Research On Modeling Of Current Transformer And Effect Of Its Saturation On Line Protection

With the continuous expansion of the scale of power grids, the short circuit current of system increases year by year. At present, most of the protective current transformers are designed using the P type electromagnetic current transformer in the 110 k V and below power system built. Due to the nonlinear of electromagnetic transformer iron core, when the short circuit current is greater than even than the rated accuracy limit primary current of the current transformer, the current transformer saturation will happen and resulting in larger composite error. When the distortion secondary current is bubbled into the relay protection device, it will cause protection disoperation, which will be a serious threat to the power supply reliability.There are quite a number of such current transformers which don’t meet the anti-saturation requirement in the 110 k V and below power grid built, if replacing all it will certainly be a huge cost, and not it may be a risk of the security and stability of power grid operation. So it is necessary to further study the action of relay protection when the current transformer is saturated, and analyzing what conditions for replacing the current transformer is necessary in engineering actually, if not change what impact will be on the line protection for theoretical guidance.To achieve the research objectives, this paper establishes the simulation model of current transformer on the base of J-A electromagnetic theory. Because this model can simulate the hysteresis characteristics of the current transformer core, it has higher accuracy than the model based on the basic excitation curve. In the theory of J-A hysteresis, the determination of five parameters is a difficulty, this paper is successful to identificate the five parameters using the genetic algorithm. This article study the effect of current transformer saturation on current protection by experiment and simulation comprehensively, which shows that as long as the accuracy limit current of the current transformer is greater than the largest setting values of current protection, which can correct operation; When the short-circuit current increase further, even if the current transformer is saturation,the second current is still keep increasing trend, which still can guarantee the protection device reliable operation, so the biggest set values of protection can determine the accuracy limit factor of current transformer. By the current transformer and transmission line distance protection combined simulation model built in this paper, simulate the action of the protection for real-time when current transformer is saturation, and analyze the effects of current transformer saturation on the distance protection of transmission line. For the distance protection of transmission line, current transformer slightly saturated will cause the relay action of jitter, and seriously lead to miss operation.