Key Technology Research On Operation Risk Control For Relay Protection During The Start-up Of Transformers

The reliable operation of relay protection is a prerequisite for the safe and stable operation of large power grids.With the continuous development of the power system and the upgrading of protection device technology,more and more power transmission and transformation equipment have been started and put into operation.The insufficiency of risk control technology during operation of corresponding relay protection is becoming more and more prominent.As a core component of the grid architecture,the operation risk control of relay protection during the transformer startup process is particularly important.In the actual operation of the power system,there have been many instances of temporary relay protection mis-action that led to the transformer cannot be put into operation in time.In severe cases,it even affected the safe and stable operation of the upper level grid.The two key technical problems to be solved urgently are the difficulty of no-load polarity verification of transformer protection and the temporary protection mis-action caused by inrush current.In order to solve the above problems,this paper analyzed the relationship between the currents on each side of the transformer during no-load closing,and obtains the relative polarity relationship between the self-generated zero-mode inrush current on the high-voltage side,the neutral current and the circulating current of the low-voltage side.It laid a theoretical foundation for the subsequent research on the no-load polarity verification method based on the inrush current.The corresponding zero-mode inrush current expression was derived,and the zero-mode inrush current characteristics were analyzed by numerical calculation.The results show that the second harmonic ratio of zero-mode inrush current is generally high,and the waveform near the characteristic opening and closing angles has obvious non-sinusoidal characteristics.The analysis results are verified by simulation,which is laid a theoretical foundation for the subsequent improvement of the temporary zero-sequence overcurrent protection.Secondly,to solve the problem of no-load polarity verification when the transformer is put into operation,a transformer protection polarity verification method based on inrush current is proposed.The first is the polarity verification of the Y-side of the transformer.The self-generated zero-mode inrush current i_0p on the Y-side and the neutral point zero-mode inrush current i_0n are essentially the same current,and the polarity relationship between these two currents determines the polarity of the transformer zero-sequence differential protection.Therefore,this article uses the"hypothesis-test-correction"method to verify the polarity by comparing the similarity of the two waveforms.Then there is the polarity verification of the low-voltage side and the neutral point.Since the low-voltage side circulating current i _D and the neutral point current i_0n have the same polarity,it can be verified by using the same similarity algorithm.Finally,the neutral point current i_0n is used as a benchmark to complete the verification of the overall polarity of the transformer protection.Then,in order to solve the problem of imperfect temporary protection scheme,based on the no-load polarity check strategy,this paper proposes an improved temporary protection scheme,which set the differential protection with polarity checked as the temporary main protection,and the zero-sequence overcurrent protection as the backup protection.Secondly,the influence of inrush current on the improved temporary protection scheme is analyzed.The results show that when the polarity is correct,the differential protection usually does not malfunction,but the zero-sequence overcurrent protection is most likely to malfunction.In order to solve this problem,this paper proposes an improved method of zero-sequence overcurrent protection based on waveform sine correlation and second harmonic restraint.Simulation analysis was carried out under various conditions,and the results verified the effectiveness of the improved method.In summary,for the key technology of operation risk control for transformer protection,this paper has carried out systematic research from multiple aspects,and put forward effective countermeasures,which are of great significance to the safe and stable operation of the power system.