Research On The Effect On AC Grid By UHVDC System And The Application Of Point Estimate Method

Because of the advantages in large transmission capacity and little transmission losses, UHVDC has a rapid development in China. It’s widely used in long-distance, large-capacity, high-voltage transmission projects and effectively helps the electric power development strategy of "power transmission from West to East, power exchange between South and North, interconnection in the whole country". When DC transmission system is under mono-pole operation, debugging, failure or maintenance, the DC current will flow into the neutral earthed tranformers and cause DC bias. DC bias leads to the asymmetry of excitation current, which will cause many influences to the operation and safety of tranformers and AC grid, like large harmonic, increase of transformer losses, local overheat, severer vibration and so on. Therefore, it has a great practical significance to do research on the effect on AC grid by mono-pole operation of UHVDC system.First, the paper analyzes the theory of DC bias and the cause of DC current into the transformer neutral point by the mono-pole operation of UHVDC system. Then, the paper researches on the harm and effect of DC bias on the transformers, power grid and related equipment.The paper deduces the calculation principle of earth surface potential distribution and the DC current distribution by using boundary element method and node voltage method. Then the paper analyzes the influence of various factors on the modeling and simulation in CDEGS. At last, as an example of±800kV Xiang-Shang UHVDC System, the paper does some research on the modeling, and the calculation of potential distribution and the DC current distribution in the AC grid around.The paper introduces point estimate method and related theories, analyzes the numerial characteristics of random variables and the calculation principle of point estimate method and Monte Carlo method. Then the paper does research on the principle and differences of two point estimate method and three point estimate method, and investigates their advantages and disadvantages. According to the point estimate method and related theories, the paper puts forward an efficient method based on point estimate method to calculate the DC current distribution in the AC grid. It’s combined with CDEGS to calculate the grounding resistance of substations and the DC current distribution in the AC grid. The paper analyzes and verifies the method by examples. The examples prove that this method gives the results with probability meaning, and make the results have more practical and reference significance than the deterministic method.