Why must the neutral point of the transformer be grounded when it is shut down?
This is mainly a measure taken to prevent overvoltage damage to the transformer.
When the circuit breaker of the receiving transformer with a power supply on one side is not fully off and on, if the neutral point is not grounded, there are the following dangers:
(1) The neutral voltage to ground on the power side of the transformer can reach the maximum phase voltage, which may damage the transformer insulation.
(2) When there is a capacitor between the high and low voltage windings of the transformer, this capacitor will cause the "transfer overvoltage" of high voltage to low voltage.
(3) When the capacitive coupling between the high and low voltage windings of the transformer, the voltage on the low voltage side will reach the resonant condition, and the resonant overvoltage may occur, damaging the insulation.
For a power supply transformer on the low voltage side:
(1) Because there is a power supply on the low voltage side, the single phase ground occurs on the high voltage side of the transformer before it is incorporated into the system. If the neutral point is not grounded, the neutral point voltage to the ground will be the phase voltage, which may damage the transformer insulation.
(2) When not fully integrated into the system, when a phase is connected to the system, due to the different frequency of the generator and the system, the transformer neutral point is not grounded, the transformer neutral point to the ground voltage will be twice the phase voltage, and the uncombined voltage will be up to 2.73 times the phase voltage, which will cause insulation damage.
How is transformer neutral gap ground protection constituted?
A: The neutral point gap grounding protection of the transformer adopts zero sequence current relay and zero sequence voltage relay in parallel, with a 0.5S time limit. When there is zero sequence current in the discharge gap when the system is grounded, the zero sequence current relay of the special current transformer located at the grounding end of the discharge gap is activated. If the discharge gap does not discharge, the zero-sequence voltage relay is used. When the gap arc grounding occurs, the time component shared by the gap protection must not return halfway to ensure the reliable operation of the gap grounding protection.
Must the neutral point of the transformer be grounded when transmitting power to the no-load transformer?
A: When sending power to no-load transformer, if the neutral point is not grounded, there will be the following dangers:
(1) The maximum reachable phase voltage of the neutral point to ground on the power side of the transformer, which may damage the transformer insulation;
(2) There is a capacitor between the high and low voltage windings of the transformer, which will cause the "transfer overvoltage" of high voltage to low voltage;
(3) When capacitive coupling between high and low voltage windings of the transformer occurs, resonant overvoltage may occur and damage insulation.
Therefore, when transmitting power to the no-load transformer, the neutral point of the transformer must be grounded.
How to consider the arrangement of the neutral ground mode of the transformer?
A: The arrangement of the neutral ground mode of the transformer should try to keep the zero sequence impedance of the substation basically unchanged. When encountering the special operation mode that the zero sequence impedance of the substation has a large change due to transformer maintenance and other reasons, it should be temporarily dealt with according to the regulations or the actual situation.
1) There is only one transformer in the substation, the neutral point should be directly grounded, and when calculating the normal protection setting value, only the normal operation mode of the transformer neutral point grounding can be considered. When the transformer is repaired, it can be treated in a special operation mode, such as changing the value or stopping and starting the relevant protection section according to the regulations.
2) When the transformer has two or more transformers, only the neutral point of one transformer should be directly grounded, and when the transformer is shut down, the other neutral point ungrounded transformer should be directly grounded. If for some reason, the substation must normally have two transformers with direct grounded neutral points, when one of the transformers with direct grounded neutral points is shut down, if there is a third transformer, the third transformer will be changed to direct grounded neutral point operation. Otherwise, it is handled in special operation mode.
3) When the transformer with double bus operation has three or more transformers, it should be run in the mode of direct grounding of the neutral points of the two transformers, and they are connected to different bus bars. When one of the neutral point direct grounding transformers is shut down, the other neutral point ungrounded transformer is directly grounded. If it is not possible to maintain a ground point on different bus bars, it is treated as a special operation mode.
4) In order to improve the protection cooperation relationship, when a short line is overhauled and shut down, the effect of line outage on the zero-sequence current distribution relationship can be offset by increasing the number of neutral ground transformers.
5) The neutral point of the autotransformer and the transformer with insulation requirements must be directly grounded.
Why do 110KV and above transformers have to ground their neutral points before power failure and transmission?
A: China's 110KV power grid generally uses a neutral direct grounding system. In operation, in order to meet the requirements of the sensitivity of the relay protection device, the neutral point of some transformers is not grounded. However, because the over-voltage caused by the non-synchronous operation of the circuit breaker will endanger the insulation of these transformers, it is required to ground the neutral point of the transformer directly when cutting and closing the no-load transformer of 110KV and above.
