Power transformer is one of the most important power equipment in power system. Compared with generator and high-voltage transmission line, its failure probability is relatively low. But once the fault occurs, it will bring serious influence to the power supply reliability and operation safety of the system. In order to ensure the safe operation of transformer and prevent the expansion of accidents, the transformer should be provided with reliable, fast and sensitive and selective protection according to its capacity, structure and fault type. According to the regulations of "technical regulations for relay protection and automatic device of power system", effective protection methods are adopted for transformer faults:
(1) gas protection. To reflect the internal failure of the oil tank and the reduction of oil level, oil-immersed transformers with A capacity of 0.8MV·A and above and transformers with A capacity of 0.4MV·A and above should be equipped with gas protection in the workshop. When the internal fault of the oil tank produces slight gas or the oil level drops, the instantaneous action should be on the signal; When a large amount of gas is produced, the circuit breaker on each side of the transformer should be disconnected. When there is no circuit breaker or short-circuit switch on the power side of the transformer installation, the signal can be acted on.
(2) longitudinal differential protection or current break protection. Used to reflect short circuit of transformer winding, casing and lead wire. Direct grounding system side winding, casing and lead wire grounding short circuit and winding between turns short circuit. Transformers with capacity of 10MV·A and above should be operated separately, and transformers with capacity of 6.3MV·A and above should be operated in parallel, and factory transformers with capacity of 6.3MV·A and above should be equipped with longitudinal differential protection. Transformers below 10MV·A can be equipped with current current break protection and overcurrent protection; When the sensitivity coefficient of current current break protection does not meet the requirements, it is advisable to install longitudinal differential protection.
(3) overcurrent protection. For the overcurrent caused by short circuit between external phases, corresponding protective devices should be installed. For example, compound voltage start overcurrent protection device or negative sequence overcurrent protection device, suitable for booster transformer; Overcurrent protection is suitable for step-down transformers.
(4) zero-sequence current protection. In the 110kV power grid with direct grounding of neutral point, when the neutral point of the transformer with power supply on the low-voltage side is directly grounded, zero sequence current protection shall be installed for the over-current caused by external single-phase grounding.
(5) overload protection. Reflects the transformer overload state. When several transformers above 400kV·A are running side by side or separately and serve as backup power supply for other loads, overload protection shall be installed according to the possible over-load situation. When the transformer is overloading, it should use the overload protection to send out the signal, and it can also be applied to the tripping device or automatic load reducing device in the substation which is not on duty.
(6) overexcitation protection. For transformers with a voltage of 500kV or above on the high voltage side, an over-excitation protection should be installed to reflect the over-excitation state of the transformer. In the over-excitation range allowed by the transformer, the protection ACTS on the signal, and when the over-excitation exceeds the allowable value, it can operate on the tripping device. Overexcitation protection is reflected in the ratio of actual working flux density to rated working flux density (called overexcitation multiple).
(7) backup protection. Such as impedance protection, compound voltage starting overcurrent protection device, undervoltage starting overcurrent protection device can reflect the transformer overcurrent state, but their sensitivity is not the same, impedance protection sensitivity is the highest, the sensitivity of the lowest overcurrent protection.
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