Transformer Main Protection and Backup Protection

1. Main protection

1. Main protection is a protection that meets system stability and equipment safety requirements and can selectively remove protected equipment and line faults as quickly as possible.

2. The main protection of the transformer is differential protection and gas protection.

1) Differential protection

The differential protection of the transformer is designed based on the principle of circulating current. It can correctly distinguish between internal and external faults of the transformer, and can instantly remove faults in the protection zone. Differential protection is the protection for phase-to-phase short circuit of the bushing lead wire, the protection inside the transformer, and the single-phase ground short circuit protection on the side of the neutral point directly grounding system. It can also react to the inter-turn short circuit of the internal winding of the transformer. This protection method cannot respond to poor contact caused by the winding or overheating of the internal core of the transformer. It may also not respond when the number of turns short-circuited between the winding turns is small.

2) Gas protection

When a fault occurs inside the transformer, due to the action of short-circuit current and short-circuit point arc, a large amount of gas will be generated inside the transformer, and the oil flow of the transformer will accelerate. The protection achieved by using gas and oil flow is called gas protection. This protection method can respond to faults such as oil leakage or overheating of the iron core and other components, short circuit between winding turns, single-phase grounding short circuit, and winding phase-to-phase short circuit. Although it can respond to various faults inside the transformer tank, it cannot respond to faults in the bushing lead wires. Therefore, 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 a minor fault or abnormality occurs inside the transformer, the fault point is locally overheated, causing part of the oil to expand. The gas in the oil forms bubbles and enters the gas relay. The light gas protection operates and sends out a light gas signal.

(2) Heavy gas protection

When a serious fault occurs in the transformer oil tank, the fault current is large, and the arc causes the transformer oil to decompose in large quantities, producing a large amount of gas and oil flow. The impact of the baffle causes the heavy gas relay protection to act, send out a heavy gas signal and trip the outlet, cutting off the transformer. Heavy gas protection is the main protection for internal faults in the oil tank and can reflect various faults inside the transformer. When a few inter-turn short circuits occur in the transformer, 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 operate. Therefore, for internal faults in the transformer, it is necessary to rely on heavy gas protection to remove the fault.

2. Backup protection

1. Backup protection is the protection used to remove faults when the main protection or circuit breaker refuses to operate. Backup protection can be divided into two types: far backup protection and near backup protection.

1) Remote backup protection

Remote backup protection is a backup protection implemented by the protection of adjacent power equipment or lines when the main protection or circuit breaker refuses to operate.

2) Near backup protection

Near backup protection is implemented by another set of protection of the power equipment or line when the main protection refuses to operate; when the circuit breaker refuses to operate, the backup protection is implemented by circuit breaker failure protection.

2. The backup protection of the transformer generally includes zero sequence current protection, external phase-to-phase short circuit backup protection, grounding backup protection and overload protection.

1) Zero sequence current protection

Transformers operating in power grids with directly grounded neutral points are equipped with zero-sequence current protection. When the high-voltage side of the transformer or the high-voltage side line is grounded, a zero-sequence current is generated and the zero-sequence current protection operates. It is considered here that the low-voltage side winding of the transformer is delta-connected. It can be used as the protection for the high-voltage winding lead-out line of the transformer and the busbar grounding short circuit. It can also be used as the backup protection for the adjacent lines and the main protection of the transformer itself.

2) Backup protection for external phase-to-phase short circuit

In order to respond to the overcurrent of the transformer caused by an external short circuit of the transformer, and to serve as the main protection of the transformer and the backup protection of adjacent busbars or lines, the transformer needs to be equipped with overcurrent protection. Depending on the transformer capacity and system short-circuit current level, over-current protection, low-voltage starting over-current protection, composite voltage starting over-current protection and negative sequence current protection can be used.

3) Ground backup protection

For transformers with a neutral point directly grounded system with a voltage level of 110kV and above, grounding backup protection should be installed as a near backup for ground faults on the directly grounded side of the transformer and as a remote backup for ground faults on external adjacent components, including busbars and lines.

4) Overload protection

Mainly used in situations where transformer overload causes overcurrent.

HZJ Oil Type Testing Transformer