OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS

UNIT I

Mr. S. VINODLECTURER

EEE DEPARTMENT

Causes of over voltages and its effect on power system

• Lightning• switching surges• temporary over voltages

protection against over voltages.

INTRODUCTION• The examination of over voltages on the power system includes a study of

their– Magnitudes– Shapes– durations – frequency of occurrence

• study should be performed not only at the point where an over voltage originates but also at all other points along the transmission network to which the surges may travel.

• Lightning rapidly heats the air in its immediate vicinity to about 20,000 ° C (36,000 °F), about three times the temperature of the surface of the Sun. This compresses the surrounding clear air and creates a supersonic shock wave which decays to an acoustic wave that is heard as thunder Lightning rapidly heats the air in its immediate vicinity to about 20,000 °C (36,000 °F) — about three times the temperature of the surface of the Sun. This compresses the surrounding clear air and creates a supersonic shock wave which decays to an acoustic wave that is heard as thunder

TYPE OF OVER VOLTAGE• 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, However, they remain the initial factor that determines its dimensions. Over voltages stressing a power system can generally be classified into two main types:

• 1. External over voltages: generated by atmospheric disturbances. Of these disturbances, lightning is the most common and the most severe.

• 2. Internal over voltages: generated by changes in the operating conditions of the network. Internal over voltages can be divided into (a) switching over voltages and (b) temporary over voltages.

LIGHTING PHENOMENON• Lightning phenomenon is a peak discharge in which charge accumulated

in the clouds discharges into a neighboring cloud or in the ground,

between clouds and cloud to ground which is very large.

• Some of the facts of clouds formation during thunder storms and lightning.

1. Upper region of the cloud is positive charge and the lower region of the

cloud are negatively charge except the local region near the base.

2. Height of the cloud from ground is about 200 to 10,000m and the charge

center at the distance of 300 to 20000m.

3. Charge inside the cloud is 1 to 100C.

4. Energy associate with the cloud is 250kWh

Theory of Charge formation

• Simpson’s theory.• Renold and Mason

theory.

• Below region A : Air current travel above 800cm/sec and no rain drops fall through.

• In region A: Air velocity is high enough to break the falling rain drops causing a +ve charge in the air. As the spray blown upwards, but as the velocity of air decreases.

• Region B : Above region A becomes negatively charged by air current.

• Region C : The temperature is low and only ice crystal exits. The impact of air on these crystals make them negatively charge and air is positively charge.

RENOLD AND MASON THEORY

• The explanation presented by Simpson theory was not satisfactory.

• Thunder clouds are developed at heights of 1 to 2 kms above the ground level and may extend 12 to 14 km above the ground.

• For thunder clouds and charge formation air current, moisture and specific temperature are required.

• Air current controlled by the temperature gradient move upwards carrying moisture and water droplets.

• The temperature is 0º degree about 4km from ground.• -50º about 12km from ground.• Water droplets in the thunder clouds are blown up in the air

currents and gets super cooled over a range of height and temperature.

CHARACTERISTICS OF LIGHTNING

• Static Electricity• Ultra-High Voltage Generation• The Discharge• Surface Dispersion• Basic Laws of Electricity

LIGHTNING OVER VOLTAGE• According to theories generally accepted, lightning is produced in an attempt

by nature to maintain a dynamic balance between ionosphere and the earth

• During thunderstorms, positive and negative charges are separated by the movements of air currents forming ice crystals in the upper layer of a cloud and rain in the lower part. The cloud becomes negatively charged and has a larger layer of positive charge at its top. As the separation of charge proceeds in the cloud, the potential difference between the concentrations of charges increases and the vertical electric field along the cloud also increases.

• The total potential difference between the two main charge centers may vary from 1500V 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

ANATOMY OF A LIGHTNING STROKE

Electrons begin zigzagging downward in a forked pattern. This is the “stepped leader.”

As the stepped leader nears the ground, it draws a streamer of positive charge upward.

