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Measurement of earthing resistance of earth grid

time:2021/9/20   source:华天电力  reading:235 time

When measuring grounded equipment, due to various interferences, such as unbalanced zero-sequence current and radio frequency, the error of the test results is relatively large. Especially the grounding resistance of the earth grid is usually very small (usually less than 0.5Ω). The relative error caused by interference is relatively large. In order to reduce the influence of on-site interference, two methods are mainly used at present, one is to increase the test current, and the other is to use a different frequency method. The first method is to increase the test. The current is used to increase the signal voltage and signal current, thereby improving the signal-to-noise ratio and reducing measurement errors. This method is very troublesome due to the large test current used.


A method of measuring the grounding resistance of the earth grid is introduced. Under the conditions of testing current frequency and system power frequency, this method can obtain stable and reliable measurement results. The source and characteristics of external interference are analyzed, and simulation test and field are combined. The measurement results are studied, and the influence of the test current frequency and the measurement wire layout on the Z, R and X of the test results are discussed.


The earth grid grounding resistance tester adopts the power frequency current method and the different frequency current method to measure. The former is a traditional method. The power frequency test current is used to test the power supply capacity. The current line has a large cross-sectional area and the equipment is heavy. The cross-frequency measurement used in the past ten years The test current frequency range is about 40-250Hz. Since the test frequency is closer to the power frequency, the measurement instrument is more technically difficult in hardware and software. Therefore, the test frequency of most inter-frequency test instruments is far away from 50Hz, inter-frequency test results and power-frequency test. The equivalence of the results is worthy of attention. First, developed a unique hardware and software anti-jamming method,


Operation steps and methods


1. The instrument measures the ground impedance by measuring the ratio of the potential rise of the grounding equipment to the current flowing into the grounding equipment.


2. The different frequency constant current power supply can output a sine wave test current with a frequency of 45Hz or 55Hz. The output frequency is controlled by the microcomputer system, and the output is isolated and output through the E and C terminals on the dashboard. The voltage amplifier is very high. The input impedance amplifier amplifies the voltage across P1 and P2 and sends it to the filter. The current amplifier amplifies the current signal obtained from the current transformer and sends it to the filter. The filter is used to filter out interference signals and only allows 45Hz, while the A/D converter uses 55Hz signals to convert the voltage and current signals into digital signals , For microcomputer system analysis.


3. After the user starts to measure the ground impedance, the instrument first turns on the different frequency power supply to make the output frequency 45Hz. After the current is stable, the microcomputer system obtains the voltage and current waveform data through the A/D converter, and performs digital filtering to calculate the voltage V45 and the current I45 and their phase difference, and further calculate the impedance Z45, the resistance component R45 and the reactance component X45. Then, the output frequency of the power supply between the switching frequencies is 55 Hz. After the same steps, the impedance Z55 and the resistance component can be calculated. R55 and reactance component X55. Use the average value of Z45 and Z55 as the power frequency grounding impedance Z50. Finally, turn off the inter-frequency power supply and display the measurement result on the LCD screen. Therefore, when measuring the impedance, the instrument will measure the two voltage input terminals P1 and P2. The ratio of the voltage of the power supply to the output current.


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