UNIT 1 – OVER VOLTAGES IN ELECTRICAL POWER SYSTEM

1. Causes for Over voltages (or) Sources of Over voltages:

Transients:-
 Sudden change in system condition (i.e) voltage.
 Temporary excess of voltage and/or current in an electrical circuit, when a fault occurs in
a system or switch opens or closed
 Transient time period is very short
Causes of Transients:
1. Internal Causes
i) Switching surges
a) Current chopping
b) Switching of unloaded line
c) Switching of loaded line
d) Ferro resonance
ii) Insulation failure
iii) Arcing ground
2. External Causes
Internal Causes or Internal Sources
 Internal causes do not produce transients of large magnitude; the transients produced due
to internal causes may increase the system voltage to twice the normal value.
 By providing proper insulation of the equipment, the switching transients are taken care.
 The internal causes of transients are mainly due to oscillations set up by the sudden
changes in the circuit due to breaker operation.
(i) Switching surges
The making and breaking of electrical circuits with switch gear may result in abnormal
transient over voltage in power system have large inductances and capacitance.
 Current Chopping:
 When breaking low current, due to deionizing effect of air blast causes the current
abruptly to zero well before the natural current zero is reached. This phenomenon
is called “current chopping”.
 Current chopping results in the production of high voltage transients across the contacts
of air blast C.B
 The transient overvoltage due to current chopping is prevented by resistance switching.

 Switching of unloaded line:

  When the unloaded line is connected to the voltage source a voltage wave is set up
which travels along the line.
 On reaching the terminal point, the wave is reflected back to the supply end without
change in sign. This is called “voltage doubling”. Voltage on the line becomes twice
the normal value. This transient voltage is of temporary voltage.
 If a unloaded line is switched off, the line will attain a voltage of 2 .

 Switching of loaded line

 Over voltages will also be produced during switching operations of loaded line.
 If loaded line is interrupted, the voltage will be raised to 2ZI across the breaker.
 I- instantaneous current, Z- Natural impedance
 Ferro Resonance:
 Resonance in an electrical system occurs when inductive reactance of the circuit
becomes equal to the capacitive reactance.
 When XL=XC, Power factor is unity.
 If generator emf wave is distorted, the trouble of resonance may occur due to 5 th or
higher harmonics.
 Energization of loaded line:
 Over voltage will be produced during switching operation or sudden interruption of
loaded line. This will set up a voltage of 2ZnI across the breaker.

 Natural impedance =

(ii) Insulation failure

 The most common case of over voltage is insulation failure between line and earth
which causes high voltage in the system.
 Suppose a line at potential V is earthed at point C. the earthing of line causes two equal
voltages –V travel along CA and CB. Both current pass through C to earth so that

current to earth is .
(iii) Arcing ground
 If the neutral of the three phase wire was not earthed in long high voltage
transmission line a serious problem called arcing ground.
 The arcing ground produces severe oscillations of three to four times the normal
voltage.

External Causes or External Sources:

 An electrical discharge between cloud and earth (or) between clouds(or)between the
charges centre of the same cloud is known as lightning.
 Lightning is a huge spark and takes place when clouds are charged to such a high
potential with respect to earth or neighboring cloud.

Lightning
Lightning is an unavoidable event that affects power system through several mechanisms.
(i) Direct flash
In the case of a direct stroke to the electrical system, the immediate thread is the flow
of lightning current through the earthing impedance, resulting in over voltage.
(ii) Near flash
In case of a near flash, the immediate thread is the voltage induced in circuit
loops,which in turn can produce surge currents.
(iii) Far flash
In the case of a far flash, the thread is limited to induced voltage. Therefore, the response
of an electrical system to the lightning event is an important consideration in assessing the
thread.

