Study Of Risk Analysis And Countermeasure Of Mal-Operation On A Family Of Zero-Sequence Overcurrent Protection Due To High-Voltage Built-in High-Impedance Transformer Energization

High-voltage built-in high-impedance transformers are effective in limiting the short circuit current so it is widely used in areas where the short circuit current exceeds the standard.However,amplitude of inrush generated by this kind of transformer is more likely higher compared with conventional transformers.This increases the risk of mal-operation of relay protection.In particular,the mal-operation of the zero-sequence overcurrent protection for busbar and transmission line often occurred in recent years.To solve this problem,this paper evaluates the risk of mal-operation on a family of zero-sequence overcurrent protection caused by high-voltage built-in transformer no-load closing,and explores reasonable strategies for dealing with mal-operation of protection based on the characteristics of zero-sequence inrush current.Firstly,the generation mechanism of zero-sequence inrush is analyzed,and dual decay time constant is used to describe the evolution process of inrush.Then a model is proposed to evaluate the mal-operation risk of mal-operation on a family of zero-sequence overcurrent protection caused by high-voltage built-in transformer.On the basis of the model,a routine is written,which can generate large amounts of inrush data and process them in batch automatically.Inrush database cover all kinds of energizing situations.Next,massive inrush database for high-voltage built-in transformer and normal transformer are generated and processed base on above procedure.The amplitude of inrush current is compared with threshold.Then the risk of protection mal-operation is analyzed.In addition,based on the above mass inrush data and taking phase A remanence and closing angle as independent variables,the characteristics of zero-sequence inrush caused by high-voltage built-in transformer,include dead angle and second harmonic component,are plotted for studying,which provides reference for designing zero-sequence inrush current identification criteria.Secondly,in order to deal with the mal-operation of line zero-sequence overcurrent section III protection caused by zero-sequence inrush,two-dimensional phase space reconstruction of zero-sequence current was carried out based on the basic principle of phase space.The morphological differences of phase space locus for zero-sequence current under different conditions are analyzed.According to the feature,the criterion of "cross region" is designed.The number of sampling points of phase space locus in the region is calculated,and compared with the threshold to distinguish inrush from fault.Then a zero-sequence inrush identification scheme is proposed,the applicability of the proposed scheme is verified through simulation analysis under various scenarios such as inrush and fault.Finally,zero-sequence inrush blocking criterion based on phase space locus may be affected by the decaying DC component,causing time delay.To deal with the problem,another criterion is proposed.This scheme amends the secondary harmonic brake ratio and use it to recognize inrush for zero-sequence over-current protection.In order to improve the anti-CT saturation performance,an auxiliary criterion based on second harmonic component trends is added to the original blocking criterion.Finally,the effectiveness of the scheme is verified based on PSCAD/EMTDC simulation platform.