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Second, the grounding resistance test principle and method When testing the grounding device grounding impedance, the current pole should be arranged as far as possible, usually the distance dcG between the current pole and the tested grounding device edge should be the maximum diagonal length D of the tested grounding device. 5 times (parallel wiring method), 2 times or more can be taken in areas where the soil resistivity is uniform (triangular wiring method), voltage lead length is 0.618 times the length of current lead (flat wire wiring method) or equal to current wire (triangle wiring method) .
1. Potential drop method Ground potential impedance of the grounding device is to arrange the test circuit according to Figure 1, and meet the requirements of the test circuit layout.
G—tested grounding device; C—current pole; P—potential pole; D—maximum diagonal length of the grounding device under test; dCG—distance between the current pole and the edge of the grounded device under test; and x-potential pole and grounding under test The distance of the device edge; d - the test distance interval;
Current I flowing through the tested grounding device G and current electrode C changes the ground potential, and the potential pole P moves outward from the edge of G in the direction of 30° to 45° with the current loop, every interval d (50m or 100m). Or 200m) Test the potential difference U between P and G once, draw the curve of U and x. The flat point of the curve is the potential zero point, and the potential between the curve point is the potential of the grounded device under test increases U, and the grounding impedance of the grounding device is: Z=Um/I.
If it is really difficult for the potential test line and the current line to be laid out at an angle, it may be set up along the same path, but keep as far as possible.
If the potential is difficult to determine the flat point of the curve, it may be affected by the grounding device under test or the current pole C, considering the extension of the current loop; or the underground situation is complex, and other methods are considered for testing and verification.
2. Current-voltage meter three-pole method a) Line method Current lines and potential lines in the same direction (with the same path) are called the linear method in the three-pole method, schematic diagram 2; dcG meets the requirements of the test circuit layout, dPG is usually For (0.5 ~ 0.6) dcG. Potential P should be moved three times in the direction of the connection of the grounding device G and the current electrode C to be measured, each moving distance is about 5% of dcG, when the error of the three tests is within 5% You can.
Large-scale grounding devices are generally not suitable for straight-line testing. If conditions are limited and must be used, care should be taken to keep the current and potential lines as far away as possible to reduce the effect of mutual inductance on the test results.
G—Test grounding device; C—Current pole; P—Potente potential; D—Destend grounding device maximum diagonal length dCG—Distance between current pole and edge of tested grounding device; dPG—Potente electrode and tested grounding device The distance of the edge;
b) Inclusion angle method As long as the conditions permit, the test of the grounding impedance of large-scale grounding devices adopts the method of current-potential wire angle arrangement. The dcG meets the requirements of the layout of the test circuit, generally 4D~5D, and is as far as possible for the super-large grounding device; the length of dPG is similar to dcG. The grounding impedance can be corrected using the formula (2): θ—the angle between the current line and the potential line;
Z''--- ground impedance test value.
If the resistivity of the soil is uniform, an isosceles triangle with equal dcG and dpG can be used. At this time, θ is approximately 30°, and dcG=dpG=2D ground correction formula 2.
3, grounding resistance tester method.
The grounding resistance tester tests the connection method of the grounding resistance of the grounding network; the testing principle, wiring, and requirements are similar to the three-pole method.
1. The E pole must be short-circuited with P1 when using the three-pole method, but the local network grounding resistance is small and the local network grounding resistance is small (≤ 0.5Ω). In order to improve the measurement accuracy, the instrument and the ground network measurement are reduced. The effect of lead resistance and contact resistance on the measurement result can be set as E. P-short circuit untwist; Reduce the error caused by the contact resistance, need separate lead and ground net test point is connected.
Note:
1, E - connected to the measured network;
2, P1 - connected to the measured network;
3, P2 - then measure the voltage line (its length to take the current line length of 0.618);
4, C - then measure the current line (the length of the network to take the diagonal of the network 4 to 5 times);
Third, the test notes and the significance of grounding device parameters are mostly closely related to the degree of soil moisture, so the ground device condition assessment and acceptance test should be carried out as far as possible in the dry season and the soil is not frozen, not in the thunder, rain, In the snow or immediately after the rain and snow. Through actual measurement, we provide a reliable basis for our rectification. For the substation grounding network grounding situation, a rectification and optimization scheme is proposed to make the grounding network grounding resistance meet the requirements, thereby effectively preventing the step-induced voltage caused by the equipment insulation damage from causing personal injury or further damage to the equipment. To ensure the safe operation of electrical equipment, to create a safe and reliable working environment for substation workers.
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Grounding resistance tester test principle and the use of matters needing attention First, the article overview In recent years, many domestic substations due to lightning strikes to expand the accident, most of the earth grounding resistance is not related to the grounding grid play a working ground and protective grounding The effect is that when the grounding resistance is too large, when the grounding fault occurs, the voltage offset of the neutral point increases, which may cause the voltages of the sound phase and the neutral point to be too high, causing the equipment to be damaged beyond the level required by the insulation. In the case of lightning or lightning strikes, high residual currents are generated due to large currents, causing nearby equipment to be counter-attacked and reducing the protective equipment (overhead transmission lines and substation electrical equipment) of the grounding grid itself. The level of lightning resistance cannot meet the design requirements and damage the equipment. At the same time, whether the grounding resistance of the grounding system is qualified directly relates to the personal safety of the substation operating personnel and the transformer maintenance personnel; however, due to the corrosive effect of the soil on the grounding device, the grounding device has been corroded with the extension of the running time, which affects the safe operation of the substation. Therefore, it is necessary to strengthen the regular monitoring of the grounding resistance of the grounding network; the grounding resistance measurement of the grounding station during operation is caused by the interference of the system's current into the grounding network and the interference between the test lead wires, which results in a large error in the test results. . In particular, the grounding resistance of large-scale grounding grids is very small (usually below 0.5Ω). Even subtle interferences will have a great impact on the test results; if the grounding grid resistance tests on the grounding grid are inaccurate, not only the equipment will be damaged, but it will also cause problems such as The unnecessary loss of ground network mistakes, etc., combined with my research on grounding network grounding impedance test methods, is summarized as follows:
September 30, 2019