Study On The Strategies Of Valve Group Block And Deblock In Uhvdc Power Transmission Systems

The biggest difference between DC±800kV UHVDC(ultra high voltage direct current) and traditional±500kV DC is the former need bipolar 12-pulse valve to form a series circuit, and has more complex and diverse operation modes. This requires UHVDC projects must provide a far more complex routine, and the function of a more comprehensive control strategy than conventional±500kV DC,to ensure AC-DC system should not cause excessive disturbance. Therefore,analysis and research on strategies of valve group block and deblock in UHVDC Power Transmission systems is important significant for power dispatch and system operation.In this paper, by means of real time digital simulator (RTDS), using Yunnan-Guangdong±800 kV UHVDC project as reference object the control characteristic for two situation are researched i.e.,the second valve group is deblocked in on-line way while the first valve group in a pole of UHVDC project is running or one of two valve groups is blocked in on-line way while two valve groups in a pole of UHVDC project are running. The strategies of valve group block and deblock in UHVDC power transmission systems are proposed. This study has also discussed possible improvement to the problems in the process.The research topics are listed as follows:The structural features of UHVDC transmission system is introduced, the strategies of valve group block and deblock in UHVDC power transmission systems are proposed, the control characteristic of valve group block and deblock in UHVDC power transmission systems are expounded and analyzed in detail.The operation of valve group block and deblock in UHVDC power transmission systems need to combined with the operating characteristics of valve bypass switches, and transfer the DC current between the valve and the high-speed bypass switch in parallel with the valve. This paper studies the logic of valve group block and deblock in UHVDC Power Transmission systems, analyzes the sequence of valve group block and deblock, and the sequential coordination of bypass switches, fire pulse, bypass pairs and so on control signal.Firing angle of two valve at rectifier has seriously unbalanced operation when deblock the second valve group, their root causes are expounded and analyzed in detail, and proposed regulation of voltage and current balancing means;the original valve at inverter may occur commutation failures when deblock the second valve group. The measures to prevent commutation failure are introduced, and commutation failure prevention strategies are proposed for deblocking the second valve group. Finally, the strategy of deblocking the second valve group after improved is proposed, the simulation results demonstrate that the strategies is feasible:Finally, the strategy of deblocking the second valve group is that:1)The DC system is power control mode;2)The fire angle value and limited time should be 70 degree and 10 ms;3)The deblocking is firstly carry out at rectifier side then at inverter side;4)The function of voltage and current balance must be put in at rectifier side after releasing fire angle limited;5)The function of lower voltage reference value must be put in at inverter side after sending out command of opening high speed bypass switch.Finally, the strategy of deblock the first valve group is that:The DC system is power control mode;The bypass pair is firstly implemented at rectifier side then at inverter side.