Auxiliary branch reuse LCC topology and control strategy based on reverse voltage blocking

Abstract

The present invention relates to the technical field of high-voltage direct current transmission, and discloses an auxiliary branch multiplexing LCC topology based on reverse voltage blocking and its control strategy. Each phase includes a thyristor valve of an upper bridge arm and a thyristor valve of a lower bridge arm, and auxiliary branch thyristors are connected in parallel at both ends of the original upper and lower bridge arm thyristor valves for current transfer in the event of a fault; and a fully controlled device IGBT is reused in the upper and lower bridge arms of the same phase, which is used to transfer the fault current to the lightning arrester connected in parallel by actively shutting down the IGBT, triggering the lightning arrester to operate and provide auxiliary commutation voltage, compensating for the voltage drop caused by the AC system fault, thereby resisting commutation failure. The present invention combines a fully controlled device with a traditional LCC converter valve for topological transformation, and combines a corresponding fault control strategy to resist the commutation failure problem existing in the traditional LCC-HVDC system in the event of a receiving-end power grid fault.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current transmission, and in particular to an auxiliary branch multiplexing LCC (Line-commutated Converter) topology based on reverse voltage blocking and a control strategy thereof. Background Art

[0002] The traditional high-voltage direct current transmission system (LCC-HVDC) based on the grid-commutated converter (LCC) has been widely used around the world due to its advantages of low loss, long-distance and large-capacity transmission. With the continuous expansion of the scale of direct current transmission in my country, the impact of the stable operation of direct current projects on the stability of power grid operation is increasing. However, the core commutation device thyristor of the traditional LCC commutator valve cannot be controlled to close by itself, and it needs to withstand a period of reverse pressure before its locking characteristics can be restored. Therefore, it is easy to cause commutation failure when the inverter fails.

[0003] The essence of commutation failure is that the free electrons and holes in the high-power thyristor have an inherent deionization recovery time of about 400 microseconds, which is converted into an electrical angle of 7 degrees. This electrical angle is called the inherent critical turn-off angle γ min , if the inverter turn-off angle γ is less than γ min , commutation failure will occur.

[0004] After the commutation failure occurs, it will cause many adverse effects on the AC and DC systems. After the commutation failure occurs, the DC side power will drop sharply, and in severe cases it will even cause DC power interruption. When the fault disappears after the commutation failure and during the commutation failure recovery period, the DC inverter station will still absorb a large amount of reactive power from the AC system, which will cause the inverter side commutation bus voltage to drop for the second time.

[0005] In the protection and control strategy of the inverter-side converter station, low-voltage current limiting control (VDCOL) and commutation failure prevention (CFPREV) control are the main means to reduce the risk of commutation failure. The core idea of ​​CFPREV is to reduce the trigger angle immediately when an AC fault is detected, and trigger in advance to leave a larger commutation margin. However, the reduction in the trigger angle will increase the reactive power demand level of the converter station, causing the reactive power of the AC power grid to flow to the converter station, weakening the current bus voltage maintenance capability, and increasing the risk of commutation failure. VDCOL can limit the surge in DC current according to the voltage drop degree after the AC fault at the receiving end, reduce the reactive power demand of the converter station, and improve the commutation conditions.

[0006] VDCOL is an important part of the constant current control scheme on the inverter side of the high-voltage direct current transmission system. After an AC fault occurs, VDCOL can dynamically change the DC current command to limit the DC current according to the voltage drop degree, change the reactive power consumption level of the converter station, thereby promoting the recovery of the AC bus voltage and increasing the maximum commutation area S. max However, as the voltage drops, the maximum commutation area S max The voltage will decrease accordingly, and the risk of commutation failure will increase, which may lead to the situation that VDCOL has not started but commutation failure has already occurred. After an AC fault occurs, the DC current tends to increase rapidly. Therefore, the DC current prediction can be used to predict the DC current value after a period of time before the current surges according to the current change trend, and convert it into a DC voltage change and input it into VDCOL, so that VDCOL can enter the slope section of the VDCOL static characteristic curve in advance before the commutation failure occurs to reduce the DC current and suppress the commutation failure.

