Research On Protective Schemes And Power Regulation Characteristics Of Thyristor Controlled Transformer-type Controllable Shunt Reactor

Controllable shunt reactor is one of the key elements used in power system for reactive power compensation.Since the stable operation of power system depends largely on the safe operation of controllable shunt reactor,thorough and reliable protection schemes is highly required.With some incomparable advantages,the thyristor controlled transformer-type controllable shunt reactor(TCT-CSR)is the direction of development of shunt reactors.The protection for TCT-CSR,which has rarely been studied,is much more complex because of its complex configuration.Research on protective schemes and transient characteristics during power regulation of TCT-CSR is significant to guarantee safe operation of both reactor and transmission systems.Firstly,this paper introduces the configuration and operating principles of TCT-CSR and chooses the 6-pulse triggered TCT-CSR with control windings in Y-connection and valve branches in Y-connection form as the foundation for further study,because it has the advantages of relatively flat control curve,maximum adjustment range and best harmonic characteristics.Based on the configuration and parameters of 35 k V TCT-CSR,a digital simulation model is built in MATLAB/SIMULINK and simulations of all possible faults within TCT-CSR are carried out.Combined with fault analysis,the overall protective schemes for the TCT-CSR is designed,with emphasis on the protection schemes exploration for turn-to-turn fault of control windings,impacts of large range of working current variation on sensitivities of its protection schemes,and impacts of power regulation transient on its protection schemes.For turn-to-turn fault of control windings,zero-sequence overcurrent protection with zero-sequence voltage blocked is proposed.For impacts of large range of working current variation on sensitivities of its protection schemes,stepped setting strategy is employed in the selection of setting values.For impacts of power regulation transient on its protection schemes,the approach of increasing the resistance of control windings is proposed.It's indicated that traditional protection schemes may not be reliable or sensible to the turn-to-turn fault on control windings during to the special structure of TCT-CSR.Instead,zero-sequence overcurrent protection scheme with zero-sequence voltage blocked can improve identifiable range of short-circuit turns ratio in the case of turn-to-turn faults on control windings.In addition,operating conditions of thyristors are asymmetry during power regulation transient,which will lead to mal-operation of zero-sequence overcurrent protection.The approach of increasing the resistance of control windings proposed in this paper can effectively restrain impacts of power regulation on protection schemes.