8/10/2019 Power System (1)

1/14

Overvoltage phenomena in

Power systems

By,

Onkar nath Pandey

11104EN079

8/10/2019 Power System (1)

2/14

Types of Overvoltages The voltage stresses on transmission network insulation are found

to have a variety of Origins. In normal operation AC (or DC) voltages do not stress the insulation

severely. Over voltage stressing a power system can be classified into two

main types:A. External overvoltage: generated by atmospheric disturbances of

these disturbances, lightning is the most common and the mostsevere.

B. Internal overvoltages: generated by changes in the operating

conditions of the network. Internal over voltages can be dividedinto(a) switching overvoltages and(b) temporary overvoltages.

8/10/2019 Power System (1)

3/14

Lightening overvoltage. Lightning is produced in an attempt by nature to maintain a dynamic

balance between the positively charged ionosphere and the negativelycharged earth.

Over fair-weather areas there is a downward transfer of positive chargesthrough the global air-earth current.

This is then counteracted by thunderstorms, during which positive chargesare transferred upward in the form of lightning.

During thunderstorms, positive and negative charges are separated by themovements of air currents forming ice crystals in the upper layer of acloud and rain in the lower part.

The cloud becomes negatively charged and has a larger layer of positivecharge at its top.

8/10/2019 Power System (1)

4/14

The total potential difference between the two main charge centers mayvary from l00 to 1000 MV.

Only a part of the total charge-several hundred coulombs-is released to

earth by lightning; the rest is consumed in inter-cloud discharges. The height of the thundercloud dipole above earth may reach 5 km in

tropical regions. As the separation of charge proceeds in the cloud, the potential difference

between the centers of charges increases and the vertical electric field

along the cloud also increases.

8/10/2019 Power System (1)

5/14

A strike can average 100 million volts of electricity. Current ofup to 100,000 amperes.

Can generate 54,000 F Lightning strikes somewhere on the Earth every second and

kills hundreds of people every year. The most severe lightning stroke is that which strikes a phase

conductor on the transmission line. It produces the highestovervoltage for a given stroke current.

The lightning stroke injects its current into a terminationimpedance Z, which in this case is half the line surgeimpedance Zo since the current will flow in both directions as

shown in Figure. Therefore, the voltage surge magnitude atthe striking point is V =( )I Z (1)

8/10/2019 Power System (1)

6/14

INDUCED LIGHTNING STROKE When the thunderstorm generates negative charge at its ground end, the

earth objects develop induced positive charges. The earth objects of interest to electrical engineers are transmission lines

and towers

Normally the lines are unaffected because they are insulated by stringinsulators However, because of high field gradients involved, the positive charges

leak from the tower along the insulator surfaces to the line conductors

8/10/2019 Power System (1)

7/14

This process may take quite a long time, of the order of somehundreds of seconds.

When the cloud discharges to some earthed object other than theline, the transmission line is left with a huge concentration of charge(positive) which cannot leak suddenly.

The transmission line and the ground will act as a huge capacitorcharged with a positive charge and hence overvoltages occur due tothese induced charges.

This would result in a stroke and hence the name induced lightningstroke .

8/10/2019 Power System (1)

8/14

The lightning current magnitude is rarely less than 10 kA. Fortypical overhead line surge impedance Zo of 300 , the

lightning surge voltage will probably have a magnitude inexcess of 1500 kV.

8/10/2019 Power System (1)

9/14

Switching surges. There is a great variety of events that would initiate a switching

surge in a power network. The switching operations of greatest relevance to insulation design

can be classified as follows:

Energization of transmission lines and cables. The following specificswitching operations are some of the most common in thiscategory:

a) a. Energization of a line that is open circuited at the far end

b) b. Energization of a line that is terminated by an unloadedtransformer

c) c. Energization of a line through the low-voltage side of atransformer

8/10/2019 Power System (1)

10/14

Re-energization of a line - This means the energizationof transmission line carrying charges trapped byprevious line interruptions when high-speed reclosures

are used. Load rejection - This is affected by a circuit breaker

opening at the far end of the line. This may also befollowed by opening the line at the sending end inwhat is called a line dropping operation.

Switching on and off of equipment - All switchingoperations involving an element of the transmissionnetwork will produce a switching surge.

a) Switching of high-voltage reactors

b) Switching of transformers that are loaded by a reactoron their tertiary winding

c) Switching of a transformer at no load Fault initiation and clearing.

8/10/2019 Power System (1)

11/14

8/10/2019 Power System (1)

12/14

Temporary overvoltages. Temporary overvoltages (sustained overvoltages) differ from

transient switching overvoltages in that they last for longerdurations, typically from a few cycles to a few seconds.

They take the form of undamped or slightly damped oscillations at

a frequency equal or close to the power frequency. The classification of temporary overvoltages as distinct from

transient switching overvoltages is due mainly to the fact that theresponses of power network insulation and surge arresters to theirwave shapes are different.

When a transmission line or a large inductive load that is fed from apower station is suddenly switched off, the generator will speed upand the bus bar voltage will rise.

8/10/2019 Power System (1)

13/14

A single line-to-ground fault will cause the voltagesto ground of the healthy phases to rise.

In the case of a line-to-ground fault, systems withneutrals isolated or grounded through highimpedance may develop overvoltages on healthyphases higher than normal line-to-line voltages.

Solidly grounded systems will only permit phase-to-ground overvoltages well below the line-to-linevalue .

Ferranti effect also causes temporaryovervoltages.

8/10/2019 Power System (1)

14/14

References.

High voltage engineering: Fundamentals E.Kuffel, W. S. Zangl, J. Kuffel.

High voltage Engineering M. S. Naidu, V.Kamaraju