| With the development of Chinese economy, the demand for power is growing. In order to solve the delivery problems of energy from energy base to load center, the construction of power grid in China has entered an ultra-high voltage age of long-distance and high-capacity transmission. Some problems in low-voltage power grid are more prominent and can no longer be ignored nowin the UHV power grid. If these problems are not resolved, they will endanger the security of grids. So the higher requirements for protection are proposed. The protection is the first line of defense for power system safe and stable operation, and must adapt to these requirements. So the protection can quickly cut short circuit fault, protect electrical equipment and prevent the occurrence of large area blackout.The capacitor charging current, of long-distance transmission lines in UHV grids, has negative effects on the current differential protection This no ignorable issue can cause pro-tective relays to start frequently and even maloperating. The series capacitor compensation is often used to improve the power system stability, reduce system line losses and improve line transmission capacity. The development of grid-scale system make impedance decrease. At the same time, the series capacitor compensation may remain unchanged. As a result, when the system impedance is less than the capacitive reactance of series compensation, the issue of current inverting will occur. And the conventional current differential protection may lead to refuse tripping because of decreased sensitivity. The magnetizing inrush current is often due to current differential protection maloperating during no-load operating. It is always the problem the protective engineers are concerned about. In addition, the UHV transmission lines, which transmit the power from west to east region, are operating in the poor condition areas (such as steep mountains, cold weather, poor geology, transportation difficulties, etc) in Schuan Province. These are the great difficulties to find fault location when the transmission line fault. So this paper addresses the influence on the conventional current differential pro-tection, of the charging capacitor current on UHV transmission lines, the series compensa-tion capacitor, the magnetizing inrush current of transformer and the fault location accuracy of transmission line. The main work is as follows:(1) About the influence of distributed capacitive current of transmission line on longitu-dinal current differential protection, the calculated power differential protection is presented. Firstly, the definitions of calculated power and differential protection criterion based on it are presented. Based on the transmission line two-port network model, the differential and re-straint component of calculated power, during normal operating (or external faults) and all kinds of internal faults, are analyzed theoretically. The simple setting principle of calculated power threshold without line parameters is presented. In order to simplify the algorithm but keep the transient electrical quantilies of initial fault, an engineering applicable algorithm, which calculates the differential and restraint component of calculated power with the virtual voltage and virtual current, is presented. Lastly, the test with PSCAD/EMTDC software sim-ulates the Jingmen 1000 kV UHV transmission line. These results verify the validity and ef-fectiveness of the proposed algorithm.(2) Because the system impedance is less than the capacitive reactance series compen-sation, current inverting may occur, making smaller the sensitivity of traditional steady cur-rent differential protection. To solve this, an improved algorithm of current differential pro-tection is presented. The sensitivity of two steady current differential protection criterion used in protective relays is analyzed. It is shown that the protective relay would maloperate because of insufficient sensitivity. The differential and restraint current of improvement al-gorithm during normal operation and fault are analyzed. The test with PSCAD/EMTDC software simulates the sensitivity coefficient of all kinds of faults at 500kV Lanpu circ.1 in Sichuan grid. The test condition includes different fault type, different fault position, differ-ent system impedance at series capacitor compensation side, different series compensation degree of line. These results verify the validity and effectiveness of the proposed algorithm.(3) The accurate fault location algorithm using transmission line distribution parameter Ⅱ type model is presented. The parameters of transmission line is calculated on-line during normal operating to avoid inaccurate fault location, usually caused by transmission line pa-rameters change due to various factors. Also taken into account are the unsynchronized sam-pling on both sides of the protective devices the coefficient of non-synchronous sampling is calculated to compensate for synchronous sampling. So that results of fault location is not affected by non-synchronous sampling. The calculation flow chart of the software modules for fault location in protective relay is presented also. The test with PSCAD/EMTDC soft-ware simulates the line parameters online in different asynchronous sampling angle and fault location in different location and different grounding resistances. These results verify the va-lidity and effectiveness of the proposed algorithm.(4) Free from the influence of the ground shunt reactors branch, the calculated power differential protection is used in this paper for transformer differential protection. The dif-ferential and restraint component of calculated power is analyzed with the electromagnetic characteristics and equivalent circuit of transformer at inter-turn short circuit fault during charging operation. The simple setting principle of calculated power threshold is presented. The test of large-capacity transformer with PSCAD/EMTDC software verify the validity and effectiveness of the proposed algorithm. |
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