Study On Transfer Characteristics Of Current Transformers With DC Bias

The current transformer is a special transformer used to transform the current. It transmits information to the measuring instruments, instrument, or relay control device by changing the current. They are the key factors to keep power system security and stability. Current transformer running correctly, reliably is very important to the power system. However, during the power grid operation, the current transformer error usually appears. It affects the safe and stable operation of the grid and fairness of the electricity metering. The DC bias phenomenon is one of the reasons causing the changes.When the normal flux of the current transformer exist DC flux, people call the DC bias phenomenon. Then transfer characteristics of the current transformer of the DC bias phenomenon will change. Because of DC magnetic flux, the current transformer excitation characteristics become asymmetric. Excitation impedance is changed accordingly. Current accuracy measured by the current transformer secondary can not meet the standard requirements. The current transformer appears DC bias phenomena for a variety of reasons. For example,1) natural causes:as a result of the geomagnetic field changes caused by geomagnetic storms, there is a Quasi-DC (0.0001Hz～0.1Hz) under earth;2) Transmission and distribution projects:Unbalanced operation of HVDC, AC power supply system of the tram, etc., flow into the ground in DC. DC bias phenomenon has attracted the attention of domestic and foreign. And now, people have been done a lot of research and quantitative analysis of the power transformer DC bias mechanism and Achieved important results. But study on transfer characteristics of current transformers with DC bias is relatively small. This study is still in its early stages.The purpose of this paper is to study the transfer characteristics changes on the measurement and protection current transformer with DC bias. Through simulation analyzing the model, the transfer characteristics law of the metering and5P20-level protective current transformers with DC bias is analyzed, and the transfer characteristics law of the TPY-level protective current transformers under the double impact of the remnant magnetization and DC magnetic bias on the protection are researched. Through testing, the transfer characteristics of current transformer in operating conditions with DC bias are studied.Four parts and seven chapters are involved in this paper. The first part (chapter1and2):This part introduces the definition and causation of the DC bias phenomenon, and the research achievements in the world wide. The current transformer DC bias mechanism has been discussed. Current transformer excitation characteristics tests and fitting methods have been discussed. The excitation characteristic curves have been fitted.The second part (Chapter3and4):The current transformer theory, and error causes and influencing factors have been discussed. Current transformer calculation model has been established. Then the steady-state measurement error of metering current transformer has been calculated with DC bias. Test program of metering current transformer has been discussed with DC bias. The test has been analyzed. The measurement error of metering current transformer becomes larger with DC bias.The third part (chapters5and6):The measurement error of protective current transformer has been calculated with DC bias. Test program of5P20-level and TPY-level protective current transformers have been discussed with DC bias. The test has been analyzed. The error of5P20-level and TPY-level protective current transformers increases with DC bias.The third part (chapters7):The main achievements of the dissertation have been concluded. The prospect of the current transformer with DC bias has been forecasted.