Important interview questions
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Ans:Turbo alternator had eddy current damping mechanism
The buchholz (gas/liquid operated) relay is connected in the pipe work between the transformer tank and the conservator. If a valve is fitted to isolate the conservator, the buchholz relay is usually on the transformer tank side of the valve.
Under normal conditions, the relay is full of oil. Most faults occurring within oil filled transformers are accompanied by the generation of gas by the oil, due to liberation of heat. Such generated gas accumulates in the relay, thereby operating the float/flap switches.
With an incipient fault, gas is produced at a very slow rate and the upper float (alarm) switch will operate after a specified volume of gas has accumulated. The upper float (alarm) switch will also indicate the low oil level of the conservator.
When a major fault occurs, the gas is produced rapidly, resulting in a sudden surge of oil up to the conservator, thereby operating the lower float (trip) switch. The lower float (trip) switch will also indicate the drained oil level of the conservator.
The tank should be permanently and effectively connected to earth by means of flat, flexible conductor of suitable size and materials (galvanized steel, copper, or acceptable equivalent) terminated on earthing terminals or pads provided at the bottom of the tank, cable box, marshalling box, switch gear box, etc.
Earthing connection(s) with a good low resistance are essential for adequate protection against electrical faults. All the earth connections should be of sufficient physical size to carry the line current for 30 seconds
De-energize the transformer by disconnecting both High and Low Voltage lines. • Ground the terminal lines of the transformer.
Place locks and warning cards stating that the interrupters or terminal boxes must not be operated (LOTO procedures).
Disconnect the power supply of the control cabinet.
Before going inside to any transformer, verify that the internal environment contains a minimum of 19.5% oxygen and maximum of 50 ppm CO for exposures up to 8 hours period. For further information, refer the OSHA regulation. Utilize dry air (less than 0.05% impurities and – 58°F/- 50a C or lower dew point) for flowing through the transformer while inside the transformer tank.
Use only explosion proof lamps inside the tank and verify the insulation condition of the electrical cord – it must be free of damage.
Plain pipe breather is generally sealed with cap for transit purpose and the same should be removed ONLY during commissioning.
Alternatively, a Dehydrating Breather should be fitted to ensure that air entering the tank during breathing is dry, thus preventing insulation losses due to internal condensation.
A Dehydrating Breather, usually Envirogel (self indicating silica-gel) or Blue Silica-Gel, is shipped as a separate item to prevent unnecessary accumulation of moisture via two-way valve/oil-seal cup of the breather during transit.
Lightning arrestors and lightning arrestor brackets will be mounted in accordance with the outline drawing provided. Care should be taken that all ground connections are securely made in accordance with all applicable local and national codes.