Page 95 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

4.24 CHAPTER FOUR

● A gas accumulation chamber is located at the top of the relay. It consists of an oil cham-
ber with a gas bleeder needle valve. A float in the oil chamber operates a magnetic oil
gauge with an alarm switch.
● A pressure chamber is located at the bottom of the relay. It has two parts. An oil cham-
ber is seen at the rear of the relay. The chamber is connected to the transformer by a pipe
entering the back of the relay. A test valve is located at the base. It is used for making
operation checks. Sensitive brass bellows separate the first section from the second.
There is an air chamber in the front. It contains stops for the bellows to prevent overtravel,
a flexible diaphragm, and a microswitch. When the bellows move, they compress the air
behind the diaphragm. This action actuates the microswitch which is fastened to the
diaphragm. When arcing occurs in the transformer, it causes a rapid evolution of gas in
the oil. A pressure wave is generated through the oil. This wave will reach the relay through the
pipe. It will compress the flexible bellows. The air in the chamber is compressed by the dis-
placement of the bellows. Since the air cannot pass quickly through the bypass valve, it forces
the flexible diaphragm to close the contact of the trip switch. This action disconnects the
transformer.
It is essential to find the fault when the pressure contact trips the transformer. The fault
should be corrected before the transformer is put back in service.

RELIEF DEVICES

Very high pressure is generated following an electrical fault under oil. These pressures
could easily burst the steel tank if they are not relieved. An explosion vent was used until
the early 1970s. It consists of a large-diameter pipe extending slightly above the conserva-
tor tank of the transformer and curved in the direction of the ground. A diaphragm (made
of glass usually) is installed at the curved end. It ruptures at a relatively low pressure,
releasing any force buildup inside the transformer. Since the early 1970s, a self-resetting
pressure relief vent was installed on transformers. When the pressure in the transformer
reaches a predetermined level following a fault, it forces the seal open under spring pres-
sure. This relieves the pressure to atmosphere.

INTERCONNECTION WITH THE GRID

Figures 4.15 and 4.16 illustrate interconnections of power plants with the grid. The trans-
former that connects the plant with the grid is normally called the main output transformer
(MOT) or generator step-up transformer. The transformer that connects the output of the
generator with the plant itself (feeding power back to the plant) is called the unit service
transformer (UST) or station auxiliary transformer. The rating of the UST is normally about
6 to 7 percent of the MOT. The transformer that connects the grid to the station loads (allow-
ing power to be fed back to the unit) is called the station service transformer (SST). The
design philosophy for supplying power to the loads in the plant varies. Some power plants
supply one-half of their loads through the UST and the other half through the SST. However,
modern power plants have opted to supply all their power through the UST. They use the
incoming power through the SST as backup. In the plants where the plant loads are supplied
equally from the UST and SST (Fig. 4.16), the tie breakers connecting the buses inside the
plant close when the unit is disconnected from the grid. This is done to ensure that the loads
inside the plant continue to be supplied with power when the unit is taken off line.

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