JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III SWITCH GEAR AND PROTECTION Previous Question Paper (August - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
Discuss the process of arc extinction in the medium of oil and what characteristics it should have? Discuss various types of OCBs used in power network.
Answer
When an arc is formed in oil, the intense heat decomposes the oil into hydrogen gas (approx 70-80%), methane, ethylene, and carbon. The arc is surrounded by a hydrogen gas bubble which is highly conductive at high temperatures but has high thermal conductivity and de-ionizing properties. The arc is extinguished by the cooling effect of the hydrogen gas and the movement of fresh oil into the arc path, which sweeps away the ionized particles. Ideal oil for circuit breakers must possess high dielectric strength, low viscosity for rapid flow, high flash point to prevent fire, and high chemical stability.
Types of Oil Circuit Breakers (OCBs) include:
- Bulk Oil Circuit Breakers (BOCB): The entire circuit breaker tank is filled with oil, which acts as both the arc quenching medium and the insulation between live parts and the earthed tank.
- Minimum Oil Circuit Breakers (MOCB): These use a much smaller quantity of oil, confined to a small arc control chamber (interrupting chamber), significantly reducing fire hazards and oil maintenance requirements.
In a short circuit test on a circuit breaker of 132 KV 3 phase the following results were obtained. Power Factor of the fault 0.4, recovery voltage is 0.9 times the full line voltage, the breaking current is symmetrical, and the frequency of oscillation of restriking voltage is 16 kHz. Determine the RRRV and its average values, assuming the fault involves ground.
Answer
Given: Line voltage (V_L) = 132 kV, Frequency of oscillation (f_n) = 16 kHz = 16,000 Hz, Recovery voltage (V_r) = 0.9 × V_L = 0.9 × 132 kV = 118.8 kV.
Since the fault involves ground, the peak recovery voltage (V_p) is the phase voltage:
The maximum restriking voltage is 2V_p = 2 × 68.59 = 137.18 kV.
The time to reach the peak restriking voltage is t = 1 / (2f_n) = 1 / (2 × 16,000) = 31.25 μs.
The Average RRRV is calculated as:
The Peak RRRV is given by the formula:
Define Active Recovery Voltage and discuss three important factors which affect the severity of it on circuit breakers.
Answer
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What are the applications of SF6 circuit breaker? Explain its working principle.
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With neat diagram, explain the principle of operation of an Induction disc type Over Current relay. How can a directional feature introduce in the same?
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An IDMT relay is used to protect an alternator of 33KV (Star) rating, through 500/1 Amp C.T. The relay has plug setting of 125% and TMS of 0.75. A three-phase short circuit fault occurs near the alternator such that the impedance from alternator to the fault point is j6 Ohms. Find the fault current and the time of operation of the relay. The IDMT characteristic is given below, which is assumed to be linear between two consecutive points.
| PSM | 2 | 4 | 6 | 8 | 10 | 12 |
| Time(s) | 7 | 6 | 4.2 | 3.5 | 3.1 | 2.9 |
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What are the various types of over current relays? Discuss their area of applications.
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Explain the requirement of primary and back up protection in any equipment. What are the demerits of differential relay? Write two main disadvantages of using Merz Price differential scheme.
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Describe the three-zone distance relay protection of the line using impedance relays.
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Explain the operation of Buchholtz relay with a neat diagram.
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How can the maloperation of differential protection scheme due to magnetizing inrush current be avoided in the protection of transformers?
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A 3-phase, 11 / 220 KV Delta – star connected transformer is protected by differential protection. The current transformer connected on high voltage side is having a ratio of 300 / 5 A. Find the ratio of current transformer connected on the low voltage side also draw the connection diagram?
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State the advantages of neutral grounding of an electrical system. Give a connection diagram of typical arc suppression coil.
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A 230 kV, 3 phase, 50 Hz, 200 km transmission line has a capacitance to earth of 0.04 micro farad/km per phase. Find the inductance and rating of the Peterson coil used for earthing the above system.
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Discuss the internal and external causes of over voltages in a power system.
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Explain the characteristics of standard impulse voltage. Why is it needed for testing?
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