Write notes on any TWO of the following: (i) Parallel operation of transformers.

(ii) Speed control of induction motors.

(iii) Applications of induction machines. (2 x 7 = 14)

Ans:

(i) Parallel operation of transformers :

Certain conditions must be satisfied in order to avoid any local circulating currents and to ensure that the transformers share the common load in proportion to their kVA ratings, The conditions are :

- The Primary winding of the transformers should be suitable for the supply system voltage & frequency.

- The transformer should be properly connected with regard to polarity.

- The voltage rating of both primaries & secondaries should be identical (same turn ratio) - Percentage impedances should be equal in magnitude and have the same X/R ratio in

order to avoid circulating currents and operation at different Power factors.

- With transformers having different kVA ratings, the equivalents impedances should be inversely proportional to the individual kVA ratings, if circulating currents are to be avoided

In case of 3 phase transformers’ parallel operation, the following conditions are also added with the above conditions.

- The phase displacement between primary & secondary voltages must be the same for all transformers, which are to be connected for parallel operation.

- The Phase sequence must be the same.

- All 3-Φ transformers must have same construction either core or shell. (ii) Speed control of induction motors

The speed control of induction motor is obtained by various methods : i. Frequency variation.

ii. Pole changing.

iii. Changing motor circuit resistance.

A slip ring (wound) motor is an adjustable speed motor. The rotor winding terminals are connected to slip rings. A 3-Φ star connected rheostat is provided separately with motor. By changing the resistance in rotor circuit, the torque speed characteristics of machine can be changed. The Speed & Torque can be changed as:

T α r2 / s and Slip ‘s’ α r2

By increasing rotor resistance, The torque increases and slip ‘s’ increases, Therefore, speed decrease as:

T α 1/N

The speed reduces in slip ring induction motor through rheostat control; and this can be obtained only if the motor is loaded under no load condition. The no-load speed changes are very little with the variation in the load resistance.

(iii) Applications of Induction Motors : The squirrel cage induction motor with single or double pole changing windings are available as follows:

CLASS APPLICATIONS

(1) Variable torque, power output α N2. Fans, centrifugal pumps.

(2) Constant –torque power output α N. Conveyors, stokers, reciprocating

Compressors, printing presses.

(3) Inverse torque power output rating constant. Machine Tools, lathes, boring mills, drill, planers,.

The Multi-speed motors are of slip ring type used for hoist, conveyor and elevator. Q.51 Highlight the role of following in the generation of power

(i) Cogeneration.

Ans: Co-generation

The interconnection of hydro and thermal power generation plant or reuse of waste product of first generator as a fuel of second generator or other machine :

The large power system comprising several power stations load centres interconnected to form a single grid operation of such a grid is controlled from a load centre or load dispatch centre. The national load control centre is linked with various regional despatching stations. The regional load centres send commands from power stations to control room periodically by telemetric data transmission system.

The automatic load frequency control in the control system of generator turbine governor basically aim to maintain a constant frequency/speed as a primary control. But setting of the governor for turbine is changed according to instruction of regional control centres. Therefore, The input of turbine from governor gets automatically adjusted by primary load frequency control and frequency is maintained. The governor setting is determined by economy load dispatch instructions from regional load control centres. The total load cogeneration control is achieved by:

- Load control centre.

- Telemetry or telecontrol equipment. - Power station control room.

(ii) Diesel Generator: These are used in medium power plants upto 25MW for industrial and marine applications. The transportable diesel generator sets are for remote location, and small power plants for small lawns /farms etc.

Generators are usually gear driven to speeds of 1000 to 1500 rpm for 50 Hz. Diesel engines have a speed range of 1500 rpm to 2250 rpm (higher speed for gas turbine driven set)

The critical speed and torsional or natural frequencies are checked for diesel generator set to avoid resonance. Brushless excitation is commonly used. The diesel generators are mounted on same bed plate of prime mover.

Q.52(a) Explain the functions and basic requirements of a protective relay. (7) (b) Compare the merits and demerits of overhead lines with an underground distribution

system. (7)

Ans: (a) Functions & basic requirements of a protective relay :

Functions:

The protective relaying senses the abnormal conditions as a part of the power system and gives an alarm or isolate that part from the healthy system. The relays are compact, self-

contained devices, which respond to abnormal conditions. Whenever abnormal conditions occur, the relay close its contacts. Thereby trip circuit of the circuit breaker is closed and circuit breaker will open. So that, the faulty part will be disconnected from the supply.

Basic requirements

- Selectivity, discrimination: The protective relaying should select the faulty part of the system and should isolate, as far as possible only faulty part from the remaining healthy system.

- Speed, Time: It is the time between fault instant and closing of relay. A rapid contact fault cleaning i.e. 0.07 second with 60 kA rms value of current, has no damage to the system but if it is 7 sec, the bus bar will destroy complete. Therefore, relay time must be minimum as much as possible (i.e. in millisecond.)

Fault clearing time : Relay time + Breaker time

- Sensitivity, power consumption: It refer to the smallest value of actuating quantity at which protection starts operating in relation with the minimum value of fault current in the protection zone

Minimum short circuit current

Sensitivity factor ‘KS’ = IS /IO =

Minimum operating current in protection.

Stability : A quality of protective system by the virtue of which , the protective system remains operating and stable under certain specified conditions such as system disturbances, through faults, transient etc.

Reliability : The protective relaying should not fail to operate in the event of faults in protected zone. The reliability of protective systems depends on diverse aspects such as protective gear manufactures, Electricity Boards & Associates.

Adequateness :The adequateness of protection is judged by considering following aspects: - Rating of protecting machines.

- Location of protecting machines.

- Probability of abnormal condition due to internal and external causes. - Cost of machine, importance.

(b)

Overhead power transmission lines

Under ground Power cable

(A) Merit 1. Easy maintenance & Repair

2. Low cost of installation 3. Mostly used in transmission system due to effective voltage upto 400 kV

4. Less skilled staff is required.

1. Less disturbances for other system 2. system looks neat and beautiful 3. Mostly used in distribution system . (Medium and low voltages upto 11kW) 4. Less lightening thunder effect

(B) De- Merits

1. Chances of frequent failure due disturbance from other system.

2 less safe

3 Lightning protection is required.

4. Chances of tapping or theft more

1. High initial cost

2. High maintenance and repair cost 3. Problem of charging current in

HV transmission.

4. Highly skilled staff is required.

Q.53 (a) Explain the working of a welding machine. (7)

(b) What is inductive interference? How is it caused and what steps are necessary to

reduce its effect? (7)

Ans: (a) Working of a welding machine:

A welding process is required to join two metal parts by heating them to melting point. Electrical welding is used for joining of fabricating structures, machinery parts, pipes, bus bars, bridges, ships railway bogies etc.

Electrical welding has two distinct types:

- Resistance welding : In resistance welding the current is passed through the joint to be welded and the heat is caused by I2 R losses in the joint, melting of metal and subsequent welding under pressure between faces to be welded.

- Arc welding : The heat is produced by the arc struck between the welding electrode and the metal to be welded. Pressure is not applied between faces to be welded.

(b) Inductive interference:

Inductive interference depends upon gradient of voltage at surface of conductor or corona. The inductive reactance is affected from conductor attenuation at 1000 KHz and varies from 0.3 to 0.9 dB/mtr.

Q.54 Identify the motor being used in the ceiling fan and explain the method of its control.