Analysis Of Operation Performance Of Zero-sequence Differential Protection For Converter Transformer And Its Countermeasures Under Complex Electromagnetic Transients

In the complex electromagnetic transient situations,the operation performance of zero-sequence differential protection for converter transformers is affected,and there are risks of mal-operation of zero-sequence differential protection in serious cases.However,at present,the research on the mal-operation of the zero-sequence differential protection is only the field accident report and the relatively simple analysis.The influence mechanism of the disturbance on protection performance under the complex electromagnetic transients is not completely clear,and there is no targeted solution.Starting from the case of mal-operation of zero sequence differential protection for converter transformers in HVDC project,this paper studies the mal-operation mechanism of zero-sequence differential protection in HVDC project under the inrush current case caused by the energization of unloaded,the recovery inrush case caused by external fault removal and the case caused by long-term unipolar-earth operation accompanied with weak fault of AC systems.The change characteristics and rules of the electric quantity of zero-sequence differential protection under the complex electromagnetic transients are excavated.Based on the above problems,a specific solution is proposed to improve the reliability of zero-sequence differential protection for converter transformers.The phenomenon that zero-sequence currents are provided with quite high magnitude and decays slowly during inrush current caused by the energization of unloaded and the recovery inrush caused by external fault removal for converter transformers are studied.The reason of the mal-operation of the zero-sequence differential protection is revealed from two aspects of theoretical analysis and simulation analysis.By identifying differences in similarity characteristics between zero-sequence current on the neutral line and the self-produced zero-sequence current under inrush and internal faults is identified,a novel criterion of the zero-sequence differential protection,which is based on dynamic time warping distance of zero-sequence currents,is proposed.The validity of the criterion is verified by the simulation model of HVDC transmission based on the actual engineering parameters.Through theoretical calculation and derivation,the saturation mechanism of the current transformer(CT)on the neutral line for converter transformers under the long-term unipolar-earth operation accompanied with weak fault of the AC systems is studied,and risks of mal-operation of the zero-sequence differential protection for converter transformers under the complex DC bias is analyzed.According to the polarity difference between the self-produced zero-sequence current and the zero-sequence current on the neutral line on both sides of the protection under internal and external faults,a novel blocking criterion of zero-sequence differential protection for converter transformers based on S-transform phase angle difference is proposed.The validity of the criterion is verified by the simulation model.