[0007] In order to improve the ability of LCC converters to resist commutation failure, forced commutation converter topologies have been proposed in the topology transformation. Forced commutation converters include capacitor commutated converters (CCC) and controlled series capacitor commutated converters (CSCC). Capacitor commutated converters are converter topologies composed of capacitors connected in series between the converter valve and the converter transformer of traditional DC transmission. Figure 1 As shown in the figure, the capacitor in the controllable series capacitor commutation converter is connected in series on the primary side of the converter transformer (between the converter bus and the transformer), and an anti-parallel thyristor is added in parallel with the capacitor. However, these two technologies have defects such as capacitor voltage imbalance and high withstand voltage requirements for converter valves, and have not been widely used in actual projects.

[0008] Existing measure 1: Use the rapid prediction and detection method of commutation failure to trigger the thyristor valve in advance to increase the turn-off angle. Such as the measures proposed by Chen Minquan et al. in "Research on Parameter Optimization of Multi-DC Inter-commutation Failure Prediction Module Based on Numerical Approximation and Value Set Analysis". However, this measure can only reduce the probability of commutation failure to a limited extent, and the large adjustment of the trigger angle exacerbates the fluctuation of active and reactive power, causing impact on the power grid and easily leading to subsequent commutation failure.

[0009] Existing measure 2: Transform the converter topology to improve the system's ability to resist commutation failure. Such as the measures proposed by Deng Lingfeng et al. in "A New Commutation Failure Suppression Method for Series Voltage Commutation Converters". However, when a serious AC fault occurs, this measure cannot effectively control the capacitor charging process, resulting in capacitor overvoltage problems, which in turn leads to inevitable commutation failure. Summary of the invention

[0010] In view of the above problems, the purpose of the present invention is to provide an auxiliary branch multiplexing LCC converter topology based on reverse voltage blocking and its control strategy, which is combined with fully controlled devices on the basis of traditional LCC converter valves for topological transformation, and combined with corresponding fault control strategies, in the case of receiving-end power grid failure, it can resist the commutation failure problem existing in the traditional LCC-HVDC system. The technical solution is as follows:

[0011] An auxiliary branch multiplexing LCC topology based on reverse voltage blocking, each phase includes a thyristor valve of the upper bridge arm and a thyristor valve of the lower bridge arm, and auxiliary branch thyristors are connected in parallel at both ends of the original upper and lower bridge arm thyristor valves for current transfer in the event of a fault. The specific connection method is:

[0012] Two ends of the thyristor valve VT4 of the upper bridge arm of one phase are connected in parallel with auxiliary branch thyristors THY42 and THY43 connected in reverse series, and the negative electrode of the auxiliary branch thyristor THY42 is connected to the negative electrode of THY43, and the positive electrode of THY42 is connected to the positive electrode of the DC input terminal;

[0013] Two ends of the thyristor valve VT1 of the lower bridge arm of the same phase are connected in parallel with auxiliary branch thyristors THY12 and THY13 connected in reverse series, and the positive electrode of the auxiliary branch thyristor THY12 is connected to the positive electrode of THY13, and the negative electrode of THY12 is connected to the negative electrode of the DC input terminal;

[0014] In addition, a fully controlled device IGBT is reused in the upper and lower bridge arms of the same phase to transfer the fault current to the lightning arrester connected in parallel with it by actively shutting down the IGBT. The specific connection method is:

[0015] The auxiliary branch IGBT1 is connected between the negative electrodes of the upper bridge arm auxiliary branch thyristors THY42 and THY43 and the positive electrodes of the lower bridge arm auxiliary branch thyristors THY12 and THY13. The emitter and collector of IGBT1 are connected in parallel with a resistor R1 and a capacitor C d The branch circuit is formed in series and has a lightning arrester in parallel.

[0016] Furthermore, the thyristor valve VT4 includes a main branch thyristor THY41 and an IGBT41 connected in series.

[0017] A working method of an auxiliary branch multiplexing LCC topology based on reverse voltage blocking, comprising:

[0018] When a fault in the receiving-end AC system is detected, the main branch thyristor THY41 is turned on for 120° and then turned off under the original trigger logic, and its current path is consistent with normal commutation; at this time, the auxiliary branch thyristors THY42 and IGBT1 are turned on, and the fault current is transferred to the auxiliary branch. The main branch thyristor THY41 is turned off after the current passes through zero after a period of time. After the main branch thyristor THY41 restores its blocking capacity, the auxiliary branch IGBT1 is turned off and the current is transferred to its parallel lightning arrester. The lightning arrester establishes an operating voltage and enhances the commutation voltage, successfully completing the commutation between the bridge arms and avoiding commutation failure.

