Transformers are static equipment that runs continuously, and their operation is relatively reliable with fewer chances of failure. However, since most transformers are installed outdoors and are affected by the load they bear during operation and the short-circuit faults of the power system, various faults and abnormalities are inevitable during operation.
1. Common faults and abnormalities of transformers
Transformer faults can be divided into internal faults and external faults.
Internal faults refer to faults that occur inside the casing, including phase-to-phase short-circuit faults of windings, turn-to-turn short-circuit faults of one-phase windings, short-circuit faults between windings and iron cores, and wire break faults of windings.
External faults refer to various phase-to-phase short-circuit faults between external lead wires of the transformer, and single-phase grounding faults that occur when the lead wire insulation bushing network passes through the casing.
Transformer failures are very harmful. Especially when internal faults occur, the high-temperature arc generated by the short-circuit current will not only burn the insulation and iron core of the transformer windings, but also cause the transformer oil to decompose by heat to produce a large amount of gas, causing the transformer casing to deform or even explode. Therefore, the transformer must be removed when it fails.
The abnormal conditions of transformers mainly include overload, overcurrent caused by external short circuit, over-temperature of transformer oil, over-temperature of winding, over-pressure of transformer, and cooling system failure. When the transformer is in abnormal operation, an alarm signal should be given.
2. Configuration of transformer protection
Main protection for short-circuit fault: mainly longitudinal differential protection, heavy gas protection, etc.
Backup protection for short-circuit fault: mainly composite voltage lockout overcurrent protection, zero-sequence (direction) overcurrent protection, low impedance protection, etc.
Abnormal operation protection: mainly overload protection, overexcitation protection, light gas protection, neutral point gap protection, temperature oil level and cooling system failure protection, etc.
3. Non-electrical quantity protection
Transformer protection composed of non-electrical quantities such as oil, gas and temperature of transformer is called non-electrical quantity protection. There are mainly gas protection, pressure protection, temperature protection, oil level protection and full stop protection of cooler.
4. Differential protection
Transformer differential protection is the main protection of transformer electrical quantity, and its protection range is the part surrounded by current transformers on each side. When faults such as interphase short circuit and interturn short circuit occur within this range, the differential protection must be activated.
5. Grounding protection of transformer
The backup protection for grounding short circuit faults of large and medium-sized transformers usually includes: zero-sequence overcurrent protection, zero-sequence overvoltage protection, gap protection, etc.
(1) Direct neutral point grounding
For transformers with voltage of 110kV and above and direct neutral point grounding, zero-sequence current protection that responds to ground faults should be set on the high current grounding system side. For transformers with direct grounding on both high and medium sides, the zero-sequence current protection should have direction, and the direction should preferably point to the busbars on each side.
(2) Neutral point ungrounded mode
The zero-sequence current forms a zero-sequence loop through the neutral point of the transformer. However, if the neutral points of all transformers are grounded, the short-circuit current at the grounding point will be diverted to each transformer, which will reduce the sensitivity of the zero-sequence overcurrent protection. Therefore, in order to limit the zero-sequence current within a certain range, there are regulations on the number of transformers with neutral point grounding.
(3) Neutral point grounded through discharge gap
UHV transformers are all semi-insulated transformers, in which the neutral point coil has weaker insulation to ground than other parts. Neutral point insulation is easily broken down. Therefore, gap protection is required.
The function of gap protection is to protect the insulation safety of the neutral point of the transformer with ungrounded neutral point.
