Full Shutdown Protection of Coolers
The full shutdown protection of coolers is a kind of relay protection method used to detect faults in the transformer cooling system. When the cooler malfunctions or stops operating, the temperature of the transformer may rise, leading to damage to the transformer. The full shutdown protection of coolers detects faults in the cooling system by monitoring the flow rate or temperature of the cooling water. When an abnormality is detected, the trip circuit for the full shutdown of the cooler is activated. The protection device will trigger an alarm or trip.
The transformers used in modern power plants generally have a relatively small magnetic resistance. Therefore, only large transformers adopt the method of tripping when the coolers are fully shut down for transformer protection, and the time is generally 20 minutes, which has nothing to do with the temperature of the transformer oil and the winding. For forced oil circulation air-cooled transformers, when all coolers are removed due to a cooling system failure, it is allowed to operate with the rated load for 20 minutes. If the top oil temperature has not reached 75°C after 20 minutes, it is allowed to rise to 75°C, but the maximum operating time in this state shall not exceed 1 hour. When the set value is reached, the non-electric quantity protection acts to fully stop the transformer.
How to Deal with the Full Shutdown of Transformer Coolers
(1) Report to the leader in a timely manner, notify the maintenance personnel, and closely monitor the change of the upper oil temperature of the transformer.
(2) If both power supplies disappear or malfunction, efforts should be made immediately to restore the power supply.
(3) If one power supply disappears or malfunctions and the automatic switching-on of the other standby power supply fails, check whether the standby power supply is normal. If it is normal, go to the site immediately and manually close the standby power supply switch.
(4) When the cooler stops operating due to the fusing fault of the cable terminal, directly pull open the faulty power supply switch in the station service power distribution room. If the automatic switching-on of the standby power supply fails, go to the site and manually close the standby power supply switch.
(5) If the main power supply (or standby power supply) switch trips and the automatic switching-on of the standby power supply switch fails at the same time, manually close the standby power supply switch. If it trips again after being closed, it indicates that there is an obvious fault in the common control circuit. In this case, emergency measures should be taken (close the emergency power switch or temporarily connect a power cord to bypass the faulty part).
(6) If the small switch of the control circuit trips, try to close it once. If it trips again, it indicates that there is an obvious fault in the control circuit, and it can be dealt with according to the aforementioned method.
(7) If there is a fault in the automatic switching-in circuit of the standby power supply or the control operation circuit for power supply switching-in, it should be changed to manual control for the switching-in of the standby power supply or directly manually operate to close the power supply switch.
(8) If it is difficult to find out and eliminate the fault in a short time and the cooler device cannot be restored to operation quickly before the transformer trips, preparations should be made to put the standby transformer or standby power supply into operation.
(9) When the full shutdown time of the cooler approaches the specified time (20 minutes), and there is no standby transformer or the standby transformer cannot carry the full load, if the upper oil temperature has not reached 75°C (for the transformer with the fully shut-down cooler), according to the leader's order, the pressure plate of the trip circuit for the full shutdown of the cooler can be temporarily removed, and continue to deal with the problem to restore the cooling device; if the upper oil temperature of the transformer rises and exceeds 70°C or although it has not exceeded 75°C but the full shutdown time has reached 1 hour and the problem has not been solved, the standby transformer should be put into operation, the load should be transferred, and the faulty transformer should stop operating.
IV. Oil Pressure Over-speed Protection
The quick-acting oil pressure protection of the transformer refers to that when the transformer has a short circuit or overload, in order to protect the transformer from being damaged, it is controlled to act through the quick-acting oil pressure relay; it acts by utilizing the dynamic pressure increase rate in the oil tank caused by the accident. When the oil level or oil pressure reaches a certain value, the switch acts to cut off the power of the transformer and avoid the occurrence of an accident.
Protection Measurement and Starting Element: Sudden Pressure Relay (Quick-acting Oil Pressure Relay)
Operating Principle: It acts by utilizing the dynamic pressure increase rate (change in oil pressure) in the oil tank caused by the accident. The faster the oil pressure increases, the quicker it acts; since the propagation speed of the oil pressure wave in the transformer oil is extremely fast, the quick-acting oil pressure relay is sensitive in response, accurate in action, and can quickly send out a signal and cut off the power. If a quick-acting oil pressure relay is installed on the transformer, once a serious internal short circuit fault occurs, it can prevent the explosion of the oil tank.
When the transformer is operating normally, because the installation position of the quick-acting oil pressure relay is lower than the oil level line of the transformer oil, the oil chamber of the relay is connected to the oil tank of the transformer. Under normal circumstances, the quick-acting oil pressure relay does not act. When a fault occurs inside the transformer, the pressure in the oil chamber suddenly rises. When the rising speed exceeds a certain value and the pressure reaches the operating value, the pressure switch acts, sends out a signal and cuts off the power to make the transformer withdraw from operation. A balancer is installed inside the relay. Through the function of the balancer, when the internal pressure of the transformer slowly increases due to non-fault reasons but is lower than a certain value, the pressure switch will not act.
V. Explosion-proof Protection
Currently, the explosion-proof devices adopted for transformers include explosion-proof pipes and pressure relief valves. The main function of the explosion-proof pipe of the transformer is to prevent the oil tank from cracking. The pressure relief valve is used to release the gas and excessive pressure generated by the transformer fault, and the pressure is released through the pressure relief valve to ensure the safety of the tank body and prevent accidents such as transformer explosion.
Protection Measurement and Starting Element: Operating Principle of the Transformer Pressure Relief Valve
The pressure relief valve can enable the transformer to quickly open within 2 ms when the pressure inside the oil tank rises to the opening pressure of the pressure relief valve due to a fault inside the oil tank, so that the pressure inside the transformer oil tank can be quickly reduced. When the pressure drops to the closing pressure value, the pressure relief valve will reliably close, so that the inside of the transformer oil tank always maintains a positive pressure, effectively preventing external air, moisture and other miscellaneous substances from entering the oil tank.
Characteristics of the Pressure Relief Valve
(1) Quick opening performance.
(2) Automatic reset function.
(3) Automatic signaling function.
Pressure Relief Capacity of the Pressure Relief Valve
The pressure relief valve is a mechanical pressure relief device. To adapt to the particularity of the fully sealed operation of the transformer, the relief diameter can only reach 130mm. The designed pressure relief capacity of the pressure relief valve with this diameter can only protect the pressure generated by the fault when the fault pressure rate inside the transformer is lower than 15kPa/ms. If the fault pressure rate exceeds this limit value, it is likely to cause damage to the transformer oil tank.
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