# Transformers Questions and Answers – Operation under Abnormal Conditions

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This set of Transformers online test focuses on “Operation under Abnormal Conditions”.

1. Increased transformer insulation stresses are due to _______________
a) third harmonic currents
b) third harmonic voltages
c) fifth harmonic currents
d) fifth harmonic voltages

Explanation: Following are the effects due to third-harmonic voltages: Increased transformer insulation stresses, electrostatic charging of adjacent lines and telephone cables, possible resonance at third-harmonic frequency of transformer windings and line capacitance.

2. Overheating of transformer windings and of load are due to ________
a) third harmonic currents
b) third harmonic voltages
c) fifth harmonic currents
d) fifth harmonic voltages

Explanation: Following are the effects due to third-harmonic currents: Overheating of transformer windings and of load, telephone and discriminative protective gear magnetic disturbances, increased iron loss in transformers.

3. Operation other than rated power will lead to ______________
a) increase in temperature
b) decrease in temperature
c) short circuit
d) open circuit

Explanation: Operation at other than rated load will result in hot-spot temperature rises differing from those corresponding to rated conditions and rated temperature rise is based on a hot-spot temperature of 98°C with a 20°C ambient.

4. Time required for an insulation to reach at end is given by __________________
a) Kirchhoff
b) Joule
c) Ampere
d) Arrhenius

Explanation: Expressed in quantitative terms the time required for insulation to reach its end of life condition is given by the Arrhenius law of chemical reaction rate. Within a limited range of temperatures this can be approximated to the simpler Montsinger relationship.

5. Arrhenius law is given by ___________________
a) L=αβT
b) L=αβ/T
c) L=α+βT
d) L=α+β/T

Explanation: L is defined as the time for the reaction to reach a given stage, but which might in this case be defined as end of life, while α and β are constants. T is absolute temperature. In limited range of temperatures, it can be approximated to Montsinger relationship.

6. Rate of aging is __________________
a) v= M e-pθ
b) v= M e
c) v= M θe-pθ
d) v= M e

Explanation: Montsinger relationship is defined as L= e-pθ , where p is a constant, θ is the temperature in degrees Celsius. Investigators have not always agreed on the criteria for which L is representative of end of life, but for the purposes of this evaluation this is not relevant and of more significance is the rate of ageing. This is the inverse of the lifetime.

7. Relative age rating is ___________________
a) 2(θ – 98) * 6
b) 2(θ – 98) – 6
c) 2(θ – 98) / 6
d) 2(θ – 98)

Explanation: If 98°C is then taken as the temperature at which normal ageing rate occurs, then the relative ageing rate at any other temperature θ is given by the expression:
V= ageing rate at θ/ ageing rate at 98°C= 2(θ – 98) / 6

8. Hot-spot temperature is dependent on ____________
a) 4 categories
b) 6 categories
c) 8 categories
d) 10 categories

Explanation: Hot-spot temperature is made up of the following components: Ambient temperature, top oil temperature rise, average gradient, difference between average and maximum gradient of the windings.

9. In 4-wire star connection 3rd harmonic currents may flow through ______________
a) lines
b) phases
c) from fourth wire to neutral
d) all connections

Explanation: With a four-wire star connection, third-harmonic voltages from lines to neutral or earth are suppressed partially or completely according to the impedance of the third-harmonic circuit. With a four-wire star connection, third-harmonic currents may flow through the phases and through the line wires and fourth wire from the neutral.

Sanfoundry Global Education & Learning Series – Transformers.

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