Research On Backward Grounding Resistance Measurement Method With Short Lead Wire

With the construction of UHV power transmission system and the expansion of the power grid, the number of large hub substation have increased greatly, the size of the grounding grid is also far larger than ten years ago. Hub substation plays an important role in the transmission of UHV power transmission system and the protection of social power supply. The reliability of its grounding grid is of great significance to the safety of China’s electric power system.Grounding resistance measurement is an important experiment and must be tested every few years. The now widely used fall-of-potential method and its derivative methods in grounding resistance measurement need to set a long lead wire(4~5D)in order to find the compensation point or zero potential point, D is the equivalent diameter of grounding grid. With more big substation and Converter station are built and begun operation, the workload and test difficulty have increased significantly,how to simply and precisely measure the grounding resistance of a grounding system especially large grounding grid is an important problem in power systems. The existing grounding resistance measurement methods rely heavily on the accuracy of judgments and calculation of potential electrode position. The lead wire should be long enough so that the compensating site of the voltage electrode can be calculated and located correctly. The aim to find the proper location of compensation point and zero potential point is the reason why the existing method cannot shorten the lead wire. A novel method for measuring the grounding resistance is proposed in this paper, which can be applied with short lead wire. In this method, the potential distribution near the auxiliary current electrode is tested and used as the key analysis and measurement parameter. This method can avoid the complex job for an accurate location of compensation point in traditional measurement methods, and can be used easily and quickly with 2~2.5D lead wire length. This method can decrease the workload and can be applied to measure large grounding grid.In this paper, firstly, the characteristic of earth potential distribution of grounding resistance measurement is simulated, the earth potential distribution near current electrode is analyzed;secondly, the measurement method to test potential near current electrode is proposed and numerical function of test data is deduced, measurement results are used to calculate the undetermined coefficients of the function, and the grounding resistance value can be deduced;Thirdly the equivalent model of grounding grid and current electrode is analyzed to determine the lead wire length; Finally, the test instrument is developed and validate of the novel method by numerical simulation and field test are applied. The conclusions are as follows:(1) The analysis of simulation results and the characteristics of earth potential distribution have proved that the potentials near current electrode has more advantages than the test of zero potential point and compensation point. The arrangement of test electrodes in the novel method is put forward.(2) The theory of backward method is put forward. Use the test data near current electrode to calculate the undetermined coefficients of analytic functions. Levenberg-Marquardt method is used in solving the objective function. Then the actual GPR of grounding grid can be calculated. Numerical simulation and field test are applied to validate the novel method.(3) Use the hemispherical electrode as equivalent model, the equivalent conditions of grounding grid and current electrode is analyzed. To most kinds of grid, the equivalent distance should be about 2D;when the distance to current electrode is longer than 3m, the current electrode can be regarded as point current source. The lead wire length is proposed to be 2~2.5D.(4) The measurement device is developed. The output voltage and power is analyzed. We reduced grounding resistance of current electrode to ensure power output;Electromagnetic interference signal in substation is tested to develop the filtering device; GNSS-RTK measuring device is used to improve the location accuracy of electrodes.