Research On Zero Mode Inrush-current Characteristics Of High-impedance Transformer And Countermeasures Of Its Associated Protection

With the rapid development of economy and the rapid progress of society,China’s power demand and grid scale are expanding,but it brings problems such as system impedance reduction,short-circuit current exceeding the standard seriously.The high impedance transformer put into operation widely at present can increase the short-circuit impedance to reduce the short-circuit current without increasing the primary equipment.However,in recent years,there have been many reports of misoperation of zero sequence overcurrent protection caused by inrush current of high impedance transformer in China’s power system,which makes the reliability and application of high impedance transformer suspect,and poses a serious challenge to the power supply reliability of the power grid,and also poses a huge threat to the safe and stable operation of the power system.Therefore,it is urgent to study the zero mode inrush current characteristics of high impedance transformer and the Countermeasures of its associated protection.At present,many high impedance transformers used in the field mainly include high-voltage built-in type and low-voltage series reactance type.The difference between them lies in the different winding structure.The mechanism of zero mode inrush of high impedance transformer and the root cause of misoperation of zero sequence overcurrent protection are lack of in-depth study.The research of inrush current has not considered the difference of winding structure and leakage inductance parameters.Because the traditional T-type equivalent circuit of three winding transformer ignores the excitation branch and decoupling transformation,it has application limitations in the analysis of inrush current,so it can not directly use the short circuit test data to obtain the equivalent circuit parameters.Therefore,it is necessary to propose a new method to obtain the parameters.At the same time,it is necessary to study the zero mode inrush current characteristic which reflects the parameter difference of high impedance transformer and its influence on its associated protection,and put forward the corresponding countermeasures.In this paper,from the perspectives of mathematical analysis,modeling simulation,dynamic model test and on-site wave recording,the system research work is carried out around the zero mode inrush current characteristics of high impedance transformer and its impact on the associated zero sequence overcurrent protection,and then practical and effective countermeasures are proposed.It is of great practical significance to ensure the safe and stable operation of power system and improve the overall safety protection level of power grid.In order to solve the problem that the short circuit test data can not be directly used to calculate the parameters in the process of inrush current analysis,the proportional relationship between the original square zero mode current and the triangular winding circulating current is obtained through the derivation of the transformer loop equation,and a method for obtaining the leakage inductance parameters based on the proportional relationship between the zero mode current and the circulating current is proposed.This method takes into account the problem that the circulating current of the triangle winding of 220 k V transformer can not be measured.Using the characteristics of the balance between the circulating current of the triangle winding and the original unsaturated phase current,the original unsaturated phase current is replaced by the circulating current,and the leakage inductance is calculated in the unsaturated time interval.The correctness of the method is verified by modeling simulation and field recording data.According to the analysis of zero mode inrush current characteristics of high impedance transformer,the expression of zero mode inrush current of transformer is derived.Combined with the difference of parameters of high impedance transformer,the difference of zero mode inrush current characteristics of different kinds of transformer is analyzed.The characteristics of high voltage winding built-in high impedance transformer with larger zero mode inrush current amplitude,faster attenuation but longer attenuation time are obtained.The theoretical analysis is verified by simulation.Through the dynamic model test system with physical model of transformer,the dynamic model tests of the influence of remanence(opening angle),closing angle,system voltage and other factors on zero mode inrush current of different types of transformer are carried out,and the relevant theory and simulation conclusions are verified.The test data show that the second harmonic content of zero mode inrush current is high,which provides a reliable test basis and a feasible design idea for the follow-up study of related protection countermeasures.In view of the influence of the lack of zero mode inrush current of high impedance transformer on its associated zero sequence over-current protection and the countermeasure research,the influence of high-voltage built-in high impedance transformer parameters on the zero sequence over-current protection is further simulated and analyzed,and the improvement strategies are discussed from the aspects of optimizing the winding structure of transformer,optimizing the opening and closing control mode,improving the system protection principle,etc.An improved zero sequence over-current protection based on restraint algorithm of the second harmonic component in zero mode inrush is proposed,and the validity of the method is verified by simulation test and recording data.