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Over voltage protection is recommended for hydro-electric generators, steam turbine generators, gas turbine generators.

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The inverse definite mean time relays are used for over current and earth fault protection of transformer against internal short-circuits.

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Which of the following is the least expensive protection for over current in low voltage system?  (A) Rewireable fuse (B) Isolator (C) Oil Circuit breaker (D) Air break circuit breaker

Answer :

Which of the following is the least expensive protection for over current in low voltage system?  (A) Rewireable fuse (B) Isolator (C) Oil Circuit breaker (D) Air break circuit breaker

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Overfluxing protection is recommended for generator transformer of the power plant.

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In over compounded generator, full load terminal voltage is more than no load terminal voltage.

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What is meant by flash over voltage?

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Explain in detail over-voltage protection.

Answer :

Over-voltage protection: Over-voltage protection to the thyristor is provided using: i) R-C Snubber circuit ii) Non-linear resistor Thyrector A) Snubber Circuit: It is basically a ... returns to its high resistance state. Thyrector is normally placed across the supply terminals.

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A relay used for protection of motors against overload is :  (A) Impedance relay (B) Electromagnetic attraction type (C) Thermal relay (D) Buchholz's relay

Answer :

A relay used for protection of motors against overload is : Thermal relay

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Description :

A negative sequence relay is used for protection of  (1) Generator (3) Motor (2) Transformer (4) Long Transmission lines

Answer :

A negative sequence relay is used for protection of Generator 

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For protection of rotating machines against lightning surges _______ is used.  (1) lightning arrester (2) capacitor (3) combination of lightning arrester and capacitor (4) lightning conductor and arrester

Answer :

For protection of rotating machines against lightning surges combination of lightning arrester and capacitor is used. 

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Reactance relay is normally preferred for protection against

Answer :

Reactance relay is normally preferred for protection against earth faults

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Surge absorbers are used for protection against

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Surge absorbers are used for protection against low voltage high frequency oscillations

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The single phasing relays are used for the protection of three phase motors.

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For the protection of power station buildings against direct strokes the requirements are interception, conduction and dissipation.

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The delay fuses are used for the protection of motors.

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The surge impedance for over head line is taken as  (1) 10-20 ohms (2) 50-60 ohms (3) 100-200 ohms (4) 1000-2000 ohms

Answer :

The surge impedance for over head line is taken as 100-200 ohms

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Blind spot is a point in the zones of protection where

Answer :

Blind spot is a point in the zones of protection where No protection is available.

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Protection by fuses is generally not used beyond 100 A .

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A fuse in a motor circuit provides protection against short-circuit and overload.

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Discrimination between main and back up protection is provided by the use of relays which are slow.

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A thermal protection switch can protect against overload.

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Primary and Backup Protection

Answer :

Primary and Backup Protection  B,D,G,I are the primary protection. A,C,H,J are the backup protection. About 90% of faults are clear by Primary Protection. So ... ordinated backup (primary is at various station  and backup is at control room with computer)

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In the induction type directional over current relay, when a short-circuit occurs in the circuit (A) power flows in reverse direction (B) power flows in normal direction (C) directional power element does not operate (D) over current element is not energized

Answer :

In the induction type directional over current relay, when a short-circuit occurs in the circuit power flows in reverse direction

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Select insulating materials for following parts : (i) Insulation between heating element and base plate of electric iron. (ii) Insulation used over copper or aluminium conductor used for making coils. (iii) Transformer bushings. (iv) Insulation between transmission line and pole.

Answer :

Parts Insulating Materials  Insulation between heating element and base plate of electric iron. Mica Insulation used over copper or aluminium conductor used for making coils.  ... Transformer bushings Porcelain  Insulation between transmission line and pole. Porcelain

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State any two advantages of MCB over Fuse. State the standard specifications of MCB available in the market.  

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Following advantages of MCB over Fuse: 1. No need of replacement of fuse wire. 2. Manually restore of supply is simple. 3. Fault understanding by visual inspection 4. More safe than fuse The standard specifications of ... 6A, 10A,16A,20A, 25A, 30A, 32A, 40A, 50A, 60A and 63A 6. 250V and 450 v

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State advantages of short pitch winding over full pitch winding in alternators. 

Answer :

Advantages of Short Pitch Winding over Full Pitch Winding in Alternators: 1) Short pitching reduces the amount of copper needed for end connection when compared with full pitched coil. 2) They ... reduced, thereby increasing the efficiency. 4) The power quality of generated emf is improved. 

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What do you mean by active filter? State its advantages over passive filter . Define pass band & stop band with respect to filter.

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Active Filter: Active Filter is formed by using active element along with the passive components.  Advantages of Active filter over Passive filter: High value of pass band gain can be obtained. It is ... pass band.  Stop Band Filter: The frequency range which is attenuated is known as stop band.

