Research On Control Strategy Of Multi-termimal HVDC-Flexible System

HVDC-Flexible can control active and reactive power independently,which is suitable for renewable energy grid connection,island power supply,asynchronous AC power grid interconnection and other fields.As the world’s first multi-terminal HVDC-flexible transmission project,the Nan’ao project was put into operation at the end of 2013.This paper studies the operation reliability index of Nan’ao HVDC-flexible transmission project after three years of operation.In view of the typical system outage problems,puts forward optimization suggestions of control strategy.At the same time,aiming at the main problems in the control strategy,the influence of DC system on AC protection and the control method after installing DC circuit breaker are analyzed through RT-LAB simulation.The main work of this paper is as follows:(1)This paper studies the operation reliability index and typical system outage cases of Nan’ao HVDC-flexible transmission project three years after its operation,and puts forward specific measures for optimizing the system control strategy.Aiming at the problem of converter bridge arm over current and DC system outage after unlocking of Qing’ao converter station,propose that the initial parameters of the output results of circulation suppression should be set in a reasonable range.Aiming at the problem that the system operation mode switch is not successful,propose that the state detection device connectes to the tripping relay and closing relay of the switch at the same time.Aiming at the problem of system blocking and shutdown caused by a packet of wrong data,propose to increase the logic delay of blocking converter when the two sets of DC protection exit.(2)This paper studies the influence of DC system access on 110 k V AC system protection.Through the analysis of five typical working conditions of DC non operation,small load operation,large load operation,Jin’niu converter station power reverse transmission and STATCOM operation,simulate different types of faults of AC lines and the main conclusions are drawn: After the DC system is put into operation,the existing setting principles of optical fiber current differential protection,grounding distance protection,phase to phase distance protection and zero sequence current protection will not be affected;while for PT broken line over-current protection,DC system can be regarded as power supply during fault period,and the time setting value of PT broken line over-current protection should be set according to the principle that the farther away from the converter station,the smaller the time setting value.(3)This paper studies the change of control strategy of Nan’ao HVDC-flexible transmission system after installing DC circuit breaker,proposes the control mode of on-line input and on-line exit,and puts forward that blocking the converter before the on-line input and on-line exit of Qing’ao converter station can effectively protect theequipment,and is verified by simulation.