UHVDC Transmission DC Magnetic Bias Impact Of Transformer Protection Study

Ultra-high voltage AC/DC transmission system working in the monopole ground circuit operation mode or bipolar unbalanced operation, after the line-fault resection using the earth return operation, then there will be appear thousands of Ampere`s DC current into the earth,Part of the DC current through the neutral grounding of power transformer then formed the loop in the alternating current system. Then the situation will cause DC magnetic bias of transformer, result in transformer loss increase, inner metal component temperature increased, the noise and vibration increased, distortion of current and voltage waveform, protection misoperation etc. Seriously affect the safe and stable operation of the power grid. At present,power transformer DC bias research mainly concentrated in the field current and harmonic analysis when the transformer no-load running, hardly any research about the DC magnetic bias influence of transformer protection and how to control or eliminate and reason of DC magnetic bias, for this in this paper use the 750 kV autotransformer as a research object, Using PSCAD(power system simulation software),750/330/60 kV autotransformer established in bilateral power supply in the extra-high voltage power transmission system simulation model. In the simulation, not considered the DC current distribution in the power grid.directly applying the DC potential simulate in the transformer neutral point to the DC magnetic bias, focuses on the DC magnetic bias of autotransformer at ground fault impact on protection that based on the principle of zero-sequence current phase comparision. Simulation results show that the autotransformer based on zero sequence current phase comparision.in the cases of different levels of DC magnetic bias in transformer did not appear the misjudgment in the protection criterion, so this Protection method is not affected by the influence of transformer DC magnetic bias. At the same time the simulation verified the neutral series resistor and capacitor can effectively reduce or inhibit effect of DC magnetic bias.