Research On Dc Biased Transformers Caused By HVDC Or Geomagnetic Storm

More and more AC and DC power transmission lines will be constructed in next decade in china. And the power grid will be weaved together as a tight network, what works to make the electric power industry strong. Obviously, it can improve the power systems'stability and make benefit to the economical operation. Ironically, what has worked to make the electric power industry strong for ordinary failure introduces key vulnerabilities to space weather and HVDC.While HVDC systems operating in ground return mode, considerable dc currents will flow in the earth through the dc electrode. The current in the earth forms DC current between the neutral points of AC transformers located in different earth potential, and the dc currents flow through the transformer winding. The interaction of the solar wind and the earth's magnetic field causes geomagnetic disturbances; an electric potential gradient is produced at the surface of the earth in response to the geomagnetic disturbances. It is the electric potential difference between ground points on the power system which drive the GIC (Geomagnetically Induced Current) through the power system equipment and facilities. GIC is a quasi current with frequency between 0.001~0.1Hz. GIC and HVDC earthed current have been proved to bring about dc biased excitation phenomena of transformers. Only a few amps of this type dc or quasi dc current are needed to disrupt transformer normal operation and increase the susceptibility in electric power system, sometimes may even induced blackout of the power system.In this paper, Finite Element Method (FEM) is used to analyze distributions of Earth Surface Potentials (ESP) near the DC earthed pole when the HVDC power transmission systems operating in ground return mode. By utilizing simplified earth resistivity model, several factors that may affect distributions of ESP are taken into consideration, such as landform, resistivities of different earth layers, DC earthed pole designs, several poles working together etc. Furthermore, due to the extremely complex electric conditions of earth and interactions between earth and power grids, ESP that calculated based on the simplified earth resistivity model are not applicable to evaluate the degrees of the dc currents flowing through the earthed points of AC transformers accurately. So a new method to evaluate the degrees of dc currents is mentioned. According to the new method, Equivalent impedances between DC earthed pole and transformers'earthed points are used to evaluate the distributions of dc currents. Therefore, distributions of dc currents would be calculated without considering complex earth conditions. And the equivalent impedances can be worked out if adequate monitoring devices are installed on the transformers'earthed point, which are used to detecting dc currents flowing in power grids.FEM also is used to analyze the dc biased transformers: field-circuit coupled method is used to compute the dc biased transformers'excitation currents and distribution of the flux. Different behaves of single-phase transformer and three-phase three-leg are compared in the paper.