As the leader and streamer come together, a powerful electrical current begins flowing.

Current begins the return stroke, an intense wave of positive charge traveling upward about 60,000 miles per second.

LIGHTNING VOLTAGE SURGES• The most severe lightning stroke is that which strikes a phase conductor on

the transmission line as it produces the highest over voltage for a given stroke current. The lightning stroke injects its current into a termination impedance Z, which in this case is half the line surge impedance Zo since the current will flow in both directions. Therefore, the voltage surge magnitude at the striking point is

• The lightning current magnitude is rarely less than 10 kA and thus, for a typical overhead line surge impedance Zo of 300Ω, the lightning surge voltage will probably have a magnitude in excess of 1500 kV.

• The shapes and magnitudes of lightning surge waves get modified by their reflections at points of discontinuity as they travel along transmission lines

CAUSES OF OVER VOLTAGES DUE TO LIGHTNING STRIKES

A transient over voltage is a voltage peak with a maximum duration of less than one millisecond.

• Over voltages due to direct lightning strikes• Over voltages due to the indirect effects of lightning strikes

OVER VOLTAGES DUE TO DIRECT LIGHTNING STRIKESThese can take two forms:

• When lightning strikes a lightning conductor or the roof of a building which is earthed, the lightning current is dissipated into the ground. The impedance of the ground and the current flowing through it create large difference of potential: this is the over voltage. This over voltage then propagates throughout the building via the cables, damaging equipment along the way.

• When lightning strikes an overhead low voltage line, the latter conducts high currents which penetrate into the building creating large overvoltages. The damage caused by this type of over voltage is usually spectacular (e.g. fire in the electrical switchboard causing the destruction of buildings and industrial equipment) and results in explosions.

Over Voltages Due To The Indirect Effects Of Lightning Strikes

• The over voltages previously mentioned are also found when lightning strikes in the vicinity of a building, due to the increase in potential of the ground at the point of impact. The electromagnetic fields created by the lightning current generate inductive and capacitive coupling, leading to other over voltages.

• Within a radius up to several kilometers, the electromagnetic field caused by lightning in clouds can also create sudden increases in voltage.

• Although less spectacular than in the previous case, irreparable damage is also caused to so called sensitive equipment such as fax machines, computer power supplies and communication systems.

Increase in ground potential, Electrostatic field, Magnetic field

Representation of the various disturbances on electrical networks

Diagram of an installation protected against lightning and its indirect effects

PRINCIPLES OF PROTECTION

• Lightning Protection System• Air Terminal• Discharge Conductor• Lightning Arresters

» 1. Rod arrester» 2.Horn gap arrester» 3. Multigap arrester » 4. Expulsion type lightning arrester» 5. Valve type lightning arrester

• Counterpoise

AIR TERMINAL

• Also known as a Lightning Rod• Traditionally ¼ inch copper rod

– With sharpened point– Six inches above object to be protected

Discharge Conductor• Not less than #4 AWG, SWG, BWG- 0.23in

– Uninsulated stranded copper wire• “Straight” from Air Terminal to Ground

– No sharp bends (bend radius of 1 foot)– Should be run outside of hull

• Electrical wiring should be at right angles

#4

Lightning Arresters• The earthing screen and ground wires can well

protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves.

• The lightning arresters or surge diverters provide protection against such surges.

• A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground.

• surge diverter consists of a spark gap in series with a non-linear resistor.

• One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.

• The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc.

ROD GAP ARRESTER• It is a very simple type of diverter and

consists of two 1.5 cm rods, which are bent at right angles with a gap in between.

• One rod is connected to the line circuit and the other rod is connected to earth

• The distance between gap and insulator must not be less than one third of the gap length so that the arc may not reach the insulator and damage it.

• Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth

LIMITATIONS OF ROD GAP ARRESTER

• (i)After the surge is over, the arc in the gap is maintained by the normal supply voltage, leading to short-circuit on the system.

