Research on the influence of HVDC transmission on measuring accuracy of current transformers

When high voltage direct current (HVDC) system is under the monopolar operation mode, huge direct current (DC) is injected into the earth through the DC grounding electrode, which will result in earth potential changes in a wide range of area. In this case, DC will be coupled into the nearby alternating current (AC) power grid and have some bad influences, called DC bias phenomenon, on the measuring accuracy of current transformers (CT) in this area. The two commonly used methods, formula derivation method and experimental test method, can only be used to assess quantitatively, but not give an insight into, aforementioned influence of DC bias phenomenon. Hence, for revealing the action mechanism of DC bias, a digital simulation model of CT is established based on Preisach theory and its correctness has been verified by error experiments. Then, the inherent mechanism of DC bias phenomenon is analyzed and concluded by the simulation of the coupling relationship of flux density of CT iron core, exciting current and DC bias. Most importantly, the first DC bias monitoring network for HVDC transmission project in China has been established, and then based on the real DC monitoring data and CT characteristic parameters, the influence status and tendency of the DC bias phenomenon on CT measuring accuracy are evaluated quantitatively.