Reserch On The Influence Of DC Bias Induced By The Change Of ESP On AC Transformer In Mono-polar Of UHVDC System Operation

Due to the distribution difference between energy and electric ity load, the power development strategy on sending power from west to east, North-South supplementation and nationwide interconnection is established. High voltage direct current has been developed as an important way of high-capacity and long-range electric power transmission and area interconnection because of its good deal of advantages such as high-capacity transmission, high voltage, without the problem of stability of synchronous operation and so on. Generally, HVDC system works in bipolar operation, however, HVDC system also operates in monopole in the early stage of construction and when mono-polar locking occurs or equipment is maintained. DC bias caused by mono-polar operation of HVDC system with ground return has serious impacts on stable operation of HVDC system. In this case, deep analyses of the distribution of earth surface potential(ESP) and the measure of restraining DC bias become significant in the development of power.In this paper, the transformer DC bias problem of 500 kV substation and related power plants induced by ±800 kV UHVDC system from the north of Shanxi to Jiangsu has been conducted. The main contents are as follows:(1) The influence of the change of local soil resistivity which is in the area where grounding electrode exists in, the change of local soil resistivity which is in the area where grounding electrode does not exist in, the change of different layer soil resistivity and the change of soil res istivity under different environments on the distribution of ESP. It is found that with the increase of soil resistivity of local cylinder area with 10000 m radius and 3 m height where grounding electrode exists in, the ESP rises gradually within the range of 5000 m from the center of grounding electrode, however, the ESP changes slightly outside the range of 5000 m from the center of grounding electrode with the rise of local soil resistivity. The impact of the change of local soil res istivity which is in the area where grounding electrode does not exist in on the distribution of ESP is negligible. It is also found that first layer soil res istivity within a 200 m height impacts on the distribution of ESP obviously. ESP increases when drought or rain occurs. The ESP with taele is distinctly lower than that without taele within the range of 91500 m from the center of grounding electrode, but the ESP with taele is slightly higher than that without taele outside the range of 91500 m from the center of grounding electrode.(2) The DC bias of 500 kV substations and related power plants in the north of Shanxi is calculated in different c limate environments through establishing AC system model with MATLAB, and the measure of series low resistance is put forward to suppress DC bias. The results show that the DC bias of all the substations and related power plants in the north of Shanxi can’t be decreased at the same time after individual substations and related power plants in the north of Shanxi use the measure of series low res istance, therefore, when the measure of series low res istance used to restrain DC bias of some substations and related power plants is considered, the impact of DC bias on other surrounding substations and related power plants should be took into account as well. The DC bias of all substations and related power plants in the north of Shanxi is calculated in different environments lower than 5 A and the operation standard is safe and stable, when a series 3 Ω low resistance within transformer neutral in Shuozhou and Pingyao substations, a series 5 Ω low resistance within transformer neutral in the second power plant of Shentou and Xinzhou substation, a series 10 Ω low resistance within transformer neutral in the Shenquan power plant are employed.