The Influence Of DC Bias On Current Transformers And Power Metering

The HVDC transmission technology has been developed rapidly in manyregions of China. When HVDC project operates in monopolar mode or violentgeomagnetically induced currents (GICs) appear, huge direct current will flowthrough the ground. Severe DC bias effects will be brought to the pivotalelectromagnetic devices of power systems such as current transformers (CT). And itdirectly impacts the accuracy of measurement and power metering, which influencethe economic benefit of power consumers and fairness of market. This paperpresents the comprehensive research on the problems induced by DC bias of CT.This paper qualitatively analysis the transfer characteristics of CTs uses the J-Atheory and simulation of hysteresis loop. Based on a full-wave Fourier algorithm, asuitable approximate solution of electromagnetic equation provides expressions forthe ratio error and phase displacement. Simulation and experiment study were givento verify the above analysis results, and the computed results agree well with thetested ones. The ability to sustain DC bias and3D error curves are quantitativelyobtained to show that the ratio error and phase displacement increase with the DCbias. Using the wavelet transform and fast Fourier transform combination ofcomprehensive testing program was proposed to detect DC bias. Measuring DCcomponent in the first winding current based on the flux-gate principle andconclusion of second harmonic excitation current is the sign of DC bias exists.Finally, based on the simulation model which is established by Real/Reactive PowerMeter to prove that DC bias makes electric energy metering small.