| High Voltage Direct Current (HVDC) is an important sector of power industry in China. Together with the rapid development of HVDC technology in our country several transmission lines such as Ning-dong HVDC have been built with frontline relevant technologies globally in recent years, HVDC system achieves power transportation of a long-distance and large-scale, improving scheduling capabilities and economic benefits, thus promoting the development of national economy. Power industry, as the fundamental sector in the national economy, has been providing necessary support for the development of other energy industries. Further research on related technology of HVDC is important for the healthy development of power industry and national economy.HVDC transmission system includes a rectifier valve, inverter valve, direct current transmission line, converter transformers, filters and control equipment. The rectifier and inverter valve are generally referred to converter valve. Like in AC transmission system, some problems concerning practical operation also exist in HVDC transmission system, among which an outstanding one is commutation failure of the converter valve. Commutation failure fault of converter valve is common among the faults of HVDC transmission system, mostly occurring in the inverter valve. Commutation failure refers to a process in which’should shut off’ valve fails to restore the forward blocking ability within a period of reverse voltage effect, but re-conduct under positive voltage again in the commutation process. It will cause drop of DC side voltage and increase of DC current, Without measures taken timely, it may trigger continuous commutation failure even power transmission interruption, affecting the sound operation of power grid.The induced mechanism of commutation failure is complex. Many factors such as AC bus voltage, inverter input current, advance firing angle, and AC system fault, transformer ratio and leakage reactance will contribute to the occurrence of commutation failure. Identification of commutation failure mainly depends on the minimum voltage drop method and arc angle method. The common preventive measures include application of reactive power compensation to maintain the commutation voltage stability, using larger smoothing reactor to limit transient DC current increase, enlarging advance trigger angle setting value, reducing the converter transformer leakage reactance during system planning, using appropriate trigger pulse control mode, etc. It is of great significance to probe into the commutation failure of HVDC system fault warning and optimized scheme of topology in reducing the occurrence of the fault and improving the stability of HVDC transmission system.Based on the analysis of various induced factors in commutation failure, the AC bus voltage and the inverter input DC current are determined as two main variables characterizing the commutation failure. Occurrence of commutation fault will be accompanied with voltage sag and current increase; two variables can be used in the judgement and early warning of occurrence of commutation fault. As these two variables affect each other, it’s inaccurate to judge the commutation failure just by one variable. Based on the theory analysis of a three-phase full wave rectifier circuit, the paper proposed the arc angle formula based on impedance parameters, and further determined impedance threshold of commutation failure, thus generating methods of commutation fault early warning based on impedance parameters, among which the arc angle method is the core. Compared with the original technology, the early warning method can forecast commutation failure faster and more accurately due to more convenient impedance measurement in engineering with two physical quantities covered, i.e. C bus voltage and the inverter input DC current,Base on a theory that commutation failure refers to to-be-shut-off valve fails to be shut off due to too short turn-off time resulting in to-be-open valve fails to to be conducted, speeding up the turn-off process of valve bridge thyristors can reduce the occurrence of commutation failure. The determinant influencing thyristors reverse turn-off time is proposed based on analyzing the internal structure and carriers motion process of thyristors in this paper. Under the condition of a certain thyristor structure, to speed up thyristor reverse turn off process is to accelerate the movement of the carrier, to put larger and more lasting reverse voltage on the thyristors. Application of a larger reverse voltage can enhance the intensity of electric field of PN junction space charge region of the thyristor, weaken the multi-diffusion and accelerate its minority drift motion. Therefore, the space charge region will faster spread, and the thyristor faster can restore the blocking ability of the forward voltage so as to more quickly complete the shutdown process, reducing the commutation failure risk.According to the physical mechanism of the thyristor turn-off process and carrier motion process, the HVDC converter topology optimization method is proposed for commutation failure. By antiparallel diode on the series thyristors of the converter valve bridge arm, collecting the reverse voltage to focus on one or a few thyristors in the bridge arm turn-off process, the topology optimization method speeds up the turn-off process of thyristors, so as to accelerate the bridge arm turn-off process, reducing the risk of commutation failure. This method, simple and remarkable, can achieve reducing commutation failure risk just by antiparallel several power diodes. To validate this topology optimization method, the model and simulation result of series thyristors turn-off process is presented by modifying the thyristor parameter in the Matlab/Simulink software in this paper. By comparing series thyristors voltage and current waveform in the turn-off process of no reverse parallel diode and reverse parallel diode, this paper validated the feasibility and effectiveness of this method. This topology optimization method is of important practical significance for HVDC transmission system.Based on the study at home and abroad, this paper puts forward the HVDC converter valve fault warning and topology optimization method for commutation failure, and has presented the formula derivation process and simulation results but without physics experiments due to limited experimental conditions. The method proposed in this paper can effectively reduce the risk of commutation failure, holding important significance for the sound and stable operation of HVDC transmission system. |
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