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Mechanism Analysis and Simulation of DC Current Distribution Along Transformer Neutral Point Under the Condition of UHVDC/HVDC Single-pole Operation

Theoretical fundamentals of mitigating DC bias current for smooth system operation are seldom researched. We have validated and verified the relationship of system DC bias current of power transformer and system operation mode. The simulation model based on PSCAD/EMTDC contains two 500 k V DC transmission lines and 220 k V transformers adjusted to grounding pole and is built up by using the method of AC system network modeling. By DC bias current distribution simulations, it is found that the developed simulation model has an error margin of less than 10% and has the ability to recur the actual operational scenarios. Comparison is performed on DC current distribution scenarios among the neutral points of specific transformers under the conditions of single grounding pole and dual grounding pole. It is revealed that the potential difference decides the mechanism of various distribution characteristics of DC current through transformer neutral points, which provides the theoretical fundamentals for mitigating the DC bias current for the smooth system operations.