Analysis and countermeasure of the large-area differential protection mal-operation of converter transformer caused by sympathetic inrush
Converter transformer is the core component of the DC converter. With the construction of HVDC in China, the capacity of the converter transformer is increasing, and its operating characteristics are particularly noteworthy. The traditional converter transformer large-area differential protection protecting the converter transformer and the lead wire, and the second harmonic of differential current is used to recognize the inrush current during unloaded switching. But it doesn’t consider that the two sets of parallel operation converter transformers may emerge sympathetic inrush during unloaded switching. When two sets of parallel operation converter transformer are switched on no-load, the magnetizing inrush emerged in the differential current of the large-area differential protection is the sum of the magnetizing inrush of Y-? and Y-Y transformer, which is discrete and random, when remanence of two kinds of transformer are different, the sympathetic inrush may emerge. The conventional second harmonic restraint used in the large-area differential protection is insufficient in distinguishing the sympathetic inrush, which may lead to mal-operation. Research shows that when converter transformer is switched on no-load, it will take some time before iron core saturation due to the transformer magnetic flux accumulation process, thus the false differential current caused by magnetizing inrush emerges later than line current, generally 3ms~5ms. As long as the differential current is caused by the fault, the differential time between the sudden change moment of the differential current and the line current will be very short, theoretically, they emerge at the same time. On the basis of this, a method to distinguish internal fault and sympathetic inrush is proposed. The generation mechanism of sympathetic inrush of converter transformer is studied on simulation model established in PSCAD/EMTDC, which is based on the parameter of Chu-xiong converter transformer of Yun-guang HVDC project. Simulation results show that the proposed method can distinguish sympathetic inrush from internal fault more rapidly and accurately, and avoid the mal-operation of the large-area differential protection caused by the sympathetic inrush when converter transformer is switched on.
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