The characteristics of electromagnetic current transformers with DC bias

Both geomagnetically induced currents (GIC) and high voltage direct current (HVDC) projects using the earth as its path can cause direct current flowing in the ground. Direct current flux will be produced and has detrimental effects on power systems. Few references can be found about the effects of dc bias on the current transformer that is very important to both metering and protective aims. In this paper, the characteristics of electromagnetic CTs are discussed. At first, the primary current is determined, and a decaying dc offset is found which will affect the CT's dynamic performance. Then the CT's steady state and transient state characteristics with dc bias are researched. It shows that the turns ratio, direct current and some other parameters have effects on the CT's time-to-saturation which is important to protective engineers. EMTP simulation results confirm the validity of the results.