Transformer (Transformer) is a device that uses the principle of electromagnetic induction to change the AC voltage. The main components are the primary coil, the secondary coil and the iron core (magnetic core). The main functions are: voltage transformation, current transformation, impedance transformation, isolation, voltage stabilization (magnetic saturation transformer), etc. According to the purpose, it can be divided into: power transformers and special transformers (electric furnace transformers, rectifier transformers, power frequency test transformers, voltage regulators, mining transformers, audio transformers, intermediate frequency transformers, high frequency transformers, impact transformers, instrument transformers, electronic transformers , Reactors, transformers, etc.). Circuit symbols often use T as the beginning of the number. Example: T01, T201, etc.
Configuration of transformer protectionThe main protection of short-circuit fault: mainly includes longitudinal differential protection, heavy gas protection, etc.
Back-up protection for short-circuit faults: mainly include compound voltage blocking overcurrent protection, zero sequence (directional) overcurrent protection, low impedance protection, etc.
Abnormal operation protection: mainly includes overload protection, overexcitation protection, light gas protection, neutral point gap protection, temperature oil level and cooling system failure protection, etc.
1. Gas protection
In order to reflect the internal fault of the transformer tank and the decrease of the oil level, gas protection should be installed for oil-immersed transformers with a capacity of 800kV·A and above and oil-immersed transformers in workshops with a capacity of 400kV·A and above. Gas protection is also called gas protection, which acts in response to the amount of gas or oil flow speed generated by the internal failure of the fuel tank. Among them, the heavy gas protection acts on the tripping of the circuit breakers on each power side of the transformer, and the light gas protection acts on the signal.
2. Longitudinal differential protection or current quick-break protection
Longitudinal differential protection or current quick-break protection is used to reflect the phase-to-phase short circuit of transformer windings, bushings and lead wires. The neutral point is directly grounded to the system side winding and lead wire of the system side winding and lead wire to ground short circuit and winding turn short circuit. Depending on the transformer capacity, choose to install longitudinal differential protection or current quick-break protection.
For transformers with a capacity of 6.3MV·A and above in parallel operation and transformers with a capacity of 10MV·A and above in separate operation, and for factory transformers with a capacity of 6.3MV·A and above, longitudinal differential protection shall be installed; for a capacity of 6.3MV· For transformers running in parallel below A and transformers running separately below 10MV·A, current quick-break protection should be installed when the backup protection time limit is greater than 0.5s; for transformers with a capacity of 2MV·A and above, when the current quick-break protection sensitivity does not meet the requirements At the time, the longitudinal differential protection should be installed; for the transformer with the voltage of 330kV and above on the high voltage side, double differential protection can be installed.
3. Back-up protection of transformer phase-to-phase short circuit
For transformer overcurrent caused by external phase-to-phase short circuit, the following protections should be used as backup protection for gas protection and longitudinal differential protection (or current quick-break protection).
(1) Overcurrent protection
Usually used for step-down transformers, the setting value of the protection device should consider the possible overload in the accident state.
(2) Overcurrent protection started by compound voltage
Usually used for step-up transformers, system tie transformers and step-down transformers whose overcurrent protection sensitivity does not meet the requirements.
(3) Negative sequence current protection and overcurrent protection for single-phase low voltage start
Usually used for step-up transformers with a capacity of 63MV·A and above.
(4) Impedance protection
For step-up transformers and system tie transformers, when the overcurrent protection of composite voltage startup or the overcurrent protection of negative sequence current and single-phase low voltage startup cannot meet the sensitivity and selectivity requirements, impedance protection can be used as a short-circuit between transformer phases. Backup protection.
For dual-winding transformers, impedance protection is installed on the high-voltage side; for three-winding transformers and autotransformers, impedance protection is installed on the high-voltage side and the medium-voltage side. The protection can have two time limits, which can reduce the influence range of the fault with a shorter time limit, and act on the tripping of the circuit breakers on each side of the transformer with a longer time limit.
4. Back-up protection for transformer ground fault
In the neutral point directly grounded power grid, zero-sequence protection is installed as a backup protection when the transformer is externally grounded and short-circuited.
5. Overload protection for capacity
For transformers of 0.4MV·A and above, when several units are operated in parallel or individually and used as backup power sources for other loads, overload protection should be installed according to possible overload conditions.
6. Overexcitation protection
For large-capacity transformers with voltage levels of 500kV and above, an overexcitation protection that acts in response to the rise of the Naosheng current should be installed.
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