Which device is used to relieve internal pressure in a transformer?

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Relieving internal pressure in an oil-filled transformer is handled by a pressure relief valve. When the oil and gas inside the transformer generate pressure—due to overheating, fault conditions, or gas formation—the valve stays closed until the pressure reaches its preset limit. At that point it opens and vents gas and oil to a safe path, preventing the tank from rupturing and helping to maintain safe, continued operation.

Other devices shown have different roles: a cooling fan helps remove heat but doesn’t vent pressure; a fusible link protects against overcurrent by opening the circuit; an expansion valve is used in refrigeration systems to control refrigerant flow. Some transformers may use rupture disks as an alternative or complementary relief method, but the core function is the same—to relieve internal pressure and protect the transformer.