1. What are the hazards of DC system positive grounding to operation?
The positive grounding of DC system may cause protection malfunction. Because the trip coil of the electromagnetic operating mechanism is usually connected to the negative power supply, if these circuits are grounded again, poor insulation will cause protection misoperation. When the negative pole of DC system is grounded, if there is another point grounded in the circuit, the tripping or closing circuit may be short circuited, resulting in the protection or circuit breaker refusing to move, or burning the relay, or fusing the fuse, etc.
2. What is the cause of light gas action?
The causes of light gas action are as follows:
(1) Air enters the transformer due to oil filtration, oil filling or poor cooling system.
(2) Due to temperature drop or oil leakage, the oil level is lower than the light gas float of gas relay
(3) Transformer failure produces a small amount of gas
(4) Through short circuit
(5) Gas relay or secondary circuit fault
3. How to choose the fuse of AC circuit, DC circuit control and signal circuit?
The selection of fuse should be as follows:
(1) AC circuit fuse shall be selected according to 1.2 times of rated current of protection equipment
(2) DC control, signal circuit fuse generally choose 5 ~ 10A
4. What are the possible reasons for the false oil level of transformer?
The false oil level of transformer may be caused by the following reasons:
(1) The oil standard pipe is blocked
(2) Respirator blocked
(3) The safety airway is blocked
(4) The film protection conservator does not exhaust the air when refuelling
5. What are the causes of transformer winding insulation damage?
The causes of transformer winding insulation damage are as follows:
(1) Line short circuit fault
(2) Long term overload operation leads to serious aging of insulation
(3) The winding insulation is damp
(4) Poor contact of winding connector or tap changer connector
(5) The invasion of lightning wave makes the winding over-voltage