Study On Saturation Of CT And The Effect On Relay Protection

The Current Transformers(CT) as a current signal components use the principle of electromagnetic induction by primary current proportional to the secondary current. When the excitation current saturation magnetic flux increases,the ratio of current bilateral relations could be changed,the secondary output waveform of current distortions.As China's power system capacity increased continuously,when a fault happens the feeder of the 220 kV to 500 kV substation low voltage side(10kV～66kV), the CT installed may be saturation which may arise from the protection relay to resist acting,and the main transformer backup protection can be override trip.This will extend the scope of shutdown of power supply.How to effectively combat relay malfunction due to CT saturation has not been very good solution.In this thesis,the characteristics of the current transformer(CT) and the effect on relay protection are analyzed.The main works are as follows:①The transient characteristics and saturation processing are detailedly analyzed when the primary current is very large.The simulation model including short-circuit current source and CT models are set up following the require of ATP-EMTE②The results show that the secondary side currents have serious and abnormal changes when the primary side current is much out of designed scope.In accordance with the simulation result,comparing CT's behavior characteristics in different conditions,the research proclaims that short-circuit current,aperiodic component, primary side time and secondary side magnitude and property as well as slew remanence and so on will affect CT's saturation.③Be contraposed with the CT's secondary current which the relay protection collected,the algorithm of the discrete fourier transform is used and all sorts of the numerical value are calculated.④A protection malfunction incident is discussed.According to the saturation characteristics of the current transformer,some prevention measures are proposed.