On TRANSFORMER OIL BREAK DOWN VOLTAGE test is carried out which is done on field while filling the oil in transformer. This process is also called oil filtration. 1.Take sample from top of Transformer before filtration. 2.The BDV of oil should withstood min 50 KV 3.The gap between the test electrodes sample to be 2.5mm maintained 4.At least 5 sample(top/bottom) should be checked separately while filtration. 5. The samples should withstand 50 kv for 1 min. 5. Take the average of all the samples timings.You will get value. 7.Procedure a.Voltage should be gradually increased from 0. b. At one point Flash will take place and that point is termed as Bread down Voltage

The break down characteristics depends upon initial moisture content of transformer & oil. The electrical strength of oil depends upon the water content in transformer & temperature gradient. The electrical strength of oil is reduced if the water content is more than 2%.

why earth pin diameter is higher than ohter two pin? what its purpose ? 

Answer:Because Current flow in the conductor is inversely proportional to the

conductor diameter. So if any short circuits occur in the system first high currents

bypassed in the Earthling terminal.( R=Pl/a area of the conductor increases resistance

value decreases) 

Electrical interview question:What is meant by knee point voltage?

Answer:Knee point voltage is calculated for electrical Current transformers and is very

important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-

estion:How do you select a cable size (Cu & Al) for a particular load?

Answer:At first calculate the electrical current of the load, after that derate the electrical current

considering derating factor(depending on site condition and laying of cable) after choose the

cable size from cable catalog considering derating electrical current.After that measure the

length of cable required from supply point of load to load poin. Calculate the voltage drop which

will max 3% (resistance and reactance of cable found from cable catalog of selecting cable) if

voltage drop>3%

then choose next higher size of cable.current transformer). Electrical interview

question:Why in a three pin plug the earth pin is thicker and longer than the other pins?

Answer:It depends upon R=rho l/a where area(a) is inversely proportional to resistance

(R), so if (a) increases, R decreases & if R is less the leakage current will take low

resistance path so the earth pin should be thicker. It is longer because the The First to

make the connection and Last to disconnnect should be earth Pin. This assures Safety

for the person who uses the electrical instrument.

Electrical interview question:What is Automatic Voltage regulator(AVR)?

Answer:AVR is an abbreviation for Automatic Voltage Regulator.It is important part in

Synchronous Generators, it controls theoutput voltage of the generator by controlling its

excitation current. Thus it can control the output Reactive Power of the Generator.

On-site transformer oil testing

As in most countries transformer oil testing is mandatory, suppliers of test equipment have developed portable devices for on-site transformer oil testing.

To determine the insulating property of the dielectric oil, an oil sample is taken from the device under test, and its breakdown voltage is measured on-site according the following test sequence:

In the vessel, two standard-compliant test electrodes with a typical clearance of 2.5 mm are surrounded by the insulating oil.

During the test, a test voltage is applied to the electrodes. The test voltage is continuously increased up to the breakdown voltage with a constant slew rate of e.g. 2 kV/s.

Breakdown occures in an electric arc, leading to a collapse of the test voltage. Immediately after ignition of the arc, the test voltage is switched off automatically. Ultra fast switch off is crucial, as the energy that is brought into the oil and is burning it during the

breakdown, must be limited to keep the additional pollution by carbonisation as low as possible. The root mean square value of the test voltage is measured at the very instant of the breakdown

and is reported as the breakdown voltage. After the test is completed, the insulating oil is stirred automatically and the test sequence is

performed repeatedly. The resulting breakdown voltage is calculated as mean value of the individual measurements.