The principle and significance of testing methods for grounding resistance in substations

In recent years, many substations in China have experienced expanded accidents due to lightning strikes. Most of the grounding resistance related to the grounding grid is unqualified, which plays a role in working grounding and protective grounding. When the grounding resistance is too high, a grounding fault occurs, and the neutral point voltage offset increases, which may cause the sound phase and neutral point voltage to be too high, leading to equipment damage and exceeding the required insulation level. Lightning strikes or lightning wave attacks, due to the extremely high residual voltage of the current, pose a threat to nearby equipment from counterattacks and reduce the radiation resistance level of live conductors in grounding network protection equipment (overhead transmission lines and substation electrical equipment), resulting in equipment damage if it does not meet the design requirements.

The qualification of grounding resistance in the grounding system is directly related to the personal safety of substation operators and maintenance personnel; However, due to the corrosive effect of soil on the grounding device, as the operating time prolongs, the grounding device has already corroded, affecting the operation of the substation; Therefore, it is necessary to vigorously strengthen the regular monitoring of network to ground resistance; The measurement of grounding resistance in the substation network during operation results in errors due to significant system interference, network current inflow, and interference between test leads. Especially when the grounding resistance is large and small (usually below 0.5 Ω), even minor interference test results can have a huge impact; Inaccurate testing of network grounding resistance not only damages equipment, but also causes unnecessary losses due to such grounding network errors. Based on my grounding impedance testing method, the following is a summary:

1、 Principle and method of grounding resistance tester:

The grounding impedance current of the test grounding method should be arranged as close as possible to the edge of the normal and test grounding device dcG. The distance should be 4-5 times the length D of the large diagonal of the test grounding device (parallel wiring method), and more than 2 times the ideal soil resistivity in the triangle wiring method. The length of the voltage lead should be 0.618 times the length of the current lead (flat wire wiring method) or equal to the current lead (triangle wiring method).

1. When using measurements, it is necessary to short-circuit together with P1. The local network grounding resistance is small, and the local network grounding resistance is small (≤ 0.5 Ω) to improve measurement accuracy and reduce the measurement results of lead resistance and contact resistance of the instrument and grounding network. EP short-circuit can be untied; Reduce contact resistance errors caused by connecting individual leads to grounding network test points.

Note:

1. E - Connected to the tested network;

2. P1- Connected to the tested network;

3. P2- Then measure the voltage line (0.618 times the length of the current line);

4 C - Connect the measuring current line (its length is taken as 4-5 times the diagonal length of the grounding grid);

2、 Detection precautions and significance

Most of the characteristic parameters of grounding devices are closely related to soil moisture, and ground evaluation and acceptance tests of device status should be carried out as much as possible in dry seasons and high soil moisture. It should not freeze and should not be carried out immediately after thunder, rain, snow or rain. The actual measurement provides a reliable basis for our rectification. It is recommended to rectify and optimize the grounding conditions of the substation, so that the grounding resistance of the grounding grid meets the requirements, effectively preventing personal injury or equipment damage caused by step voltage on equipment insulation. Play the role of ensuring the operation of electrical equipment and create a safe working environment for substation staff.

Weshine Electric Manufacturing Co., Ltd.