Industry professionals analyze common protection methods for transformers

The abnormal working conditions of transformers mainly include overload, overcurrent caused by external short circuit, neutral point overvoltage caused by external ground short circuit, oil level drop caused by oil tank leakage or temperature rise caused by cooling system failure, etc. In addition, large-capacity transformers have high rated working magnetic flux density, and the working magnetic flux density is proportional to the voltage-frequency ratio. When operating under overvoltage or low frequency, it may cause overexcitation faults of the transformer.

In view of the above situation, large transformers generally adopt the following protection methods :

1. Gas protection: protects the transformer from internal short circuit and oil level drop.

2. Differential protection and current quick-break protection: protect the transformer windings or lead wires from phase-to-phase short circuits, large ground current systems from ground short circuits, and winding turn-to-turn short circuits.

3. Overcurrent protection: protect against external phase-to-phase short circuit and serve as backup protection for gas protection and differential protection (or current quick-break protection).

4. Zero-sequence current protection: protect external single-phase ground short circuit of large ground current system.

5. Overload protection: protection against symmetrical overload, only acts on the signal.

6. Overexcitation protection: protect the transformer from overexcitation exceeding the allowable limit.

Transformer gas protection responds to various faults and oil level reduction inside the transformer oil tank. Oil-immersed transformers of 0.8MVA and above and oil-immersed transformers in workshops of 0.4MVA and above should be equipped with gas protection. When a fault in the oil tank produces slight gas or the oil level drops, it should act on the signal instantly; when a large amount of gas is produced, it should act to disconnect the circuit breakers on both sides of the transformer. The voltage regulating device of oil-immersed transformers with load voltage regulation should also be equipped with gas protection.

Transformer protection method 2: longitudinal differential protection or current quick-break protection

The longitudinal differential protection or current quick-break protection for transformer lead wire, bushing and internal short circuit faults. The protection acts instantly on the circuit breakers on each side of the transformer.

1. For plant transformers and parallel-operated transformers below 6.3MVA, and plant standby transformers and independently operated transformers below 10MVA, when the backup protection time is greater than 0.5s, current quick-break protection should be installed.

2. Longitudinal differential protection should be installed for plant working transformers and parallel-operated transformers of 6.3MVA and above, plant standby transformers and independently operated transformers of 10MVA and above, and transformers of 2MVA and above whose current quick-break protection sensitivity does not meet the requirements.

3. For transformers with high-voltage side voltage of 330kV and above, double longitudinal differential protection can be installed.

4. For generator transformer groups, when there is a circuit breaker between the generator and the transformer, the generator is equipped with a separate longitudinal differential protection. When there is no circuit breaker between the generator and the transformer, the generator and the transformer group of 100MVA and below share the longitudinal differential protection; for generators above 100MVA. In addition to the shared longitudinal differential protection of the generator and transformer, the generator should also be equipped with a separate longitudinal differential protection. For the generator transformer group of 200~300MVA, a separate longitudinal differential protection can also be added to the transformer, that is, double fast protection is adopted.

The reaction transformer external phase-to-phase short circuit is backed up by overcurrent protection , low voltage starting overcurrent protection , compound voltage starting overcurrent protection, negative sequence current protection and impedance protection. After the protection is actuated, it should trip with a time limit.

1. Overcurrent protection should be used for step-down transformers.

2. Compound voltage starting overcurrent protection is suitable for step-up transformers, system interconnection transformers and step-down transformers whose overcurrent protection does not meet the sensitivity requirements.

3. Negative sequence current and single-phase low voltage starting overcurrent protection can be used for step-up transformers of 63MVA and above.

4. When the protections in 2 and 3 above cannot meet the requirements of sensitivity and selectivity, impedance protection can be used. Common transformer protection method 4: zero-sequence current protection

Zero-sequence current protection for external ground short circuit of transformer in large ground current system . In large ground current system of 110kV and above, if the neutral point of transformer may be grounded, zero-sequence current protection should be installed for step-up transformer or step-down transformer with two or three power sources as backup protection for main protection of transformer and backup protection for adjacent components.

What is zero-sequence current protection?

The device that uses the zero-sequence current generated during grounding to activate protection is called zero-sequence current protection. Special zero-sequence current transformers are used on cable lines to achieve grounding protection. The zero-sequence current transformer is placed on a three-core grounding cable, and the current relay is connected to the secondary coil of the transformer. During normal operation or when there is no grounding fault, since the vector sum of the three-phase current of the cable is equal to zero, the current of the secondary coil of the zero-sequence transformer is also zero (only a small unbalanced current), so the current relay does not operate. When a grounding fault occurs, a large current will appear in the secondary coil of the zero-sequence transformer, causing the current relay to operate in order to send a signal or cut off the fault.

Common transformer protection method 5: overload protection

Overload protection for reactive transformer symmetrical overload.

For transformers of 400kVA and above , when several units are operated in parallel or individually and used as backup power for other loads, overload protection should be installed according to possible overload conditions. For autotransformers and multi-winding transformers, the protection device should be able to respond to the overload conditions of the common winding and each side. In most cases, the overload current of the transformer is three-phase symmetrical, so the overload protection only needs to be connected to one phase of current, and the current relay is realized, and acts on the signal after a certain delay. When choosing which side to install the protection, it should be considered that it can reflect the overload conditions of all coils on each side of the transformer. In substations without regular on-duty personnel, the overload protection can act on tripping or disconnecting part of the load when necessary.

Transformer protection method six: overexcitation protection

Overexcitation protection for reactive transformer overexcitation.

In the current design of large transformers, in order to save materials, reduce costs, and reduce transportation weight, the rated working magnetic flux density of the core is designed to be relatively high, about 1.7~1.8 T, close to the saturation magnetic flux density (1.9~2 T), so it is easy to produce overexcitation under overvoltage conditions. In addition, because the magnetization curve is relatively "hard", when overexcited, due to the saturation of the core, the excitation impedance decreases, and the excitation current increases rapidly. When the working magnetic flux reaches 1.3~1.4 times the normal magnetic flux, the excitation current can reach the rated current level. Secondly, since the excitation current is a non-sinusoidal wave and contains many high-order harmonic components, and the eddy current loss of the core and other metal components is proportional to the square of the frequency, it can cause serious overheating of the core, metal components, and insulating materials. If the overexcitation multiple is high and the duration is too long, the transformer may be damaged. Therefore, transformers with a high-voltage side of 500kV should be equipped with overexcitation protection.

The purpose of installing transformer overexcitation protection is to detect the overexcitation of the transformer , send out signals or trip in time, so that the overexcitation of the transformer does not exceed the allowable limit, and prevent the transformer from being damaged due to overexcitation.