Research On Busbar Sampling Value Differential Protection

Sampling value differential protection has fast speed of action,excellent anti disturbance performance and fault tolerance ability.So it has been gradually developing into the mainstream principle and standard configuration of digital bus differential protection.However,due to the occurence of CT transient saturation during busbar external fault,the sampling value differential protection needs to cooperate with the anti-CT saturation identification criterion.The saturation mechanism of CT is introduced in this paper,and the characteristics of steady state saturation and transient saturation of CT are analyzed.The existing methods of anti-CT saturation in busbar differential protection at home and abroad are summarized,and their advantages and disadvantages are compared.This paper expounds the basic principle of sampling value differential protection,analyzes the selection of the data window,the reasonable selection of R and S,and the setting of braking coefficient.The size of fuzzy area is discussed.On the basis of the above analysis,an improved bus sampling value differential protection criterion is proposed.The criterion includes the abrupt starting component,CT saturation first cycle criterion,sampling value differential protection criterion based on CT linear transmission region.The criterion takes full advantage of the waveform characteristics during CT saturation.The change rate of braking current is used to detect the the starting point of CT linear transmission region,the proportional relationship between the differential current and the braking current is used to detect the end point of CT linear transmission region.This criterion is combined with the new type of differential criterion of braking force to form a complete differential protection.A simulation model is built in PSCAD and a variety of busbar intemal and extemal faults are simulated.The algorithm is written in MATLAB to verify the performance of the differential protection criterion under various fault types.The influence of the fault initial phase difference,the fault conversion time,frequency offset and sampling frequency on the criterion is explored.The simulation results show that the new criterion can effectively prevent misoperation when CT transient saturation occurs during busbar external fault,and it will quickly issue a trip signal after the external fault has transferred to busbar internal fault.