Corresponding protection devices shall be installed for the short-circuit faults of the transformer lead wires, bushings and internals, and shall meet the following requirements:



1) The single-running transformer of 10MVA and above and the parallel running transformer of 6.3MVA and above should be equipped with longitudinal differential protection. Important transformers with 6.3MVA and below operating independently can also be equipped with longitudinal differential protection.



2) Transformers below 10MVA can be equipped with current quick-break protection and overcurrent protection. For transformers of 2MVA and above, when the current quick-break sensitivity coefficient does not meet the requirements, longitudinal differential protection should be installed.



3) 0.4MVA and above, the primary voltage is 10kV and below, and the coil is a transformer with a delta-star connection, and a two-phase three-relay type overcurrent protection can be used.



4) The various protection devices specified in this article shall operate to disconnect the circuit breakers on each side of the transformer.


During the operation of the transformer, due to internal faults, sometimes we cannot identify and take measures in time, which is easy to cause some accidents. After the protection of the gas relay, the occurrence of similar incidents can be avoided to a certain extent.



■ Introduction to Gas Protection



During the operation of the transformer, due to internal faults, sometimes we cannot identify and take measures in time, which is easy to cause some accidents. After the protection of the gas relay, the occurrence of similar incidents can be avoided to a certain extent.


Gas protection is divided into light gas protection and heavy gas protection. The action principle of light and heavy gas is different. Light gas is because the gas is accumulated in the open cup facing downward, so that the open cup floats up under the action of the buoyancy of the transformer oil, turns on the relay, and issues an alarm, which indicates that the fault in the transformer is slight, and the transformer oil Thermal decomposition produces gas. The heavy gas is due to a serious fault in the transformer, when the transformer oil is heated and rapidly expands and rushes to the oil pillow, the inner baffle of the heavy gas is flushed to a certain angle, and the relay is turned on to generate a signal. Normal input trip position. Unless there is oil filtering, oil replenishment and other operations. Generally, the non-electrical protection of large oil-immersed transformers includes: pressure release, pressure mutation, etc.



Gas protection is one of the main protections of transformers, which belongs to non-electrical protection. The main difference between the so-called light and heavy gas protection is the difference in the setting value of the gas relay. Light gas only sends an alarm signal, not tripping, and heavy gas will trip. The zero-sequence voltage is equal to the phasor sum of the three-phase voltages. The calculation method of zero sequence current is similar.

■ What is the principle of heavy gas protection



It is said that the gas protection of the transformer is based on the principle of the float reed relay. The oil cup is connected to the transformer chamber. When the transformer is overloaded, the overcurrent generates gas to make the float drop to a certain position. The first relay is connected to light. The gas alarm, the gas continues to descend when the float is increased, and the second relay is connected to the heavy gas to trip the switch.



■ The difference between the action principle of light gas protection and heavy gas protection




Gas protection is the main protection of the transformer, which can effectively reflect the internal fault of the transformer. The light gas relay is composed of an open cup, a dry reed contact, etc., and acts on the signal. Heavy gas relays are composed of baffles, springs, reed contacts, etc., which act on tripping.



During normal operation, the gas relay is filled with oil, the open cup is immersed in the oil, is in the floating position, and the reed contacts are disconnected. When the internal fault of the transformer occurs, local overheating occurs at the fault point, which causes the expansion of the nearby transformer oil, and the dissolved air in the oil is expelled, forming bubbles and rising. When the fault is slight, the discharged gas slowly rises and enters the gas relay, causing the oil level to drop. The fulcrum generated by the open cup is the counterclockwise rotation of the shaft, which makes the reed contact connect and sends a signal.


When the internal failure of the transformer is serious, a strong gas gas is generated, which causes the internal pressure of the transformer to increase suddenly, and a large oil flow impacts the direction of the oil pillow. Because the oil flow hits the baffle, the baffle overcomes the resistance of the spring and drives the magnet to dry. The direction of the reed contact moves, so that the reed contact is turned on and acts on the trip.

The relay characteristic of the relay refers to the relationship between the input quantity and the output quantity of the relay in the whole changing process. Whether it is action or return, the relay is from the starting position to the final position, it is impossible to stay in a certain intermediate position. This characteristic is called the relay characteristic of the relay.

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