[0019] Furthermore, the auxiliary branch IGBT1 is turned off after receiving the shutdown signal, and the current flowing through IGBT1 decreases rapidly, and the current is transferred to the capacitor C d When the fault current transferred from the auxiliary branch IGBT1 to the capacitor C d When the capacitor is charged until the action voltage of the lightning arrester connected in parallel on the auxiliary branch IGBT1 is reached, the current begins to transfer to the lightning arrester. When the voltage on the capacitor is charged to the action voltage of the lightning arrester on the auxiliary branch, the lightning arrester on the auxiliary branch is activated, and the fault current is transferred to the normal bridge arm through the lightning arrester.

[0020] The beneficial effects of the present invention are:

[0021] 1) The present invention proposes a new auxiliary branch multiplexing LCC converter topology based on reverse voltage blocking. On the basis of the traditional LCC converter valve, the topology is transformed by combining the fully controlled device, and combined with the corresponding fault control strategy, it can resist the commutation failure problem existing in the traditional LCC-HVDC system in the case of a receiving-end power grid fault;

[0022] 2) The present invention fully considers the coupling characteristics of fully controlled devices and semi-controlled devices, combines the advantages of both to carry out topological transformation, and the selected IGBT is a fully controlled device that can actively control the opening and closing of the auxiliary branch; the control strategy of the present invention only turns on the auxiliary branch in the event of a fault, reducing the power loss caused by frequent opening of the auxiliary branch;

[0023] 3) Compared with the existing measure 1, the present invention makes full use of the controllability advantage of topology transformation, changes the active and reactive output characteristics of the DC converter under AC fault conditions, and avoids a large shortage of active power and large fluctuation of reactive power in the DC system;

[0024] 4) Compared with the existing measure 2, the present invention takes into account the advantages of fully controlled devices and semi-controlled devices, actively shuts down the main branch in the event of a fault, switches the fault current to the auxiliary branch, and completes the commutation by providing auxiliary commutation voltage through the lightning arrester, avoiding the problem of capacitor overvoltage and having higher applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1It is a CCC inverter topology.

[0026] Figure 2 It is a branch multiplexing controllable LCC AC fault operation mode.

[0027] Figure 3 It is a branch multiplexing type controllable grid phase-commutated converter topology.

[0028] Figure 4 These are the DC voltage and current simulation results under single-phase short-circuit grounding fault conditions.

[0029] Figure 5 This is the simulation result of the AC current at the receiving end under the single-phase short-circuit grounding fault state.

[0030] Figure 6 These are the DC voltage and current simulation results under a three-phase short-circuit grounding fault state.

[0031] Figure 7 This is the simulation result of the AC current at the receiving end under the three-phase short-circuit grounding fault state. DETAILED DESCRIPTION

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

[0033] In order to solve the commutation failure problem in the traditional LCC-HVDC system, the present invention proposes a new converter topology, thereby effectively solving the commutation failure problem in the high voltage direct current transmission system.

[0034] (1) Based on the shutdown characteristics of fully controlled devices and semi-controlled devices, the traditional LCC converter is topologically modified. An auxiliary branch is added to the original bridge arm for current transfer in the event of a fault. The upper and lower bridge arms of the same phase reuse a fully controlled IGBT, and the fault current is transferred to the lightning arrester connected in parallel by actively shutting down the IGBT, triggering the lightning arrester to operate and provide auxiliary commutation voltage to compensate for the voltage drop caused by the AC system fault, thereby resisting commutation failure.

