1. Transformer fault types and characteristics
Power transformer is mainly composed of iron core and winding around the core composition. To ensure the insulation between the windings, as well as core, winding cooling needs, the core and winding placed in the tank with transformer oil. The two ends of the transformer windings are through the insulating sleeve to guide the transformer housing outside.
Power transformer fault can be divided into internal fault tank and tank external fault two categories. The main internal faults of the fuel tank are the interphase short circuits of the windings, the inter-turn short circuit between the turns of the single-phase windings, the single-phase windings short-circuited by the single-phase windings and the damage of the iron core. Transformer tank fault is very dangerous, because the fuel tank is full of transformer oil, after the failure of a strong short-circuit current transformer oil decomposition gasification may produce a large amount of flammable gas, it is easy to cause the fuel tank explosion. Faults outside the tank are mainly the short-to-phase and short-to-ground faults that occur on the bushing and pinout.
2. Transformer not in normal operation
Abnormal power transformer operating conditions include external short-circuit transformer caused by over-current, load over time caused by rated capacity overload, fan failure or leakage caused by the decline in cooling capacity and so on. For transformers with non-directly grounded neutral point, the external neutral grounding may cause over-voltage of transformer neutral point, threatening transformer insulation. For large-capacity transformers, due to core rated flux density and magnetic flux density close to the saturation, so when the external over-voltage is too high or frequency is prone to over-excitation.
3. The basic principles of transformer protection configuration
Gas protection
Gas protection is used to react the gas or oil flow generated inside the transformer tank. It can prevent all kinds of short-circuit faults and oil level reduction in the transformer tank, and has high sensitivity. Gas protection has heavy gas and light gas points. General heavy gas response is the internal oil flow rate of the transformer, the protection of the action in the jump on each side of the power transformer circuit breakers, light gas protection reaction is the internal gas volume of the transformer, the protection of the action signal.
Oil-immersed transformers of 800 kVA and above and oil-immersed transformers of 400 kVA and above shall be provided with gas protection. For load-regulating oil-immersed transformer voltage regulator should also be installed gas protection.
Differential protection or current instantaneous protection
6300kVA and above parallel operation of transformers, 10000kVA and above individually run transformers, power plant or industrial enterprises own use 6300kVA and above important transformers should be installed differential protection. Other power transformers should be installed current instantaneous protection, the over-current protection action time should be greater than 0.5S. For more than 2000kVA transformer, when the current instantaneous protection sensitivity can not meet the requirements, it should also be installed differential protection. Differential protection is used to react short-circuit faults in the power transformer windings, bushings and lead wires. The protective action is to trip the corresponding circuit breakers of each power supply side of the transformer.
Phase short-circuit backup protection
Phase short circuit protection is used to react the external short circuit caused by the transformer overcurrent protection, and as the gas protection and differential protection (or current protection) backup protection, the action time limit according to the current protection ladder principle to set the delay Action in the jump off the power transformer side circuit breakers, the corresponding signal. Generally used over-current protection, composite voltage starting over-current protection or negative sequence current single-phase low voltage protection.
Ground fault of the zero sequence protection
Zero-sequence protection shall be provided for transformers in neutral-grounded direct systems. Zero-sequence protection shall be used to react to the high voltage side (or medium voltage side) of the transformer as well as the short to ground of external components.
For autotransformer and high, medium voltage side of the neutral point are directly grounded three-winding transformer, when the selectivity requirements should be additional zero-sequence directional components.
When only part of the transformer center line grounding operation, in order to prevent the occurrence of short circuit, the neutral point grounding transformer jumps, the neutral ungrounded transformer (low voltage side of the power) may still continue to run with a ground fault, Resulting in over-voltage, the threat of insulation, it should be installed according to the circumstances zero sequence over-voltage protection, zero-sequence current protection gap.
Overexcitation protection
Since the magnetic flux density B in the transformer core is proportional to the ratio of voltage and frequency U / f, it will cause over-excitation of the transformer when the voltage increases and the frequency decreases, so that the excitation current Increase, resulting in increased iron loss, core temperature and winding temperature rise, in serious cases to cause local deformation and damage surrounding insulation medium. Overexcitation protection is reflected in the ratio of the actual working magnetic flux and the rated working magnetic flux density (over-excitation ratio). In the transformer allows the over-excitation range, over-excitation protection effect on the signal, when the over-excitation can exceed the allowable value in the trip.
Other non-power protection
The transformer is also usually equipped with non-electricity protection features that reflect the oil, gas and temperature in the fuel tank, including the temperature protection of the transformer body and the on-load voltage regulation part, the pressure release protection of the transformer, the protection of the air- Overload lock with load regulator protection.
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