Experiment Study On Control And Protection Of Hybrid Back-to-Back DC Transmission System

High voltage direct current technology will play an increasingly important role in China's west-to-east power transmission and national interconnection.Using back-to-back HVDC system to connect the asynchronous AC power network will not affect the short-circuit current of AC system,cut off the transmission of AC fault in structure,resist the mutation and oscillation of current and power,and enhance the power grid anti-disturbance and anti-accident ability.It is necessary to be equipped with a large number of reactive power compensation equipment if using multiple LCC-HVDC to comply asynchronous interconnection.When near-zone AC failure occurs,multiple HVDC commutation failure is prone to occur.The VSC-HVDC developed in recent years adopts full control type device and its valve can self-turn off,thus there is no commutation failure problem,and it can realize decoupling control of active power and reactive power,without large capacity reactive power compensation equipment.By using LCC-HVDC and VSC-HVDC hybrid back-to-back DC transmission system to achieve asynchronous interconnection,the demand of reactive power compensation equipment can be reduced,the capacity of reactive power group can be increased equivalently,and the stability of DC system can be provided.This paper describes the control and protection overall program of hybrid back-to-back DC transmission system,including the hierarchical design of control system and the complete duplex design of protection system;The typical control logic and DC protection schemes of LCC-HVDC and VSC-HVDC units in the hybrid back-to-back HVDC system are analyzed and the control strategy of the coordinated operation of the two units is illustrated;A real-time simulation test platform with control and protection hardware in-the-loop is built,and a series of control protection tests are carried out.The response characteristics of the control protection system are analyzed,and the rationality of control protection design is verified;A reactive power control strategy for hybrid back-to-back HVDC transmission system is proposed.By adjusting the output of the VSC-HVDC at the filter switching instant,the reactive power impact and voltage disturbance caused by the filter switching to the weak AC system is restrained.The stability and economy of hybrid back-to-back HVDC is enhanced;The modes of commutation failure are divided into single-bridge commutation failure,double-bridge discontinuous commutation failure and double-bridge continuous commutation failure.The mechanism of commutation failure of LCC-HVDC is analyzed.The influence of AC fault level and fault triggering time on the commutation failure type is studied.The causes of abnormal commutation failure of LCC-HVDC at a slight fault level are explained.The simplex algorithm is used to optimize the control parameters of VSC-HVDC in a hybrid HVDC transmission system to inhibit this kind of abnormal commutation failure occurred.