OVERVOLTAGE

Question 1

3 types of internal overvoltages

Answer 1

1.Transient internal overvoltages
2.Sustained internal overvoltages
3.Temporary overvoltages

Question 2

The magnetising current
of transformers or shunt reactors may be forcibly ‘chopped’ before the instananeous
value of the 50 Hz current reaches a zero value.

Answer 2

Switching out of transformers or shunt reactors

Question 3

During the interruption process in a circuit breaker, the contacts separate gradually

Answer 3

Switching of capacitors and unloaded feeders

Question 4

2 TYPES OF TRANSIENT INTERNAL OVERVOLATGES

Answer 4

1.Switching out of transformers or shunt reactors
2.Switching of capacitors and unloaded feeders

Question 5

It is hardly practicable to design a large
network in such a way that excessive lightning or switching surges do not occur
under any circumstances

Answer 5

Sustained internal overvoltages

Question 6

3 typical conditions causing sustained internal overvoltages

Answer 6

1.Neutral inversion
2.Arcing ground phenomena
3.Resonance phenomena

Question 7

The neutral point of a three-phase system under normal conditions is symmetrical in relation to the voltages of the three phases, that is the neutral is in the centre of the phasor diagram showing the phase voltages

Answer 7

Neutral inversion

Question 8

describes a condition when the neutral point assumes a position outside the perimeter of the triangle formed by the phase voltage vectors

Answer 8

Neutral inversion

Question 9

On systems without any neutral earthing, the
current in a fault arc is determined by the capacitance of the system and the arc
may be unstable

Answer 9

Arcing ground phenomena

Question 10

Such abnormal conditions may arise as a result of the open-circuiting of one of two phases in a three-phase system, for example by faulty circuit breaker or broken conductors.

Answer 10

Resonance phenomena

Question 11

TRUE or FALSE: Excessive overvoltages can appear on two open-circuited phases of a system which is energised through one phase only of a faulty circuit breaker at a voltage

Answer 11

TRUE

Question 12

This term has recently appeared in CIGRE and
IEC documents. 4°’42 It describes repetitive overvoltages maintained for several
cycles

Answer 12

Temporary overvoltages

Question 13

TRUE or FALSE: Temporary overvoltages usually originate from switching operations or faults, e.g. load rejection

Answer 13

TRUE

Question 14

TRUE or FALSE: When a voltage is applied suddenly to a transmission line (between two conductors or one conductor and earth), an energy wave will travel along the line with a speed approaching that of light.

Answer 14

TRUE

Question 15

TRUE or FALSE: As the line has capacitance, the voltage wave can advance only if accompanied by the current required to charge up the line

Answer 15

TRUE

Question 16

Thus the ratio of the travelling voltage to the travelling current is determined by

Answer 16

ratio of L to C

Question 17

The surge impedance of overhead transmission lines is usually between ___ and ___

Answer 17

300 and 500 Ohms

Question 18

TRUE or FALSE: The surge impedance of cables (due to their high capacitance and low inductance) is a small fraction only (of the order of onetenth) of these values

Answer 18

TRUE

Question 19

TRUE or FALSE: When a travelling wave reaches the end of an open-circuited line, no current can flow out at the point of open-circuit, thus the magnetic energy must be zero there

Answer 19

TRUE