• (ii) The rods may melt or get damaged due to excessive heat produced by the arc.

• (iii) The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester.

• (iv) The polarity of the f the surge also affects the performance of this arrester.

Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main arresters.

HORN GAP ARRESTER• It consists of a horn shaped metal rods A and B

separated by a small air gap.• The horns are so constructed that distance

between them gradually increases towards the top.

• The horns are mounted on porcelain insulators. One end of horn is connected to the line through a resistance and while the other end is effectively grounded.

• The resistance R helps in limiting the follow current to a small value.

• The gap between the horns is so adjusted that normal supply voltage is not enough to cause an arc across the gap.

• Under normal conditions, the gap is non-conducting.

• On the occurrence of an over voltage, spark-over takes place across the small gap G.

• The heated air around the arc and the magnetic effect of the arc cause the arc to travel up the gap. The arc moves progressively into positions 1,2 and 3. At some position of the arc (position 3), the distance may be too great for the voltage to maintain the arc; consequently, the arc is extinguished. The excess charge on the line is thus conducted through the arrester to the ground.

MULTIGAP ARRESTER • It consists of a series of metallic (generally alloy of zinc) cylinders insulated from one

another and separated by small intervals of air gaps.• The first cylinder (i.e. A) in the series is connected to the line and the others to the

ground through a series resistance. • The series resistance limits the power arc. By the inclusion of series resistance, the

degree of protection against traveling waves is reduced. • In order to overcome this difficulty, some of the gaps (B to C) are shunted by

resistance. • Under normal conditions, the point B is at earth potential and the normal supply voltage

is unable to break down the series gaps. • On the occurrence an over voltage, the breakdown of series gaps A to B occurs. • The heavy current after breakdown will choose the straight – through path to earth via

the shunted gaps B and C, instead of the alternative path through the shunt resistance.

EXPULSION TYPE LIGHTNING ARRESTER

EXPULSION TYPE LIGHTNING ARRESTER

• It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube.

• The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth.

• One expulsion arrester is placed under each line conductor.• On the occurrence of an over voltage on the line, the series gap AA’

spanned and an arc is stuck between the electrodes in the tube.• The heat of the arc vaporizes some of the fiber of tube walls resulting in

the production of neutral gas. In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow.

• As the gas leaves the tube violently it carries away ionized air around the arc. This de ionizing effect is generally so strong that the arc goes out at a current zero and will not be re-established

EXPULSION TYPE LIGHTNING ARRESTER

Advantages:(i) They are not very expensive.

(ii) They are improved form of rod gap arresters as they block the flow of power frequency follow currents

(iii) They can be easily installed.Limitations:(i) An expulsion type arrester can perform only limited number of

operations as during each operation some of the fiber material is used up.

(iii) This type of arrester cannot be mounted on enclosed equipment due to discharge of gases during operation.

COUNTERPOISE• A counterpoise is galvanized steel wire run in parallel or radial or a

combination of the two, with respect to the overhead line.• The corners of the squares indicate the location of the tower legs. The

lightning stroke as is incident on the tower, discharges to the ground through the tower and then through the counterpoises.

• the leakage resistance of the counterpoise should always be smaller than the surge impedance otherwise, positive reflections of the surge will take place and hence instead of lowering the potential of the tower (by the use of counterpoise) is will be raised.

• The leakage resistance of the counterpoise depends upon the surface area, i.e., whether we have one long continuous counterpoise say 1000 m or four smaller counterpoises of 250 m each, as far as the leakage resistance is concerned it is same.

COUNTERPOISE

Over voltage Due To Switching Surges• when the transmission voltages were about 220 kV and below, over voltages

due to lightning were of very high order and over voltages generated inside the system were not of much consequence.

• with increase in transmission voltages, (400 kV and above) the over voltages generated inside the system reached the same order of magnitude as those of lightning over voltages, or higher.