2. Mechanism of lightning stroke (or) Lightning Phenomena:

 An electric discharge between cloud and earth, between clouds or within the same
cloud is known as “lightning”.
 The discharge of the charged cloud to the ground is called lightning phenomenon.
 The large spark accompanied by light produced by an abrupt, discontinuous discharge
of electricity through air, form the clouds generally under turbulent conditions of
atmosphere is called lightning.
 The clouds get charged during thunder-storms, the high potential gradient cause’s
breakdown of insulation of air production a lightning stroke.
 A lightning discharge through air occurs when a clouds is raised to such a high potential
with respect to the ground.
 The total potential difference between the two main charge centers may vary from 100 to
1000mv.
 As the lower part of the cloud is negatively charged, the ground gets positively
charged by induction.
Mechanism:
 When a charged cloud passes over the earth, it induces equal and opposite change on the
earth below.
 The charge acquired by the cloud increase, the potential between cloud and earth
increases and gradient in the air increases.
 The potential gradient is sufficient to breakdown the surrounding air, the lightning stroke
starts.
 At this instant, a streamer called a “pilot streamer” starts from the cloud towards the
ground which is not visible.

 The current is 100 ampere and 0.15m/micro sec.

 This process continues until one of the leader strikes ground. When one of the stepped
leader strike the ground an extremely bright return streamer propagates upwards from
ground to cloud.

 After neutralization of the negative charge on the cloud any further from the nearest
charge centre. This streamer of discharge is called “dart streamer”.
 Cold lightning stroke:- the discharge current in the return streamer high it continues for
few micro seconds less energy. It contains less energy.
 Hot lightning stroke:- (Dart leader) – having less discharging current, contains more
energy.
wave shape of lightning stroke

Characteristics of lightning wave shape

 Crest or peak value and it have been observed that the max value of this current is 4,
00,000 amps.
 The wave front line which varies from 1 to 10 micro sec.
 The time at which the stroke current reduces to 50 percent of that crest value and its
estimated limit is 10 to 100µ sec.
Location:-

In the United States, for example Florida experience the largest no. of recorded strikes in a
given period.

Incidence of lightning to power lines.

3. A. Mathematical model of lightning:
The lightning stroke consider as a current source.

Where, Zs – impedance of the lightning channel

Z – impedance of stroke strikes object

ZGW – impedance of ground wire

When a streamer discharge occurs to ground by first stroke, followed by main stroke.

If the lightning strikes an object of impedance Z, the voltage built across.

The magnitude of surge voltage is

If a lightning stroke current as low as 10000A strikes line of 400Ω surge impedance, it
may cause over voltage of 4000kV. This will cause immediate flash over of the line
conductor and insulator.
3. B. Rate of Charging of thunder clouds:
Thunder clouds consist of uniform mixture of positive and negative charges. Due to air
currents and hail stones the charges separate.

If the generated charge Qg is being separated Qs at a velocity V, the rate of growth of the
electric field is

----------1
 Where, λ – effective air conductivity

v – velocity of separation of charges

ρ – charge density in the cloud

From equation 1, ---------2

-------------------3

----------4

Substitute the equations 3, 4 in equation 2

-----------5

Qg x h is the electric moment of inertia.

--------------6

4. Control of over voltages:

i) Insertion of resistor
ii) Phase controlled switching of circuit breaker
iii) Drainage of trapped charge
iv) Shunt reactor
i) Insertion of resistor
Resistor ‘R’ inserts in series with the circuit breaker, when switching ‘ON’ but closed them for after
few cycles. This will reduce the transients occurring due to switching. If the resistance is inserted for
long time, voltage reaches high value.

ii) Phase controlled switching of circuit breaker

 Life of circuit breaker depends on number of operation. Over voltage can be avoided by closing the
circuit breaker at same instant. Phase controlled technique used for exact switching.
iii) Drainage of trapped charge
when the transmission line switched off , electric charge on capacitor will leak through leakage path
of the insulator. Insertion of resistor is used to reduce the trapped charges.
iv) Shunt reactor
Shunt reactors are used limit the voltage rise due to Ferranti effect. but shunt reactors will give rise to
oscillations with capacitance. Resistor in series with the reactor is used to suppress these oscillations.

5. Reflection and Refraction of travelling wave:

Reflection is the Change in direction of waves when they bounce off carrier.

Refraction is the Change in direction of waves when they passes from one medium to
another.

When the waves arrives at discontinuity, the impedance of line changes.

The reflected wave and its and its companion current wave travel back to the line
superimposed on incident wave. But the refracted wave penetrates beyond the
discontinuity.

