I. Overview
In recent years, many substations in China have experienced expanded accidents due to lightning strikes, most of which are related to unqualified grounding resistance of the grounding grid. The grounding grid serves the functions of working grounding and protective grounding. When the grounding resistance is too large, in the event of a grounding fault, the neutral point voltage deviation increases, which may cause the healthy phase and neutral point voltage to be too high, exceeding the insulation requirements and causing equipment damage. When struck by lightning or lightning waves, due to the large current, a very high residual voltage will be generated, threatening nearby equipment with counter-attack and reducing the lightning resistance level of the grounding grid itself for protecting equipment (overhead transmission lines and substation electrical equipment) with live conductors, failing to meet design requirements and damaging equipment. At the same time, whether the grounding resistance of the grounding system is qualified is directly related to the personal safety of substation operators and maintenance personnel; however, due to the corrosive effect of soil on grounding devices, as the operation time increases, the grounding devices have been corroded, affecting the safe operation of the substation; therefore, it is necessary to strengthen the regular monitoring of the grounding resistance of the grounding grid; the measurement of the grounding resistance of the substation grounding grid during operation is subject to interference from the current flowing into the grounding grid from the system and interference between test leads, resulting in large errors in test results. Especially for large grounding grids, the grounding resistance is very small (generally below 0.5Ω), and even slight interference can have a great impact on the test results; if the grounding resistance test of the grounding grid is inaccurate, it will not only damage equipment but also cause unnecessary losses such as erroneous transformation of the grounding grid. Based on my research on the test method of grounding impedance of grounding grids, I now summarize as follows: Using the principle of series resonance for the resonant boosting device for CVT testing, adopting the technology of stepless manual inductance adjustment and the method of connecting reactors in series, the required power supply, test equipment volume and weight are greatly reduced. II. Principle and Method of Grounding Resistance Test:
When testing the grounding impedance of a grounding device, the current electrode should be placed as far as possible. Usually, the distance dcG between the current electrode and the edge of the tested grounding device should be 4 to 5 times the large diagonal length D of the tested grounding device (parallel wiring method). In areas with uniform soil resistivity, it can be 2 times or more (triangular wiring method). The length of the voltage lead is 0.618 times the length of the current lead (parallel wiring method) or equal to the current lead (triangular wiring method).
1. Potential Drop Method
The potential drop method for testing the grounding impedance of a grounding device is arranged according to Figure 1, and meets the requirements for the arrangement of the test circuit.
G—Tested grounding device; C—Current electrode; P—Potential electrode; D—Large diagonal length of the tested grounding device; dCG—Distance between the current electrode and the edge of the tested grounding device; x—Distance between the potential electrode and the edge of the tested grounding device; d—Test distance interval;
The current I flowing through the tested grounding device G and the current electrode C causes changes in the ground potential. The potential electrode P moves outward from the edge of G along a direction at an angle of 30°~45° to the current circuit. At each interval d (50m or 100m or 200m), the potential difference U between P and G is tested, and a curve of U versus x is plotted. The flat part of the curve is the zero potential point, and the potential difference between the curve points is the potential rise U of the tested grounding device under the test current. The grounding impedance of the grounding device is:
Z=Um/I
If it is difficult to lay the potential test line at an angle to the current line, it can be laid along the same path, but keep as far away as possible.
If the flat point of the potential curve is difficult to determine, it may be affected by the tested grounding device or the current electrode C. Consider extending the current circuit; or if the underground conditions are complex, consider using other methods for testing and verification.
2. Current-Voltmeter Three-Electrode Method
a) Straight Line Method
Laying the current line and potential line in the same direction (same path) is called the straight line method in the three-electrode method, as shown in Figure 2; dcG meets the requirements for the arrangement of the test circuit, and dPG is usually (0.5~0.6) dcG. The potential electrode P should move three times along the line connecting the tested grounding device G and the current electrode C, each time moving about 5% of dcG. When the results of the three tests have an error within 5%, it is acceptable.
Large grounding devices generally should not use the straight line method for testing. If conditions are limited and it must be used, attention should be paid to keeping the current line and potential line as far apart as possible to reduce the influence of mutual inductance coupling on the test results.
G—Tested grounding device; C—Current electrode; P—Potential electrode; D—Large diagonal length of the tested grounding device
dCG—Distance between the current electrode and the edge of the tested grounding device; dPG—Distance between the potential electrode and the edge of the tested grounding device;
b) Angle Method
As long as conditions permit, the test of grounding impedance of large grounding devices should adopt the arrangement of current-potential lines at an angle. dcG meets the requirements for the arrangement of the test circuit, generally 4D~5D, and for ultra-large grounding devices, as far as possible; the length of dPG is similar to dcG. The grounding impedance can be corrected using formula (2).
In formula (2)
θ---The angle between the current line and the potential line;
Z''--- Test value of grounding impedance.
If the soil resistivity is uniform, an isosceles triangle wiring with equal dcG and dpG can be adopted. At this time, θ is about 30°, dcG=dpG=2D, and the grounding correction formula 2 is used.
3. Grounding Resistance Tester Method.
Figure 3 shows the wiring method for testing the grounding resistance of the grounding grid using a grounding resistance tester; the test principle, wiring, and requirements are similar to the three-electrode method.
1. When using the three-electrode method for measurement, the E electrode must be short-circuited with P1. However, when the grounding resistance of the grounding grid is very small, when the grounding resistance of the grounding grid is small (≤0.5Ω), in order to improve measurement accuracy and reduce the influence of the resistance of the test leads and contact resistance between the instrument and the grounding grid on the measurement results, the E.P short-circuit piece can be disconnected; to reduce errors caused by contact resistance, a separate lead needs to be connected to the test point of the grounding grid.
Note:
1. E—Connect to the measured grounding grid;
2. P1—Connect to the measured grounding grid;
3. P2—Connect to the measurement voltage line (its length is 0.618 times the length of the current line);
4. C—Connect to the measurement current line (its length is 4 to 5 times the diagonal length of the grounding grid);
III. Test Precautions and Significance
Most of the characteristic parameters of grounding devices are closely related to the moisture level of the soil. Therefore, the condition assessment and acceptance test of grounding devices should be carried out as much as possible in the dry season and when the soil is not frozen, and should not be carried out immediately after thunder, rain, snow, or after rain or snow. Through actual measurement, it provides reliable basis for our rectification. For the grounding condition of the substation grounding grid, a rectification and optimization plan is proposed to make the grounding resistance of the grounding grid meet the requirements, thereby effectively preventing personal injury caused by step voltage due to equipment insulation damage or further damage to equipment. It plays a role in ensuring the safe operation of electrical equipment and creating a safe and reliable working environment for substation staff.
