Transformer Main Protection And Backup Protection

1, the main protection is to meet the requirements of system stability and equipment safety, and can selectively cut the protection of the protected equipment and line faults at the fastest speed. 2, the main protection of the transformer is differential protection and gas protection. 1) The differential protection of the transformer is designed according to the principle of circulating current, which can correctly distinguish the internal and external faults of the transformer, and can remove the faults in the protection area instantaneously. Differential protection is the interphase short circuit of the casing lead line, the protection inside the transformer, and the single-phase ground short circuit protection on the side of the neutral direct grounding system, and can also reflect the inter-turn short circuit of the winding inside the transformer. This protection method can not react to the poor contact caused by the winding, the overheating of the iron core inside the transformer, and may not react when the number of short turns between the winding turns is small. 2) Gas protection When the transformer internal failure, due to the short-circuit current and short-circuit arc, the transformer will produce a large amount of gas, while the transformer oil flow speed up, the use of gas and oil flow to achieve protection is called gas protection. This protection method can react the faults of oil leakage or overheating of iron core and other components, short-circuit between winding turns, single-phase grounding short-circuit and short-circuit between winding. Although it can react various faults inside the transformer tank, it can not react the failure of the bushing outlet line, so the gas protection and differential protection together form the main protection of the transformer. Gas protection is divided into light gas protection and heavy gas protection.
(1) Light gas protection When there is a slight fault or abnormal inside the transformer, the fault point is overheating, causing some oil to expand, the gas in the oil to form bubbles into the gas relay, the light gas protection action, and the light gas signal.
(2) Heavy gas protection When a serious fault occurs in the transformer tank, the fault current is large, the arc causes a large amount of transformer oil to decompose, resulting in a large amount of gas and oil flow, the impact plate makes the heavy gas protection action, sends a heavy gas signal and the exit trip, and cuts the transformer. Heavy gas protection is the main protection for internal faults of the tank, and can reflect various faults inside the transformer. When the transformer has a few inter-turn short circuits, although the fault current is large, the differential current generated in the differential protection may not be large, and the differential protection may refuse to move. Therefore, for the internal fault of the transformer, it is necessary to rely on heavy gas protection to remove the fault. 2, backup protection 1, backup protection is the main protection or circuit breaker refused to move, used to remove the fault protection. Backup protection can be divided into two types: far backup protection and near backup protection. 1) Far backup protection Far backup protection is the backup protection achieved by the protection of adjacent power equipment or lines when the main protection or circuit breaker refuses to move. 2) Near backup protection Near backup protection is realized by another set of protection of the power equipment or line when the main protection refuses to move; When the circuit breaker refuses to operate, the backup protection is realized by the circuit breaker failure protection. 2, the transformer backup protection generally has zero sequence current protection, external short-circuit backup protection, grounding backup protection and overload protection. 1) Zero sequence current protection Transformers operating in the neutral direct grounding network are equipped with zero sequence current protection. When the high-voltage side of the transformer or the high-voltage side line is grounded, zero sequence current is generated and zero sequence current protection action is taken. Here it is considered that the low-voltage side winding of the transformer is triangular connection. It can be used as the transformer high-voltage winding lead line, busbar grounding short circuit protection, but also can be used as the backup protection of adjacent lines and transformer itself main protection. 2) The backup protection of the external phase short circuit is the transformer overcurrent caused by the reaction transformer external short circuit, and as the backup protection of the transformer main protection, adjacent bus or line, the transformer needs to be equipped with overcurrent protection. According to the transformer capacity and the system short circuit current level, overcurrent protection, low voltage start overcurrent protection, compound voltage start overcurrent protection and negative sequence current protection can be used. 3) Grounding backup protection for the neutral direct grounding system transformer with a voltage grade of 110kV and above, grounding backup protection should be installed as the near backup for the direct grounding fault of the transformer and the far backup protection for the external adjacent components including the bus and the line grounding fault. 4) Overload protection
It is mainly used in the case of overcurrent caused by transformer overload.