Research On The Effect On AC System By Mono-polar Operation Of UHVDC System

Because of its advantages in transmission losses, transmission capacity. UHVDC is widely used in high-voltage, long-distance, high-capacity transmission project. Debugging, maintenance or failure. UHVDC would mono-pole operation, which will get the DC current into the transformers causing DC bias. DC bias makes the upper half of transformer core get into saturation, which produces large of harmonic, causes the transformer vibration intensified and noise increases, an explosion in shunt reactors. Therefore, doing research on the causes, principle, suppression measures of the effect on AC system by mono-polar operation is important in the AC and DC co-exists.The paper analyzes the DC current in transformer neutral point, and the theory of DC bias caused by mono-polar operation of UHVDC system. At the same time, the paper research on harm caused by DC bias on the transformer, shunt reactor and power grid.The paper, using boundary element method, analyzes factors, including soil layered, hillside, river and sea, affecting potential distribution. The author also proposes a new method that the system is divided into above and below ground parts, and linked by the potential on ground grid and the current in transformer neutral point to calculate the DC current in transformer neutral point. At last, as an example of Zhe-Xi grounding electrode, the author does some research on potential distribution and DC current in transformer neutral point.The paper simulates tolerance of the overvoltage in transformer neutral point under the different suppression measures. Studies show that the small resist series in transformer neutral point should smaller than7.5Ω in the500kV transformer whose neutral point is direct to ground so as to meet the insulation level of the transformer neutral point under short-circuit fault; The other grounding way all can meet the requirements At last, the paper does research on harmonics in the excitation current under DC bias. The paper also simulates different structure of the tolerance of DC bias using PSCAD\EMTDC. Simplifying the excitation curve, the author derives the maximum value calculation formula of each harmonic which can be used into project.