This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Transformer Protection”.
a) 5 conditions
b) many conditions
c) 10 conditions
d) 7 conditions
Explanation: The satisfactory parallel operation of transformers is dependent upon five principal characteristics; that is, any two or more transformers which it is desired to operate in parallel should possess: The same inherent phase angle difference between primary and secondary Terminals, The same voltage ratio, The same percentage impedance, The same polarity, The same phase sequence.
2. Voltmeter connected across two similar terminals of parallel operated transformers should give _______
a) 0 reading
b) maximum reading
c) sum of individual reading
d) division of individual readings
Explanation: Polarity applied so as to indicate the directional relationship of primary and secondary terminal voltages of a single unit. Any two single-phase transformers will have the same polarity when their instantaneous terminal voltages will be in phase. With this condition a voltmeter connected across similar terminals will indicate zero.
3. Transformers are subjected to transients because _________________
a) open-circuit currents
b) short-circuit currents
c) inrush currents
d) both OC and SC currents
Explanation: The transients to which transformers are mainly subjected are: Impact of high-voltage and high-frequency waves arising from various causes, including switching in, system switching transients with slower wavefronts than the above, switching inrush currents, short-circuit currents.
4. Any transformer needs to be protected from __________
a) transformer faults
b) faults occurring on the transformer connected systems
c) faults within and on system
d) other faults
Explanation: The subject of transformer protection falls naturally under two main headings. These are the protection of the transformer against the subsequent events of effects of faults occurring on any part of the system. Protection of the system against the effects of faults arising in the transformer.
Explanation: Considering first the means to be adopted for protecting the transformer itself against the effects of system faults, three distinct types of disturbances have to be provided for. These are: Short-circuits, high-voltage, high-frequency disturbances including lightning, pure earth faults.
6. Ferroresonanace can be added in __________________
a) faults due to system
b) faults in the transformer
c) manual faults
d) other faults
Explanation: To this list could be added ferroresonance, which can occur under certain conditions in any system containing capacitance and inductance elements such as those associated respectively with cables and transformers.
7. System-short circuits may occur due to ________________
a) Line to line contacts
b) Line to neutral contacts if neutral is not earthed
c) Line to neutral contacts if neutral is earthed
d) LL and LG faults
Explanation: System-short circuits may occur across any two or even three lines, or, if the neutral point is solidly earthed, between any one line and the earthed neutral. Thus, mechanical stress gets created in the transformer circuit.
8. Mechanical stress produced in the circuit ______________
a) directly proportional to the square of the voltage
b) inversely proportional to the square of the voltage
c) directly proportional to the square of the currents
d) inversely proportional to the square of the currents
Explanation: The short-circuit currents produce very high mechanical stresses in the equipment through which they flow, these stresses being proportional to the square of the currents. The magnitude of these short-circuit currents can be limited by increasing the system impedance, usually incorporating this into the supply transformers.
a) atmospheric disturbances
b) line faults
c) manual faults
d) line to neutral faults
Explanation: High-voltage, high-frequency surges may arise in the system due to lightning, external flashover on overhead lines, switching operations and to the effects of atmospheric disturbances. These surges principally take the form of travelling waves having high amplitudes and steep wavefronts.
10. Surge impedance can be calculated as _________
Explanation: The amplitudes of the waves in the overhead line and at the transformer terminals depend upon the respective values of their surge impedance, which is given by the formula: √(L/C); where Z=surge impedance in ohms, L=inductance in henrys, C=capacitance in farads, of the circuit concerned.
11. Surge protection can’t be implemented by the addition of rod gaps.
Explanation: Surge protection is implemented by the addition of rod gaps or surge arresters adjacent to the transformer to shunt the surges to earth. These attenuate the surge magnitudes in the view of the windings and their resulting insulation stresses to levels which can be withstood by suitably proportioned insulation distribution without causing resonant instability and dangerous oscillations within the windings.
Sanfoundry Global Education & Learning Series – Transformers.
To practice all areas of Transformers, here is complete set of 1000+ Multiple Choice Questions and Answers.