[0035] The present invention is based on the auxiliary branch multiplexing LCC topology with reverse voltage blocking. Each phase includes a thyristor valve of the upper bridge arm and a thyristor valve of the lower bridge arm. Auxiliary branch thyristors are connected in parallel at both ends of the original upper and lower bridge arm thyristor valves for current transfer in the event of a fault. The specific connection method is as follows:

[0036] Auxiliary branch thyristors THY42 and THY43 connected in reverse series are connected in parallel at both ends of the thyristor valve VT4 of the upper bridge arm of one phase, and the negative electrode of the auxiliary branch thyristor THY42 is connected to the negative electrode of THY43, and the positive electrode of THY42 is connected to the positive electrode of the DC input terminal.

[0037] Auxiliary branch thyristors THY12 and THY13 connected in reverse series are connected in parallel at both ends of the thyristor valve VT1 of the lower bridge arm of the same phase, and the positive electrode of the auxiliary branch thyristor THY12 is connected to the positive electrode of THY13, and the negative electrode of THY12 is connected to the negative electrode of the DC input terminal.

[0038] In addition, a fully controlled device IGBT is reused in the upper and lower bridge arms of the same phase to transfer the fault current to the lightning arrester connected in parallel with it by actively shutting down the IGBT. The specific connection method is:

[0039] The auxiliary branch IGBT1 is connected between the negative electrodes of the upper bridge arm auxiliary branch thyristors THY42 and THY43 and the positive electrodes of the lower bridge arm auxiliary branch thyristors THY12 and THY13. The emitter and collector of IGBT1 are connected in parallel with a resistor R1 and a capacitor C d The branch circuit is formed in series and has a lightning arrester in parallel.

[0040] (2) Based on the control strategy of fault detection, the main branch IGBT is turned off and the auxiliary branch is triggered to complete the fault current transfer when a fault is detected, ensuring that the main branch thyristor restores the forward blocking capability and thus shuts down reliably. When an AC side fault occurs, the reuse branch type LCC converter valve enables the auxiliary branch and works in the forced commutation mode. Taking the commutation of VT4 valve to VT6 as an example, its operating principle is as follows: Figure 2 As shown. When a fault is detected, the main branch is turned on for 120° and then turned off under the original trigger logic, and its current path is consistent with normal commutation. At this time, the auxiliary branch thyristor THY42 and IGBT1 are turned on, and the fault current is transferred to the auxiliary branch. The main branch thyristor THY41 is turned off after a period of time, and after the main thyristor valve restores its blocking capacity, the auxiliary branch IGBT1 valve is turned off and the current is transferred to the parallel lightning arrester. The lightning arrester establishes an action voltage and enhances the commutation voltage, successfully completing the commutation between the bridge arms and avoiding commutation failure.

[0041] In this stage, the auxiliary branch IGBT1 is turned off after receiving the shutdown signal, and the current flowing through IGBT1 decreases rapidly. The current is transferred to the capacitor Cd to charge it. The shutdown process of IGBT1 can be processed linearly, such as Figure 2 When the fault current transferred from the auxiliary branch IGBT1 charges the capacitor Cd until it reaches the action voltage of the lightning arrester connected in parallel on the auxiliary branch, the current begins to transfer to the lightning arrester, as shown in Figure 2 As shown in (c), when the voltage on the capacitor is charged to the operating voltage of the auxiliary branch arrester, the auxiliary branch arrester is activated and the fault current is transferred to the VT6 bridge arm through the arrester.

[0042] During the phase change from VT4 valve to VT6 valve, the auxiliary IGBT1 valve of VT4 valve is forced to close, and the forward current passes through the VT4 valve IGBT valve arrester. A forward voltage will be established at both ends of the VT4 valve. The phase change voltage between the valves increases the action voltage of the auxiliary branch arrester on the basis of the AC line voltage, which is more conducive to phase change.

[0043] (3) Based on time domain simulation, a three-phase short-circuit grounding fault is imposed on the inverter side of the high-voltage DC system, and the voltage and current of the DC line are measured and observed.

[0044] Example:

[0045] The new auxiliary branch multiplexing LCC converter topology based on reverse voltage blocking proposed in the present invention is applied to the inverter side of the high-voltage direct current transmission system for simulation and verification. The specific topology of the converter is as follows: Figure 3 As shown, taking the VT4 valve as an example, THY41 and IGBT41 are main branches used for current flow under normal working conditions; THY42 and IGBT1 paths are auxiliary commutation branches, which are turned on and transfer fault current under fault conditions.

