overvoltages in electrical power system

Overvoltages which appear on electrical power system can be broadly described as internal and external.

Internal overvoltages:

These type of over voltages originate in the system itself and may be transient, dynamic and stationary type.

Transient Over Voltages: Over voltages of transient nature will have frequency un related to the normal system frequency and will persist for few cycles only. They can be caused by the operation of the circuit breakers when switching inductive and capacitive loads.

Dynamic Over Voltages: This over voltages occur at normal frequency and persist for only few seconds. This may be caused by the disconnection of the generator with overspeeds, or when large part of the load is suddenly removed.

Stationary Over Voltages: This occurs at system frequency but persists for some time ranges from seconds to hours. Condition such as earth fault on one line is sustained when the neutral is connected to the earth through the arc suppression coil then the voltage levels on the other two healthy phases rises.

The over voltages of above types rarely exceed three to five times the normal phase to neutral peak voltage o the system. Apparatus are to be designed in a manner to provide adequate insulation to withstand these over voltages

External over voltages:

These over voltages are produced by the atmospheric discharges such as static charges and lightning strikes. These have the magnitude to cause the considerable stress on the insulation of the equipment ( motors, generators, transformers,etc..). To avoid the break down, the insulation strength of the equipment to be protected ( transformers, motors, generators) should be greater than the over voltage protection equipments such as surge arresters and surge divertors. In the present day practice high voltage switchgear is designed to with stand the impulse voltage which is generated due to lightning strikes and other factors.

However it should be considered that while designing the insulation for the EHV lines (greater than 400 kV) switching impulse caused during the switching of the circuits is considered as it is more severe than the lightning impulse at very higher voltages.

Engineering Services (IES) Objective Questions Answers:Machines3

Indian Engineering Services (IES) Objective Questions and Answers:

1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006) 

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit

b) Solid short circuit

c) Full load

d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s

b) -s

c) (1-s)

d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II 

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5%

b) 10%

c) 20%

d) 40% 

 Answers:

(1) b (2) b (3) b (4) d

(5) d (6) c (7) d (8) c

 

Engineering Services (IES) Questions and Answers:Machines 2

Indian Engineering Services Objective Questions and Answers

1) Match the Following   ( IES- 2003)

      List I                                        List II 

A. Transformer                           1. Slip Test

B. DC Motor                               2. Blocked Rotor Test

C. Alternator                               3. Sumpner's Test

D. Induction Motor                      4. Swinburne's Test

Codes:

         A          B          C         D

a)      3           4          1         2

b)      4           3          2         1

c)      3           4          2         1

d)      4           3          1         2

2) Possible three to three phase transformer connection for parallel operation is: ( IES- 2002)

a) Delta-Star to Delta-Star

b) Delta-Delta to Delta-Star

c) Star-Star to Delta-Star

d) Delta-Star to Star-Delta

3) Stepper Motors are widely used for  ( IES- 2002)

a) Very high power requirement

b) Very high speed operation

c) Very low speed operation

d) Control system applications

4) Armature torque of a d.c motor is a function of which of the following factors ( IES- 2003)

1. Speed

2. Field Flux

3. Armature Current

4. Residual Magnetism

Select the correct answer

a) 2 and 3

b) 1 and 4

c) 3 and 4

d) 1 and 2

5) The dummy coils in the DC machine are useful to

a) Increase the efficiency of the machine

b) Improve the commutation

c) Reduce the armature reaction

d) Maintain mechanical balance

6) The synchronous reactance is the ( IES- 2003)

a) Reactance due to armature reaction of the machine

b) Reactance due to the leakage flux

c) Combined reactance due to leakage flux and armature reaction

d) Reactance either due to armature reaction or leakage flux

7) The crawling in the induction motor is due to  ( IES- 2003)

a) Improper design of the stator laminations

b) Low voltage supply

c) Improper design of rotor laminations

d) Harmonics developed in the motor

8) In an Induction motor, when the number of stator slots is equal to an integral multiple of rotor slots  ( IES- 2003)

a) There must be a discontinuity in the torque slip charcteristics

b) A high starting torque will be available

c) The maximum torque will be high

d) The machine may fail to start

Answers:

(1) a (2) a (3) d (4) a

(5) d (6) c (7) d (8) d

Engineering Services (IES) Questions and Answers:Machines 1

Indian Engineering Services (IES) Questions and Answers:  

1) Synchronous Condenser means (IES-2001)

a) a Synchronous motor with capacitor connected across stator terminals to improve the power factor

b) a Synchronous motor operating at full load with leading power factor

c) Over excited synchronous motor partially supplying mechanical load, and also improving the power factor of system which is connected to it

d) Over excited synchronous motor operating at no load with leading power factor for improving power factor

2) DC series motor is accidentally connected to the single phase ac supply. The torque of the motor will be (IES-2002)

a) Zero average value

b) Oscillating

c) Steady and Uni  directional

d) Pulsating and Uni directional

3) The power factor of the synchronous motor (IES-2002)

a) Improves with increase in the excitation and may become leading at high excitation

b) Decrease with increase in excitation

c) Is independent of excitation

d) Increase with loading for a given excitation

4) In the case of auto transformers which of the following statements are correct (IES-2001)

a) An auto transformer requires less copper as compared to the conventional 2 winding transformer of the same capacity

b) Auto transformer provides isolation between the primary and secondary windings

c) An auto transformer has less leakage reactance compared to the conventional two winding transformer of same capacity

Select the correct answer:

a) 1,2 and 3

b) 1 and 2

c) 1 and 3

d) 2 and 3

5) When the excitation of the normally operating unloaded salient pole synchronous motor suddenly gets disconnected, it continues to run as (IES-2002)

a) Induction motor

b) Stepper motor

c) Switched reluctance motor

d) variable reluctance motor

6) The rotor power output of 3 phase induction motor is 15kW. The copper losses at a slip of 4% will be: (IES-2001)

a) 600 W

b) 625 W

c) 650 W

d) 700 W

7) An induction motor when started on load does not accelerate up to full speed but runs at 1/7 th of the rated speed. Then the motor is said to be under (IES-2001)

a) Locking

b) Plugging

c) Crawling

d) Cogging

8) The advantage of double squirrel cage induction motor over single case rotor is that (IES-2001)

a) Efficiency is higher

b) Power factor is higher

c) Slip is larger

d) Starting current is lower

Answers:

(1) d (2) d (3) a (4) c

(5) d (6) b (7) c (8) d