Research On Performance Of Dc Ground Electrode And Influence On Power Transformer By The Earthing Current

When the direct current (DC) transmission systems operate in the monopolar mode using ground return, a certain amount of direct current are injected into the earth through the ground electrodes. The human being nearby the ground electrode suffers a voltage between his standing feet, and the voltage is called step voltage. High level of the step voltage makes human feel uncomfortable, even cause life danger. On the other hand, the leakage currents of the DC ground electrode impinge the AC power system through the directly earthed neutrals of the star-connected windings. This DC interference affects the performances of the power transformers and the power grids, which is the so-called the DC bias phenomenon.This dissertation aims at the performances of the ground electrodes of a UHVDC (ultra high votage direct current) system and the influences of the DC leakage currents on the power transformer. The research is supported in part by the National Natural Science Foundation of China under project 50677016, part by the State Grid Corporation of China (SGSC) under the project SGSC[2005]99, SGSC[2005]133, SG0848. This research concentrates on the electric and magnetic model of a DC biased power transformer. The effects of eddy currents in the core material were considered. The effectiveness of the model and the computation code were validated by comparing the calculated results and the measured ones, which coincide with each other. Then the model and the code were applied to the performance analysis of several large power transformer products under DC bias. Meanwhile, the research also includes the grounding performances of the deepwell electrode and the slantwell electrode. And the research results were used in the design of a±800 kV level UHVDC community ground electrode in China. The main research achievements are as follows:1. A new reluctance formula for the silicon-steel sheet of core is presented, which takes the effects of eddy currents into account. The equivalent magnetic circuit model of the new reluctance is built based on the network synthesis theory, and the parameters of the magnetic circuit are calculated by using the Vector Fitting techniques.2. An electric and magnetic circuit model of DC biased power transformer is built, which consider the spatial geometry of the core and windings, the eddy currents in the ferromagnetic materials, and leakage flux pattern in the core joint zone. In addition, a computation code was developed for the analysis of a large power transformer under DC bias. 3. A definition called transformer DC bias index is presented. Based on the definition, it is possible to compare the capacities of DC bias endurance for transformers with different types and ratings. The characteristics of DC bias influences on three typical power transformers are obtained by using the above computation code. And their endurance capacities are sorted.4. A probability stratified model of the deep earth of a ground site based on the Gamma distribution is presented. By testing the MT sounding data of two different ground sites, the reasonableness of the model presented in the paper is validated.5. A new type of ground electrode using slant well is presented. The new slantwell electrode can be used to decrease the maximum step voltages of the ground electrodes pertaining to a large capacity DC transmission system. In addition, the grounding performances of the deepwell electrodes and normal ground electrode are analyzed.