In the power system, transformers play a vital role. They are responsible for converting electrical energy from one voltage level to another to maintain stability and efficiency during transmission and distribution. To ensure the safe operation of the transformer and extend its life, an effective protection system must be equipped. Among them, primary protection and backup protection are key elements to ensure the safe operation of the transformer.
Primary protection
Primary protection refers to the protection measure that quickly cuts off the power supply when a serious internal fault occurs in the transformer to prevent further damage or catastrophic consequences. The following are several common primary protection technologies:
Differential protection: Differential protection detects whether there is a current difference by comparing the current on different sides of the transformer. If the difference exceeds the set threshold, the system immediately cuts off the power supply to prevent damage that may be caused by continued operation.
Overcurrent protection: Overcurrent protection monitors the current of the transformer. When the current exceeds the set normal operating range, the protection system will act and cut off the power supply to prevent overload damage.
Short-circuit protection: Short-circuit protection detects possible short circuits inside the transformer. Once a short circuit is detected, the protection system quickly cuts off the power supply to avoid serious damage.
Backup protection
Backup protection is an auxiliary protection measure to ensure that the transformer is protected from damage even if the main protection system fails or fails to work properly. Backup protection usually includes the following technologies:
Differential ratio protection: Differential ratio protection is a backup protection mechanism that is activated when the main differential protection fails to work properly. It determines whether there is a fault inside the transformer by comparing the ratio of differential current to rated current.
Remote end protection: Remote end protection is a method of detecting internal faults by monitoring the relationship between the current and voltage at the remote end of the transformer. When the remote signal is abnormal, the system will initiate protection action.
Overvoltage protection: Overvoltage protection monitors the voltage of the transformer. When a voltage outside the set range is detected, the protection system will cut off the power supply to prevent equipment damage.
Transformers are static devices that run continuously, and their operation is relatively reliable and there are 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, short-circuit faults between turns of one-phase windings, short-circuit faults between windings and iron cores, and wire breakage faults of windings.
External faults refer to various phase-to-phase short-circuit faults between external lead-out wires of transformers, and single-phase grounding faults caused by flashover of lead-out insulation bushings 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 the transformer mainly include overload, overcurrent caused by external short circuits, too high oil temperature of the operating transformer, too high winding temperature, too high transformer pressure, and cooling system failure. When the transformer is in an abnormal operating state, an alarm signal should be given.
2. Configuration of transformer protection
The main protection for short-circuit faults: is longitudinal differential protection, heavy gas protection, etc.
Backup protection for short-circuit faults: 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 fault protection, etc.
3. Non-electrical quantity protection
Transformer protection that uses non-electrical quantities such as oil and gas and the temperature of the transformer is called non-electrical quantity protection. There are mainly gas protection, pressure protection, temperature protection, oil level protection, and cooler full-stop protection. Non-electrical quantity protection acts on tripping or sending signals according to on-site needs.
4. Differential protection
Transformer differential protection is the main protection of transformer electrical quantities, and its protection range is the part surrounded by the current transformers on each side. When faults such as winding phase short circuits and turn-to-turn short circuits occur within this range, the differential protection must act.
5. Transformer grounding protection
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 a voltage of 110kV and above with direct neutral point grounding, zero-sequence current protection that responds to grounding 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 be directional, and the direction should preferably point to the busbars on each side.
(2) Neutral point ungrounded mode
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 zero-sequence overcurrent protection. Therefore, 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 grounding through a discharge gap
UHV transformers are all semi-insulated transformers, in which the insulation of the neutral point coil to the ground is weaker than that of other parts. The 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 an ungrounded neutral point.
