Abstract
Through the analysis of the recovery inrush current generated by the external fault removal of the converter transformer, it is pointed out that the zero-sequence current caused by the recovery inrush may result in the saturation of the neutral current transformer (CT), whose measurement distortion contributes to the mis-operation of zero-sequence differential current protection. In this paper, a new scheme of zero-sequence differential current protection based on waveform correlation is proposed. By analyzing the characteristics of zero-sequence current under internal fault, external fault and external fault removal, the waveform correlation of the zero-sequence current measured at the terminal of the transformer and the zero-sequence current measured at the neutral point of the transformer is used for identification. The polarity of the CT is selected to guarantee the zero-sequence currents at the terminal and neutral point of the transformer exhibit a "ride through" characteristic under external fault, then the waveform similarity is high, and the correlation coefficient is positive. On the other hand, when internal fault occurs, zero-sequence current waveforms on both sides differ from each other largely, and the correlation coefficient is negative. Through a large number of simulations verified by PSCAD/EMTDC, this criterion can accurately identify internal and external faults, exempt from effects of the recovery inrush. Moreover, it presents certain ability for CT anti-saturation.
Highlights
In the ultra/extra high voltage direct current transmission system (UHVDC/EHVDC), it is essential to guarantee the safe and stable operation of converter transformers, which play the important role of connecting alternating and direct current systems
The new criterion can effectively solve the problem of the false differential current caused by external fault and its removal accompanied by current transformer (CT) saturation on one side, can operate reliably in case of internal fault
The results show present zero-sequence differential current in protection has a of certain of 2.mis-operation, whilethat the differential current protection strategy based on waveform correlation analysis proposed in this paper present zero-sequence differential current protection has a certain risk of mis-operation, while the zero-sequence differential current protection strategy based on waveform correlation analysis has a strong
Summary
In the ultra/extra high voltage direct current transmission system (UHVDC/EHVDC), it is essential to guarantee the safe and stable operation of converter transformers, which play the important role of connecting alternating and direct current systems. Analyzed and pointed out that the removal of the external faults of the converter transformer may bring about the transient saturation of the CT caused by recovery inrush, which brings the risk of mis-operation to the transformer zero-sequence differential current protection, affecting the safe operation of the DC transmission project. It is necessary to analyze the influence of recovery inrush current caused by fault removal on zero-sequence differential current protection, and put forward appropriate schemes to improve the reliability of zero-sequence differential current protection and avoid the occurrence of mis-operation. The new criterion can effectively solve the problem of the false differential current caused by external fault and its removal accompanied by CT saturation on one side, can operate reliably in case of internal fault It is not affected by the recovery inrush current, and has certain ability of CT anti-saturation.
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