The impedance of a transmission line is,

ZL = (LL/CL)1/2

The impedance of Cable is,

ZC = (LC/CC)1/2

The wave that starts travelling over the line could be considered as Incident wave.

The rise in potential (V) could be considered due to the wave which is reflected at the open
end.
Actual voltage at the open end could be considered as Refracted or Transmitted wave.

Refracted wave = Incident wave + Reflected wave

Let 1, 2 and 3 subscripts represents the incident, reflected and refracted waves respectively.

ZAis the impedance of line and ZB is the impedance cable.

The incident voltage (V1) and current waves (I1) is represented by,

I1 = V1/ ZA

Let the reflected and refracted waves be V2 and V3 respectively, the reflected and refracted
current will be

I2 = -V2/ ZA

Negative sign indicates that the wave travelling at opposite direction.

I3 = V3/ ZB

V3 = V1+V2

I3 = I1+I2

Expression for the Reflected wave:

The reflected wave in terms of incident wave,

is called reflection co-efficient and denoted as ‘a’.

It can be either positive or negative depending on the values of ZB and ZA

-1 ≤ a ≤ 1.

Expression for Refracted wave:

Refracted wave in terms of incident wave,

 is called refraction co-efficient. And denoted as ‘b’.

It can be only positive.

0≤b≤2

6. Charge formation theory:

 During thunderstorms, positive and negative charges in the clouds separated by heavy air
current with ice crystal in upper part and rain in lower part.
 Charge separation depends on the height of the clouds, which range from 200 m to 10 km
with their charge centers at a distance of 300 m to 2 km.
 Charge inside the cloud 1-100 c, Max potential of a cloud 10 mv to 100 mv, Field
gradients 100 v/cm -10 kv/cm
 Energy in a lightning stroke 250 kwhr, Max gradients at ground level 300v/cm,Fair
weather gradient is 1v/cm
Wilson’s theory of charge separation
 Wilson’s theory is based on the assumption.
 A large number of ions are present in the atmosphere
  Many of these ions attach themselves to small particles and water particles
 A normal electric field exists in the atmosphere under fair-weather condition this is
generally directed down ward towards the earth
 A relatively large raindrop (0.1cm radius) falling in this field becomes polarized
 On upper sides = acquires negative charge
 On lower sides = acquires positive charge
 According to Wilson theory
 Large negatively charged drops settle on base of clouds
 Smaller positively charged drops settle on upper positive of the clouds

Simson’stheory:

There are three regions A, B and C in the cloud to be considered for charge formation

 Below region A, air current travel above 800cm/s and no raindrops fall through
 Region B is negatively charged by air current
 Region C only ice crystal exist due to low temp
 The explanation presented on Samson’s theory is not satisfactory Recently, Reynold and
mason proposed modification

Reynolds and mason theory:

 Thunder clouds are developed at heights 1to2 km above the ground level and may
extend upto 12 to 14 km above the ground
 Temperature is 0oc at about 4 km from the ground and many reach -50oc at 12 km height
 Water droplets don’t freeze at 10oc, they freeze below -40oonly
 Thunder cloud consisting super cooled water droplets moving upwards and large hail
stones moving downwards
 Splinter moved up by air currents and carry positive charge to upper region
 Hail stones that travel downwards carry an equivalent negative charge to the lower regions
in the clouds
 Once the charge separation is complete, the conditions are now solution for a lighting
stroke

7. Protection of power system using protective device:

 Effective protection against direct strokes requires a shield to prevent lighting from
striking the electrical conductors, together with adequate drainage facilities over
insulated structures so that the discharge can drain to the ground without affecting the
conductors
 The most generally accepted and effective method protecting lines against direct strokes is
by the use of overhead ground wires
  This method of protection is known as shielding method which does not allow an area
path to from between the line conductors and ground
Ground wires:

 Ground wires are conductors running parallel to the main conductors of the
transmission line, supported on the same towers
 Ground wires are made of galvanized steel wires or Aluminum conductor steel-
reinforced (ACSR) conductors
 Overhead ground wires protect the lines by intercepting the direct strokes
 So keeping them off the phase line conductors, and by providing multiple paths for
conducting the strokes current to ground
 The multiple paths for the stroke current result in a reduced voltage drop
 In addition to the above function, ground wires also reduces the voltages induced on line
conductors