• Over voltages thus generated last for longer durations and therefore are severe and more dangerous to the system.

• Insulation co-ordination, where the protective level of any particular kind of surge diverter is proportional to the maximum voltage, the insulation level and the cost of the equipment depends on the magnitudes of these over voltages.

• Over voltages that determine the insulation level of the lines and other equipment and consequently, they also determine their dimensions and costs.

Origin of Switching Surges• The making and breaking of electric circuits with switchgear

may result in abnormal over voltages in power systems having large inductances and capacitances.

• The over voltages may go as high as six times the normal power frequency voltage.

switching surges are quite different and may have origin from any of the following sources.

1. De-energizing of transmission lines, cables, shunt capacitor, banks, etc.2. Disconnection of unloaded transformers, reactors, etc.3. Energization or reclosing of lines and reactive loads,4. Sudden switching off of loads.5. Short circuits and fault clearances.6. Resonance phenomenon like ferro-resonance, arcing grounds, etc.

over voltages due to Switching Surges are calculated By

a) mathematical modeling of a system using digital computersb) scale modeling using transient network analysersc) by conducting field tests to determine the expected maximum

amplitude of the over voltages and their duration at different points on the line.

The main factors that are investigated in the above studies are(i) the effect of line parameters, series capacitors and shunt reactors on the magnitude and duration of the transients

(ii) the damping factors needed to reduce the magnitude of over voltages

(iii) the effect of single pole closing, restriking and switching with series resistors or circuit breakers on the over voltages, and

(iv) the lightning arrester spark over characteristics.

CONTACT CLOSING• . The simple closing of

switch or of a circuit breaker can produce significant over voltage in electric system.

• The transformer in the line represented as leakage inductance, while the cable is represented as capacitance

The circuit performance after switching may be expressed by the following differential equation:

(1)The supply voltage vs(t) beyond the switching instant is given by

(2)The expression for the voltage across the line capacitance takes the form 1 and

2

(3)

MEASURES TO REDUCE THE OVER VOLTAGES

(a) Energization of transmission lines in one or more steps by inserting resistance and withdrawing them afterwards,

(b) phase controlled closing of circuit breakers,

(c) drainage of trapped charges before reclosing,

(4) use of shunt reactors, and

(e) limiting switching surges by suitable surge diverters.

Resistor Switching• It is done by initially applying the supply voltage to the line through a

resistor. After a suitable period of time, normally between one third and one-half of a cycle, the preinserted resistor is short circuited, allowing the full supply voltage to be applied to the line.

• By the end of the preinsertion period, the magnitude of the Energization surge is usually much reduced by the effect of system damping

• The initial amplitude of the energization surge when a preinsertion resistor of value R is used will be only Zo/(R + Zo) of that reached in the absence of the resistor, where Zo is the surge impedance of the line.

• When the resistor is shorted at the end of the preinsertion period, another surge will develop. If R is too small, control of the first surge becomes ineffective; if it is too large, the second surge becomes dangerous. An optimal value of R would normally be a fraction of Zo, and depends on transmission-line length.

Phase Controlled Switching• The amplitude of the energization surge depends on the

switching phase angle ωT.

• By properly timing of the closing of the circuit breaker poles, the resulting switching over voltage can be greatly reduced. Phase-controlled switching should be carried out successively for the three poles to accomplish a reduction in the initial voltages on all three phases.

• This is extremely difficult with conventional circuit breakers but is quite possible with solid-state circuit breakers,

USE OF SHUNT REACTORS• Shunt reactors are used on many high-voltage transmission

lines as a means of shunt compensation to improve the performance of the line.

• which would otherwise draw large capacitive currents from the supply. They have the additional advantage of reducing energization surge magnitudes.

• This is accomplished mainly by the reduction in temporary over voltages.

DRAINAGE OF TRAPPED CHARGES BEFORE RECLOSING

• Charges are trapped on the capacitance to ground of transmission lines after their sudden reenergization.

