Damage caused by excessive grounding resistance
(1) transformer grounding resistance resistance is too large, such as low voltage phase line insulation damage and grounding (such as L1 phase line grounding), then the transformer grounding line will have current flow, L1 phase voltage on the earth and grounding resistance, grounding resistance value is greater, the greater the partial voltage on the grounding resistance. At this time, if someone mistakenly touches the transformer grounding wire or neutral line and transformer shell, the human body will be in parallel with the grounding resistance, so the voltage added to the human body will lead to electric shock.
(2) when the neutral line grounding resistance resistance of the transformer three-phase four lines is too large or neutral line break, at this time due to the imbalance of three-phase load, the neutral point of the transformer will be offset, ground point potential is not zero, making some phase voltage rise and burn electrical equipment.
(3) When the grounding resistance value is too large, and the transformer batch arrester can not normally discharge to the ground, resulting in the arrester or transformer burn.
The grounding resistance of the transformer is too large
(1) The material of grounding device is unqualified. Excessive grounding resistance may be caused by improper embedment of the grounding body, unqualified installation process, loose connection between the grounding body and the grounding cable, and dry earth.
(2) Due to insufficient understanding of the importance of the role of the transformer grounding wire, the selection of neutral line sectional area is too small, or due to the damage of external force, grounding wire stolen and other reasons, may lead to grounding wire break or grounding resistance value is too large.
Precautions to prevent excessive grounding resistance
(1) Strict construction technology and standard burying of grounding body. The grounding device is generally made of steel pipe, Angle steel, flat steel and steel strand. The buried depth should not be less than 0.5~0.8m. The grounding device should be constructed at the same time as the foundation. The specific requirements are as follows.
① The depth of grounding groove should meet the design requirements, generally 0.5~0.8m, arable land should be laid below the tillage depth. Generally, the width of the grounding trough is 0.3 to 0.4m, and all debris in the trough that affects the contact between the grounding body and the soil should be removed.
(2) The specification of steel pipe and the depth in the soil should meet the design requirements, and the grounding body should be vertically driven into the ground and fixed, so as not to increase the resistance value of street resistance. In mountainous areas and areas with high soil resistivity, tube grounding devices should be used as little as possible, and surface embedded grounding devices should be used.
The grounding lead down line should be laid along the pole, and as short and straight as possible to reduce its impact reactance. The grounding lead down is fixed on the tower with support parts. The straight-line distance between the support parts is usually 1.0-1.5m, and the turning part is 0.1m.
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