[0046] After replacing the inverter side converter with the proposed new converter topology, a single-phase short-circuit grounding fault is imposed on the inverter side of the system during t = 1.1-1.15s. The DC voltage and current waveforms during the fault are as follows: Figure 4 As shown; at this time, the receiving end AC current waveform is as follows Figure 5 shown.

[0047] At this time, a more serious three-phase short-circuit grounding fault is imposed on the receiving-end power grid. The DC voltage and current waveforms during the fault are as follows: Figure 6 As shown, the receiving end AC current waveform is as follows Figure 7 shown.

[0048] It can be seen that after the inverter side is replaced with the proposed new converter topology, even in the extreme fault condition of three-phase short circuit and grounding of the receiving-end power grid, power transmission can still be guaranteed, the DC voltage will not drop to 0 and the AC current at the receiving end is continuous, thereby resisting commutation failure faults, proving the effectiveness of the present invention.

Claims

1. An auxiliary branch multiplexing LCC topology based on reverse voltage blocking, each phase includes a thyristor valve of an upper bridge arm and a thyristor valve of a lower bridge arm, characterized in that: Auxiliary branch thyristors are connected in parallel at both ends of the original upper and lower bridge arm thyristor valves to transfer current in the event of a fault. The specific connection method is: The two ends of the thyristor valve VT4 of the upper bridge arm of one phase are connected in parallel with the auxiliary branch thyristors THY42 and THY43 connected in series in reverse direction. The cathode of the auxiliary branch thyristor THY42 is connected to the cathode of THY43, and the anode of THY42 is connected to the anode of the DC input terminal; The two ends of the thyristor valve VT1 of the lower bridge arm of the same phase are connected in parallel with the auxiliary branch thyristors THY12 and THY13 connected in series in reverse direction. And the positive electrode of the auxiliary branch thyristor THY12 is connected to the positive electrode of THY13, and the negative electrode of THY12 is connected to the negative electrode of the DC input terminal; In addition, a fully controlled device IGBT is reused in the upper and lower bridge arms of the same phase to transfer the fault current to the lightning arrester connected in parallel with it by actively shutting down the IGBT. The specific connection method is: The auxiliary branch IGBT1 is connected between the negative electrodes of the upper bridge arm auxiliary branch thyristors THY42 and THY43 and the positive electrodes of the lower bridge arm auxiliary branch thyristors THY12 and THY13. The emitter and collector of IGBT1 are connected in parallel with a resistor R1 and a capacitor C d The branch circuit is formed in series and has a lightning arrester in parallel.

2. The auxiliary branch multiplexing LCC topology based on reverse voltage blocking according to claim 1 is characterized in that: The thyristor valve VT4 includes a main branch thyristor THY41 and an IGBT41 connected in series.

3. A control strategy for auxiliary branch multiplexing LCC topology based on reverse voltage blocking as claimed in claim 2, characterized in that: include: When a fault in the receiving-end AC system is detected, the main branch thyristor THY41 is turned on for 120° and then turned off under the original trigger logic, and its current path is consistent with normal commutation; at this time, the auxiliary branch thyristors THY42 and IGBT1 are turned on, and the fault current is transferred to the auxiliary branch. The main branch thyristor THY41 is turned off after the current passes through zero after a period of time. After the main branch thyristor THY41 restores its blocking capacity, the auxiliary branch IGBT1 is turned off and the current is transferred to its parallel lightning arrester. The lightning arrester establishes an operating voltage and enhances the commutation voltage, successfully completing the commutation between the bridge arms and avoiding commutation failure.

4. The control strategy according to claim 3, characterized in that: The auxiliary branch IGBT1 is turned off after receiving the shutdown signal, and the current flowing through IGBT1 decreases rapidly, and the current is transferred to the capacitor C d Charge it; When the fault current transferred from the auxiliary branch IGBT1 to the capacitor C d When the capacitor is charged until the action voltage of the lightning arrester connected in parallel on the auxiliary branch IGBT1 is reached, the current begins to transfer to the lightning arrester. When the voltage on the capacitor is charged to the action voltage of the lightning arrester on the auxiliary branch, the lightning arrester on the auxiliary branch is activated, and the fault current is transferred to the normal bridge arm through the lightning arrester.

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