• If the line is reenergized soon after, usually by means of automatic reclosures, these charges may cause an increase in the resulting surge.

• In practice, trapped charges may be partially drained through the switching resistors incorporated in circuit breakers, as mentioned earlier.

• Magnetic-type potential transformers also drain trapped charges via a low-frequency oscillation which is highly damped by the effect of magnetic saturation.

SURGE ARRESTER• A protective device connected between a conductor of an electrical system

and ground to limit the magnitude of transient over voltages on equipment.

• The valve arrester consists of disks of zinc oxide material that exhibit low resistance at high voltage and high resistance at low voltage.

• By selecting an appropriate configuration of disk material, the arrester will

conduct a low current of a few mA at normal system voltage. During conditions of lightning or switching surge over voltages, the surge current is limited by the circuit; and for the magnitudes of current that can be delivered to the arrester location, the resulting voltage will be limited to controlled values, and to safe levels as well.

• A typical surge arrester consists of disks of Zinc oxide material sized in cross-sectional area to provide desired energy discharge capability, and in axial length proportional to the voltage capability.

• The disks are then placed in porcelain enclosures to provide physical support and heat removal, and sealed for isolation from contamination in the electrical environment

Temporary over voltages• Temporary over voltages (i.e., sustained over voltages) differ

from transient switching over voltages in that they last for longer durations, 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.

• Some of the most important events leading to the generation of temporary over voltages are

– Load rejection– Ferranti effect– ground fault– Harmonic over voltage due to magnetic saturation

LOAD REJECTION• When a transmission line or a large

inductive load that is fed from a power station is suddenly switched off, the generator will speed up and the busbar voltage will rise

• The amplitude of the over voltage can be evaluated as

• E is the voltage behind the transient reactance

• Xs the transient reactance of the generator in series with the transformer reactance

• Xc the equivalent capacitive input reactance of the system

Ferranti effect• The Ferranti effect of an uncompensated transmission line is

given by

• where Vr and Vs are the receiving-end and sending-end voltages, respectively, and l is the line length (km). β0 is the phase shift constant of the line per unit length.

• It is equal to the imaginary part of √ZX, where Z and Y are the impedance and admittance of the line per unit length.

• For a lossless line , β = ω √LC where L and C are the inductance and capacitance of the line per unit length.

• β0 has a value of about 60 per 100 km at normalpower frequency

GROUND FAULT

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

• In the case of a line-to-ground fault, systems with neutrals isolated or grounded through a high impedance may develop overvoltages on healthy phases higher than normal line-to-line voltages.

• Solidly grounded systems, on the other hand, will only permit phase-to-ground over voltages well below the line-to-line value.

.

Harmonic over voltage due to magnetic saturation

• Harmonic oscillations in power systems are initiated by system nonlinearities whose primary source is that of the saturated magnetizing characteristics of transformers and shunt reactors.

• The magnetizing current of these components increases rapidly and contains a high percentage of harmonics for voltages above the rated voltage.

• Therefore, saturated transformers inject large harmonic currents into the system.

PART – A (2 marks)1. What are the causes of power frequency

over voltages?2. What are the causes of over voltages in

power system?3. What are the different types of fault that

may occur on power lines?4. What are harmful effects of lightning?5. Name the sources of switching surges?6. What is voltage surge? Draw a typical

lightning voltage surge.

PART – B (16 marks)

1. Explain the selection of surge arresters for E.H.V. system. Explain the V-I Characteristics of typical surge diverters.

2. What are the methods used to control of over voltages due to switching? Explain briefly.

3. What are the causes for power frequency over voltage? How they are controlled in power systems?

4. What are the causes for switching over voltage? How they are controlled in power systems?

5. What are the different methods employed for lightning protection of overhead lines?

6. Explain the characteristics of switching surges with typical wave shapes clearly.

7. Give the mathematical model of lightning discharges and explain them.

8. With neat sketches, describe the two types of lightning strokes.