

[0044] For the turn - off current - source converter topology used at the high - end, the main - branch thyristor V 41 adopts the series - connected thyristor valve used in a conventional LCC converter and has the ability to withstand high voltage and large current. The main - branch IGBT (fully - controlled device V 42 ) transfers the current I 11 to the auxiliary branch by active turn - off. As long as its turn - off voltage is higher than the on - state voltage drop of the auxiliary branch, the current can be successfully transferred. According to the current IGBT devices used in engineering, the maximum is 4.5 kV / 3 kA. Therefore, only 1 - 2 - stage series connection is required to meet the requirements, and at the same time, 1 - 2 IGBTs in parallel for each stage can meet the current - carrying requirements of DC projects with different current levels of 3000 - 5000 A.

[0045] The auxiliary branch briefly undertakes the current I 12, it will be actively turned off after a certain delay to ensure that the commutation of the bridge arm is completed and it is reliably turned off. During steady-state operation, each working cycle of the auxiliary branch only bears a short-term small current, and the long-term current-carrying capacity requirement is not high. During the AC fault of the system, the maximum DC current level on the inverter side does not exceed 2 times the rated current. Currently, the maximum turn-off capacity of IGBT exceeds 5 times its own rated current level. Therefore, the IGBT (fully controlled device V 43 ) of the auxiliary branch can be selected according to 0.4 times the rated DC current level. In terms of the withstand voltage requirement, since the auxiliary branch is connected in parallel with the main branch and needs to bear the voltage of the entire bridge arm during the non-current-carrying period, therefore, the thyristors V 44 and IGBT valves of the auxiliary branch should all be connected in series in multiple stages.

[0046] For the turn-off current source converter topology used in high-end applications, when the main branch thyristor V 41 is triggered and conducts, the main branch IGBT (fully controlled device V 42 ) is simultaneously triggered and conducts for 120°, and the auxiliary branch IGBT (fully controlled device V 43 ) is triggered and conducts for 140°. When the main branch IGBT (fully controlled device V 42 ) is turned off, the auxiliary branch thyristor V 41 is triggered and conducts. The switching between the main and auxiliary branches is realized according to the above logic. When a fault occurs, the device switching logic remains unchanged. When the auxiliary branch IGBT (fully controlled device V 43 ) is turned off, the fault current is transferred to the capacitor C d3 connected in parallel with the auxiliary branch IGBT. When the voltage of the capacitor C d3 reaches the operating voltage of the arrester, the fault current I 12 is transferred to the arrester and the fault energy is dissipated.

[0047] According to the foregoing scheme, after replacing the high-end LCC with a turn-off current source converter, a fault is set on the inverter-side commutation bus, and the simulation results are as Figures 2 - 5 shown.

[0048] When a fault occurs on the receiving-end AC side bus, the power of the conventional cascaded hybrid DC system drops to about 1000 MW. However, since the new LCC cascaded hybrid DC system does not have the problem of commutation failure, it can maintain a higher power output under faults, as Figure 2 shown.

[0049] At the same time, due to the voltage drop caused by the receiving-end AC system, the power of the high-end LCC converter of the conventional cascaded hybrid DC system drops significantly to nearly 0, while the new LCC cascaded hybrid DC system can still maintain a power transmission of more than 60%, as Figure 3 shown.

[0050] Under the fault of the receiving-end AC system, due to the significant power drop of the high-end LCC, overvoltage and overcurrent will occur in the low-end MMC valve group of the conventional cascaded hybrid DC system. However, the new LCC cascaded hybrid DC system can operate in the energy-consuming state of the auxiliary branch under fault conditions, and the fault energy is dissipated through the arrester. The specific voltage and current waveforms are as Figure 4 , 5 shown.

[0051] The results show that when a fault occurs in the commutation busbar on the inverter side, since the turn-off current source converter can force commutation and avoid the high-end LCC power from dropping to 0, the power drop of the hybrid DC system configured with the turn-off current source converter is significantly reduced, and the active power drop of the hybrid DC monopole is reduced by 53%. More importantly, due to the function of the energy-consuming branch of the turn-off current source converter, it can ensure that the DC current of the low-end and middle-end MMC in the hybrid DC does not exceed 2.0 pu (about 3.33 kA), and the DC voltage fluctuation does not exceed 5% (420 kV), avoiding the problems of overvoltage and overcurrent of the MMC. It can be seen that this scheme has practical engineering value.

Claims

1. A hybrid cascaded DC fault disturbed coordination system based on a turn-off current source converter, characterized in that, Including: Sending - end converter: Adopting a line - commutated converter, which is used to convert AC electrical energy into DC electrical energy; Receiving - end converter: Comprising a high - end turn - off current - source converter and a low - end MMC. The low - end MMC is connected in series with the high - end turn - off current - source converter and is used to regulate reactive power and suppress DC voltage fluctuations; The high - end turn - off current - source converter includes: Main branch: Composed of thyristor V 41 and fully controlled device V 42 connected in series, used to conduct current under normal working conditions; Auxiliary branch: composed of thyristor V 44 and the fully controlled device V connected in parallel 43 and a metal oxide arrester in series, used for forced commutation and energy dissipation during faults; Control unit: Configured to perform the following operations when a receiving - end AC system fault is detected: Turn off the fully-controlled device V in the main branch of the high-end turn-off current source converter 42 , and switch the fault current to the auxiliary branch; then trigger the metal oxide arrester in the auxiliary branch to act, establish the commutation voltage through the metal oxide arrester and dissipate the fault energy.

2. The hybrid cascaded DC fault disturbed coordination system based on a turn-off current source converter according to claim 1, characterized in that The thyristor V 41 is shunted by a resistor R d1 and a capacitor C d1 to form a series branch; The fully controlled device V 42 is connected in parallel with the branch formed by the series connection of resistor R d2 and capacitor C d2 ; the resistor R d2 is also connected in parallel with a diode; The fully controlled device V 43 is connected in parallel with a branch formed by series connection of a resistor R d3 and a capacitor C d3 ; the resistor R d3 is also connected in parallel with a diode; The thyristor V 44 is connected in parallel with the branch formed by the series connection of the resistor R d4 and the capacitor C d4 in series.

3. The hybrid cascaded DC fault disturbed coordination system based on a turn-off current source converter according to claim 2, wherein The working processes of the main branch and the auxiliary branch are as follows: When the main-branch thyristor V 41 is triggered and conducts, the fully-controlled device V in the main branch 42 is simultaneously triggered and conducts for 120°, and the fully-controlled device V in the auxiliary branch 43 is triggered and conducts for 140°; when the fully-controlled device V in the main branch 42 turns off, the thyristor V in the auxiliary branch 41 is triggered and conducts to achieve the switching between the main and auxiliary branches; After a fault occurs, after the fully-controlled device V in the auxiliary branch 43 is turned off, the fault current transfers to the capacitor C 43 connected in parallel with the fully-controlled device V in the auxiliary branch. d3 When the voltage of the capacitor C d3 reaches the operating voltage of the arrester, the fault current is transferred to the arrester to dissipate the fault energy.

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