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Description :

What is the advantage of AC over DC electrical transmission?

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State two advantages of digital system over analog system.

Answer :

1. They are less susceptible to noise. 2. The effect of fluctuation in the characteristic of the components, ageing of components etc. is very small in digital circuits. 3. Digital circuits have capability of memory which makes them suitable for computers. 4. More accurate.

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Thyristor can be protected from over voltages by using

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Thyristor can be protected from over voltages by using Voltage clamping device

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The main advantage of AC over DC is:  (A) Conversion flexibility from one voltage level to other (B) AC can be easily converted into DC by using a rectifier (C) AC is easy to maintain & can be easily distributed at long distances (D) All of the above

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Ac is easy to maintain and ac can be easily converted to dc by rectifier .There is no commutator for conversion in ac that's why ac machines are weight less and more efficiency.ac machines occupy less space than dc .

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An over excited synchronous motor on noload is known as synchronous condenser.

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Explain the significance of over modulation.

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For a 15-bus power system with 3 voltage controlled buses, the size of Jacobian matrix is

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For a 15-bus power system with 3 voltage controlled buses, the size of Jacobian matrix is 25 x 25

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The under voltage relay can be used for generators, busbars, transformers, motors.

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If 3000 kW power is to be transmitted over a distance of 30 km, the desirable transmission voltage will be.  (A) 11 kV (B) 33 kV (C) 66 kV (D) 132 kV

Answer :

By

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The voltage of transmission can be regulated by  (A) Use of tap changing transformers (B) Switching in shunt capacitors at the receiving end during heavy loads (C) Use of capacitors to neutralize the effect of series reactance (D) Any of the above methods

Answer :

The voltage of transmission can be regulated by  (A) Use of tap changing transformers (B) Switching in shunt capacitors at the receiving end during heavy loads (C) Use of capacitors to neutralize the effect of series reactance (D) Any of the above methods

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Answer :

The contacts of high voltage switches used in power system are submerged in oil. The main purpose of the oil is to extinguish the arc.

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The arc voltage produced in A.C. circuit breaker is always in phase with the arc current.

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The voltage appearing across the contacts after opening of the circuit breaker is called recovery voltage.

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Answer :

In order that current should flow without causing excessive heating or voltage drop, the relay contacts should have low contact resistance, be clean and smooth, be of sufficient size and proper shape.

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Description :

State the nature of protection provided for following types of faults; 1) Over voltage 2) Under voltage 3) High air pressure in air circuit 4) Low air pressure in air circuit

Answer :

Ans:  1) Over voltage: Controlled through tap changing, only an alarm needs to be provided for attention of crew.  2) Under voltage: voltage below 17.5 kV is not desirable. A no volt ... 4) Low air pressure in air crcuit: Relay senses low pressure in air circuit and trips the relevant CB.

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Description :

Explain over-current protection of transformer.

Answer :

Over-Current Protection of Transformer: Simple over-current & earth fault protection of transformer against external shortcircuit and excessive overloads is shown in the above figure. The ... trip command to the circuit breaker and the transformer is disconnected from load or fault.

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Description :

State standard voltage in India for the following : (i) Generation Voltage (ii) Primary Transmission Voltage

Answer :

i. Generation Voltage : 3.3KV, 6.6KV,11KV and 17.5 KV  ii. Primary Transmission voltage :- 220 KV, 400KV, 765 KV (750 KV) 

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For a balanced three phase star connected load for which line voltage is 230V and per phase resistance and reactance is 6 Ohm and 8 Ohm respectively. Calculate: 1) phase voltage 2) line current 3) power factor 4) total power absorbed

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Description :

Write the relation between phase and line current, phase and line voltage for Star connection.

Answer :

Line voltage is equal to root three times of phase voltage and line and phase currents are equal in star connection

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Description :

A single core cable is designed to work for a single phase system with a peak voltage of 220 kV. The maximum permissible stress in the dielectric is not to exceed 20 kV/mm. For economical size, the overall core diameter of the cable is (A) 28.2 mm (B) 26.4 mm (C) 24.4 mm (D) 22.0 mm

Answer :

A single core cable is designed to work for a single phase system with a peak voltage of 220 kV. The maximum permissible stress in the dielectric is not to exceed 20 kV/mm. For economical size, the overall core diameter of the cable is 22.0 mm

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For a voltage source?

Answer :

For a voltage source terminal voltage cannot be higher than source e.m.f.

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The voltage used for suburban trains in D.C. system is usually 600 to 750 V.

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For measurements on high voltage capacitors, the suitable bridge is Schering bridge.

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For given applied voltage, with the increase in frequency of the applied voltage?

Answer :

For given applied voltage, with the increase in frequency of the applied voltage eddy current loss will remain unchanged.

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