Studies On The Influence Of Commutation Failure Of HVDC On The Differential-type Protection Of Converter Transformer And Its Countermeasures

As a common fault in HVDC transmission systems,the commutation failure may endanger system stability in severe cases.In special cases,commutation failure has an impact on the protection of main equipment in the converter station,but the influence mechanism is not completely clear.Based on the study of the factors affecting commutation failure and its current characteristics,,the influence mechanism of commutation failure on the differential protection and zero-sequence differential protection for converter transformers under special conditions is analyzed in this paper.According to the electrical characteristics of the protection under abnormal operations,the corresponding strategies are given to improve the reliability of the main protection and ensure the stability of the system.In this paper,the current change characteristics after commutation failure and its influence on the differential protection of converter transformers are studied.Firstly,the influencing factors of commutation failure and the scenario of continuous commutation failure are analyzed.Then,the HVDC transmission system model is established according to the actual parameters.The characteristics of the differential current waveform when the commutation failure occurs during the fault recovery,the influence mechanism of the commutation failure on the differential protection of the converter transformers and the reason of the abnormal operation of the protection are analyzed respectively.The discrete Frechet distance algorithm is used to identify the difference of current characteristics in different fault situations and abnormal operation scenarios caused by commutation failure which can improve the reliability of differential protection for converter transformers.The characteristics of zero-sequence current on the neutral line that grounding current caused by commutation failure and coupled by ground network interacts with large DC intruded into the neutral line of converter transformers is analyzed in this paper,as well as its influence on transmission characteristics of zero-sequence current transformer(CT).The mal-operation risks of the zero-sequence differential protection is also researched under this condition.The ZOOM-FFT is used to subdivide the amplitude frequency characteristics of the zero-sequence differential current,and the ratio of low frequency to frequency band amplitude is applied as auxiliary criterion to prevent mal-operation of zero-sequence difference protection.