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Transformers Questions and Answers for Experienced people on “Additional Testing for Important Transformers”.
1. Which of the following is not a routine test for transformers?
a) Impedance test
b) Core insulation voltage test
c) Radio interference test
d) Polarity test
Answer: c
Clarification: Impedance test is done on a transformer in order to check net impedance offered by a transformer circuit at rated supply. Polarity test is also done before SC and OC test on transformer. Core insulation is also checked before installations in such tests.
2. Which of the following is not required for transformer testing to get an accurate result?
a) Class 0.1 current transformer
b) Class 0.1 voltage transformer
c) High power factor wattmeter
d) Voltmeters
Answer: c
Clarification: To obtain accurate results it is essential that low power factor wattmeter, precision grade ammeters, voltmeters, and class 0.1 (see BS 3938 and 3941) current and voltage transformers are used.
3. All the instruments needed for transformer testing should be tested within span of ____________
a) a month
b) 3 months
c) 6 months
d) 12 months
Answer: d
Clarification: These instruments should be checked at intervals not exceeding 12 months to ensure that the requisite accuracy is maintained. All these instruments are highly sensitive in their operation, thus to maintain accuracy testing is essential.
4. Epstein square method is used in power transformer installations, to check ___________
a) Core frame insulation
b) Core-plate checks
c) Core-loss measurement
d) Winding copper checks
Answer: a
Clarification: Incoming core plate is checked for thickness and quality of insulation coating. A sample of the material is cut and built up into a small loop known as an Epstein Square from which a measurement of specific loss is made. Such a procedure is described in BS 6404.
5. Generally Core frame insulation test is done at _________
a) 10 kV DC
b) 10 kV AC
c) 2KV RMS
d) 2KV DC
Answer: c
Clarification: This is checked by Megger and by application of a 2 kV RMS or 3 kV DC test voltage on completion of erection of the core. These checks are repeated following replacement of the top yoke after fitting the windings.
6. Why tanks are tested?
a) Stiffness
b) Vacuum withstand capacity
c) Stiffness and vacuum withstand capacity
d) Any other purpose
Answer: c
Clarification: The first tank of any new design should be checked for stiffness and vacuum-withstand capability. For 275 and 400 kV transformers, a vacuum equivalent to 25 mbar absolute pressure should be applied. This need only be held long enough to take the necessary readings and verify that the vacuum is indeed being held, which might take up to 2 hours for a large tank.
7. How much pressure is applied for transformers rated 132 kV and below ones?
a) 330 mbar
b) 300 mbar
c) 400 mbar
d) 500 mbar
Answer: a
Clarification: For transformers rated 132 kV and below a more modest vacuum test equivalent to 330 mbar absolute pressure should be applied. The permissible permanent deflections following this test should be similar to those allowed for 275 and 400 kV transformer tanks reduced pro-rata for smaller tanks.
8. Which of the following parameters don’t define that transformer is built correctly?
a) Losses
b) Polarity
c) Tap changing operation
d) Ratio
Answer: a
Clarification: Tests to prove that the transformer has been built correctly: include ratio, polarity, resistance, and tap change operation. While losses prove the that transformer is guaranteed in its operation.
9. Which of the following parameters don’t define that transformer is built correctly?
a) Losses
b) Temperature rise
c) Noise level
d) Resistance
Answer: d
Clarification: Tests to prove guarantees; these are losses, impedance, temperature rise, noise level. While resistance, tap changing, ratio, polarity are the parts of tests that prove that transformer has been built correctly.
10. Which of the following tests don’t indicate that transformer will work satisfactorily for at least 30 Years?
a) Temperature rise
b) Dielectric test
c) Overvoltage test
d) Load current runs
Answer: a
Clarification: Tests to prove that the transformer will be satisfactory in service for at least 30 years. The tests in this category are the most important and the most difficult to frame: they include all the dielectric or overvoltage tests, and load current runs.
11. Which of the following is not in the category of special test for a transformer?
a) Short-circuit test
b) Harmonics on the no-load current
c) Power taken by fan and oil-pump motors
d) Open-circuit test
Answer: d
Clarification: Special tests are tests, other than routine or type tests, agreed between manufacturer and purchaser, for example: test with lightning impulse chopped on the tail, zero-sequence impedance on three-phase transformers, and other tests included in options.
12. How temperature rise of an oil-immersed transformer is not found out?
a) Short circuit equivalent test
b) Delta/delta test
c) Back-to-back test
d) Normal SC test
Answer: d
Clarification: When a test for temperature rise is specified it is necessary to measure the temperature rise of the oil and the windings at continuous full load, and the various methods of conducting this test are as follows: short-circuit equivalent test, back-to-back test, delta/delta test, open-circuit test.