One point grounding and multi-point grounding of transformer
Writer： Hengfeng you electric Time：2023-04-07 views：times
Transformer is a very common electrical equipment in our daily life. Transformer its main functions are: voltage conversion, current conversion, impedance conversion, isolation, voltage stabilization (magnetic saturation transformer), etc. In the power industry, people's life and enterprise production need to use transformers to convert high-voltage transmission into distribution; In the power supply system, the transmission and transmission of ultra-high voltage, high voltage and power distribution are required. In the field of power generation, transformer step-up is also required for transmission. Then why should the transformer be grounded? Why not multi-point grounding?
1. Why should the transformer be grounded at one point?
During the operation of the transformer, the iron core, fixed iron core and the metal structure, parts and components of the winding are in a strong electric field. Under the action of the electric field, they have a high potential to the ground. If the iron core is not grounded, there will be potential difference between it and the grounded clamp and oil tank. Under the action of potential difference, intermittent discharge may occur.
In addition, during the operation of the transformer, there is a strong magnetic field around the winding. The iron core, metal structure, parts and components are all in a non-uniform magnetic field, and the distance between them and the winding is not equal. Therefore, the electromotive force induced by the magnetic field is also different, and there is a potential difference between them. Although the potential difference is small, it can also break down a small insulation gap, which may also cause continuous micro discharge.
Whether the intermittent discharge phenomenon may occur due to the action of potential difference or the continuous micro discharge phenomenon caused by the breakdown of small insulation gap is not allowed, and it is very difficult to check these intermittent discharge parts.
The effective solution is to reliably ground the iron core, the metal structure, parts and components of the fixed iron core and winding, so that they are at the same ground potential as the oil tank. The iron core grounding of transformer is one point grounding, and it can only be one point grounding. Because the silicon steel sheets of the iron core are insulated from each other, this is to prevent large eddy current. Therefore, all silicon steel sheets must not be grounded or grounded at multiple points, otherwise, large eddy current will be created and the iron core will be seriously heated.
The iron core of the transformer is grounded, usually any silicon steel sheet of the iron core is grounded. Although the silicon steel sheets are insulated, the insulation resistance value is very small. The uneven strong electric field and strong magnetic field can make the high-voltage charge induced in the silicon steel sheet flow from the grounding point to the earth through the silicon steel sheet, but can prevent the eddy current from flowing from one sheet to another. Therefore, as long as any silicon steel sheet of the iron core is grounded, it is equivalent to grounding the whole iron core.
It should be noted that the iron core of the transformer must be grounded at one point, not at two points, and not at many points, because multi-point grounding is one of the common faults of the transformer.
2. Why does the transformer not allow multi-point grounding? But a little grounding?
The reason why the transformer core lamination can only be grounded at one point is that if there are more than two points grounded, a loop may be formed between the grounding points. When the main track passes through the closed loop, circulating current will be generated in it, causing internal overheating and accidents.
The burned local iron core will form a short-circuit fault between iron suspension pieces, resulting in large iron loss, which will seriously affect the performance and normal operation of the transformer. Only the silicon steel sheet of the iron core can be replaced for repair. Therefore, the transformer is not allowed to be grounded at multiple points, and only one point can be grounded.
3. What are the disadvantages of transformer multipoint grounding?
During the operation of the transformer, its iron core, clamps and other metal parts are in a strong electric field, because electrostatic induction will produce a suspended potential on the iron core and metal parts, and this potential will discharge to the ground, which is of course not feasible. Therefore, the iron core and its clamps must be correctly and reliably grounded (except for through bolts).
The iron core is only allowed to be grounded at one point. If there are two or more points of grounding, the iron core will form a closed circuit with the grounding point and the earth. When the transformer is running, magnetic flux will pass through the closed circuit, which will produce the so-called circulating current, causing local overheating of the iron core and even burning the metal parts and insulating layer.
To sum up: the iron core of the transformer can only be grounded at one point, not at two or more points.
