Optimization Method Of Comprehensive Management For DC Bias In Transformers

Affected by Geomagnetically Induced Currents(GIC)or HVDC,High-voltage,high-capacity and long-span power grid are more susceptible to DC bias.In the past,studies of restraining measures against transformer DC biasing mostly focus on installation of capacitor blocking device or resistance current limiting device,and in this paper,a comprehensive control and optimization scheme is dealt with,and optimal locating of capacitors and resistors is put forward;furthermore,effects of slight changes of power grid structure on the optimization scheme are studied,with solutions proposed.Main contents of the study are as follows.(1)In the past,transformer neutral direct current was used as the basis of DC bias of transformer,which could not well reflect the risk of DC bias of auto-transformer.With series winding current of auto-transformer also considered,effective transformer biasing current is introduced to assessment basis of transformer DC bias.Thus,matrix formula of effective single-phase magnetic biasing current is inferred at length and in comprehensive manner for transformer substations across the power grid.(2)The use of series capacitor has the advantage of thoroughly blocking DC current flow.Such a series resistance method is simple,practical and economically efficient.On the basis of analyzing the bias restraining devices installed in the past,the paper examines the issue of combining capacitor blocking device and resistance current limiting device as a comprehensive solution to transformer DC biasing;hence,the paper comes with the solution formula to maximum effective biasing current over the whole power grid when those two types of bias restraining devices are installed.(3)The paper builds equivalent models for power grid in cases of magnetic storm and HVDC separately;the paper proposes optimization method of capacitors and resistors for restraining of transformer DC biasing,on the principle of transformer effective single-phase biasing current not exceeding prescribed limits,and minimized number of the capacitor blocking device and resistance current limiting device.Based on genetic algorithm,the optimal configuration scheme can be found(4)In view of slight changes of power grid structure,the paper analyzes the stability of the comprehensive optimization scheme,and firstly proposes the strategy of introducing new devices,thus successfully solving the issue of maximum effective bias current exceeding limit values in the power grid,which provides reference for coordinating the bias restraining devices.