Requirement of ground wire for effective protection:-

 There should be an adequate clearance between the line conductors and the ground or
the tower structure.
 There should be an adequate clearance between the line conductors and round wire. It’s
come under transmission line designing.
 It is very important that the tower footings resistance should be as low as possible so as to
allow lightning discharge current to flow to earth easily.
 Ground wire having sufficient mechanical strength must be situated in such a manner so
as to shield the line conductors from direct strokes.
 To avoid short circuit:The round wire s should be placed at such a height above the line
conductors that they should be well out on towers and should not be exactly over the
conductor in order to avoid any possibility of a short circuit occurring in the event of
conductor swinging under lie loading.
 Protective ratio: it is defined as the ratio of the induced voltage on a conductor with
ground wire protection to the induced voltage which would exists on the conductor
without ground wire protection.
 Protection angle or angle of protection: it is defined as the angle formed between the
normal passing through the ground wire.

Protective angle ‘α’ of an overhead ground wire

 Height of the ground wire:- for protecting against lightning, the ground wire should be
located at a height at least 10%

Equation for calculating the height

X= horizontal spacing between the conductor and ground wire

H=height of cloud
Y= height of ground wire
B=height of the conductor
Protection using ground rods and counter poise wires:
Ground rods:
 Ground rods are a number of rods about 15mm diameter and 205to 3m long driven into
ground.
 In hard soils, the rods may be driven to a depth of 50m.
 It is made up of galvanized iron or copper.
 The effective resistance may be reduced to 10Ω.
Counter poise wires:
 Counter poise wires are buried in the ground in depth of 0.5 to 1.0m, running parallel to the
line conductor and connected to tower legs.
 Lightning stroke first discharge tower to ground and through counter poise wires.
Shunt Protective Devices:
i) Expulsion gaps:
Expulsion gap is a device which consists of a spark gap together with an arc quenching device. It
extinguishes the current arc when the spark gaps break over due to over voltages.
It consists of rod gap in series with second gap enclosed within fibre tube. If the over voltage occurs,
both spark gaps breakdown simultaneously. The arc in fibre tube vapourizes a small portion of fibre
material. Due to mixture of water vapour and decomposed fibre product, a gas is produced. The gas will
drive away the arc products and ionized air.
When the follow on power frequency current reaches zero value, the arc is extinguished and the path
become open circuited.
 Figure. Expulsion gap

ii) Protector tube:

Protector tube is commonly used on system voltage up to 33kV. It consists of rod gap AA’ in series
with a second gap enclosed within fibre tube. The gap in the fibre tube is formed by two electrodes. The
upper electrode is connected to rad gap. And lower electrode is connected to earth.
When the over voltage occurs, the arc is struck between electrodes. The heat of arc vapourizes and
produces some neutral gas. The arc goes out at a current zero and will not be re-established.

Figure. Protector tube

Advantages:
 Not expansive
 Improved form of rod gap arrester
 Easily installed
Limitations:
 Limited no. of operations
 Cannot be mounted on enclosed equipment

iii) Rod gaps:

 Rod gap is very simple type of diverter. It consists of two 1.5cm rods. It bends at right angles with a
gap. One rod is connected to line conductor and other is connected to earth.
The distance between gap and insulator is more than one third of gap length. So the arc may not
reach the insulator and damage it. The gap length is so adjusted.
Under normal conditions, the gap remains non-conducting. When the over voltage occurs on the line,
the gaps closed and surge current id conducted to earth.

Figure. Rod gap arrangement

Limitations:
 The rods may melt or damaged due to excessive heat by arc
 Climatic conditions may affect the performance
 Polarity of surge also affects the performance.

iv) Surge Arresters (or) Lightning arresters:

It is used at substations and at line terminations to discharge the lightning over voltages and short
duration switching surges. It is usually mounted on the line end at the nearest to the substation.
Under normal conditions, the voltage is insufficient to breakdown the air gap. When the over voltage
occurs, the breakdown of series gap takes place and the surge current is conducted to earth.
Since the surge current is high, non-linear resistor offers a low resistance path. When the surge is
over, non-linear resistors assume high resistance to stop the flow of current.
Figure. Non-linear element Surge arrester