Oil-immersed transformer have several separate oil systems. When the oil-immersed transformer is running, the oil in these independent oil systems is not interrelated, and the oil quality and operating conditions are different. Gas chromatographic analysis of oil should be done separately to determine whether there are potential faults or not.

oil immersed transformer structure
oil immersed transformer structure

1.Main internal oil system of Oil-immersed transformer.

The oil system connected with the oil around the winding is the main internal system, including the oil in the cooler or radiator, the oil in the oil storage tank, and the oil in the oil injection casing of 35kV and below.

When injecting transformer cooler oil, the gas drain plug stored in the oil system must be released. Generally speaking, each of the above components should have its own gas drain plug. The main internal oil plays the role of insulation and cooling.

Transformer Oil can also increase the electrical strength of insulating paper or insulating cardboard. In vacuum oil injection, if some parts can not bear the same vacuum intensity as the main tank, temporary gate isolation, such as the gate valve between the tank and the main tank, should be applied. The head of the submersible pump on the cooler should be enough to avoid suction of air due to negative pressure. The oil system should have a protective system of pressure relief devices to eliminate the pressure generated by the failure of the body.

2.Oil in switching switch room of on-load tap-changer.

This part of the oil has its own protection system, namely flow relay, oil storage tank, pressure relief valve. The oil in the switch room acts as insulation and extinguishing current. Transformer Oil will go into the oil produced when the switch cuts off the load current. The oil system should have good sealing performance, even if the arc pressure is generated during the switching process, it should also protect the sealing performance.

Although the oil in the switching switch chamber of on-load tap changer is separated from the oil in the main body, in order to avoid damaging the seal of the switching switch chamber during vacuum oil injection, the oil in the main body should be injected in vacuum at the same time. When vacuum oil is injected, the two systems should have the same vacuum degree, and if necessary, the oil storage tank of the system should be separated during vacuum pumping.  For the convenience of the structure, the main oil storage unit and the oil storage tank of the switching switch room are designed as an isolated whole.

3.Fully sealed at voltage levels of 60 kV and above.

The main function of this oil system is to insulate, or to increase the electrical strength of insulating paper in oil capacitive bushing. When injecting oil into the main body, the terminal of casing end should be sealed well to prevent air intake.

4.Oil in high-pressure outlet box or point gas outlet box.

The high voltage output line of three-phase 500kV Oil-immersed transformer passes through corrugated insulation oil isolation system. This oil system mainly acts as insulation. In order to simplify the structure, the oil system can also be connected with the main internal oil system through connecting pipes or designed as a separate oil system.

5.Oil-immersed transformer insulation test

When conducting various insulation tests on Oil-immersed transformers, the first step is to exhaust gas and release the possible stored gas through the exhaust plug. Potential faults can be predicted by analyzing the gas chromatographic analysis of oil in each system. Every oil system must meet the operation requirements, such as the change of oil volume when absorbing oil expanding and contracting, the valve for oil discharge, the gas release plug, the isolation valve between cooler and radiator and main oil tank, etc. Each oil system has good sealing performance. The oil in the switching switch room of on-load tap-changer should be replaced independently without releasing the main body oil. When transporting, the main body oil can be released and filled with dry nitrogen.

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