JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III SWITCH GEAR AND PROTECTION Previous Question Paper (February/March - 2022 - R16)
Answer any five questions
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Main
Explain the operation of Plain break oil circuit breaker with a neat schematic diagram.
Answer
A plain break oil circuit breaker (OB) is the simplest type of oil circuit breaker where the contacts are separated within a tank of insulating oil. When the contacts separate, an arc is struck between them. The intense heat of the arc decomposes the surrounding oil into hydrogen gas, creating a gas bubble around the arc. The cooling effect of the oil and the turbulence of the gas help to deionize the arc path, leading to extinction at the current zero crossing.
Schematic description: The breaker consists of a grounded steel tank filled with transformer oil. Fixed and moving contacts are submerged in the oil. An operating mechanism pulls the moving contact downward to open the circuit. As the contacts part, the arc is formed in the oil. The gas bubble expands and rises, and the oil flows into the arc space to extinguish it. Because the arc control is purely dependent on the natural movement of the oil and gas, this design is limited to low voltage and low capacity applications.
Explain the terms (i) Symmetrical breaking current; (ii) Asymmetrical breaking current; and (iii) making current.
Answer
(i) Symmetrical breaking current: This is the RMS value of the AC component of the fault current at the instant the circuit breaker contacts separate. It represents the steady-state fault current without considering the DC offset.
(ii) Asymmetrical breaking current: This is the total RMS value of the current at the instant of contact separation, including both the AC component and the DC offset component. It is given by the formula:
(iii) Making current: This is the peak instantaneous value of the current during the first cycle of closure of the circuit breaker when a fault exists. It is higher than the breaking current due to the DC component and is defined as:
The factor 2.55 accounts for the maximum possible DC offset and the peak factor of the AC wave.
Explain the significance of Resistance Switching in circuit breakers.
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Describe the construction, principle of operation and application of SF6 circuit breaker. How does this breaker essentially differ from an air blast breaker?
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Show that the torque on the disc of an induction disc relay is maximum when the phase difference between the two fluxes is 90°. Indicate the direction of rotation of the disc with reference to the fluxes under the poles.
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Explain clearly the basic principle of operation of a differential relay. Explain the working of this type of relay for (a) an internal fault, and (b) a through fault.
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Explain the circulating protection scheme for earth fault protection of alternator with a neat diagram.
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Explain how the biased differential protection is applied to transformers and give its circuit and necessary requirements.
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Explain the following methods of Grounding:
Solid Grounding
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Reactance grounding.
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Explain in detail about the value type lightning arrester.
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What is insulation coordination? Explain.
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