www.jntuworld.
com
www.jwjobs.net
www.jntuworldupdates.org
Unit-VIII
[Protection against Over-Voltages]
www.specworld.in
www.jntuworld.com
www.smartzworld.com
Outline
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
Profile
Overvoltage
Voltage dip
Harmonics
Mitigative tools and solutions
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
Transient effects in power system
www.jntuworld.com
www.jntuworld.com
www.jntuworldupdates.org
TRANSIENT EFFECT is defined as the result of a
step change in an influence quantity on the steady
state values of a circuit. [1]
In power system, transient effects can be roughly
described as undesired voltages that may result in
interruption or even damage not only to system
devices but also to customer equipments.
Typically, they are related to the power quality issues
in term of overvoltage, voltage dip and harmonics.
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
Classification of Transients[3]
www.jntuworld.com
www.jntuworldupdates.org
Impulsive Transient
www.jntuworld.com
-Nanosecond 5ns rise
-Microsecond 1
1s rise
-Millisecond 0.1ms rise
O ill
Oscillatory
Transient
T
i
- Slow transient
<5 kHz
- Medium transient 55--500 kHz
- High transient
0.5
0.5--5 MHz
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
Overvoltage
g
www.jntuworld.com
www.jntuworld.com
www.jntuworldupdates.org
Lightning overvoltage is a type of transient
overvoltage
g in which a fast front voltage
g is pproduced
by lightning or fault. Such overvoltage is usually
unidirectional and of very short duration. [2]
Switching overvoltage is a transient overvoltage in
which a slow front, shortshort-duration, unidirectional or
oscillatory,
ill
highly
hi hl damped
d
d voltage
l
is
i generatedd
(usually by switching or faults).[2]
I mostt off the
In
th cases suchh surges may damage
d
the
th
Nearby transformer, generators & other equipments.
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
Causes of over voltages
www.jntuworld.com
www.jntuworldupdates.org
Internal causes
www.jntuworld.com
Switching surge
surges
Insulation failure
Arcing ground
Resonance
E t
External
l causes
Direct lightning Strokes
Atmospheric conditions
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
Switching surge:
surge: The overvoltages produced on the power
system due to switching are known as switching surge.
surge
www.jntuworld.com
www.jntuworldupdates.org
Insulation failure:
failure: The most common case of insulation
failure in a ppower system
y
is the ggrounding
g of conductors ((i.e.
insulation failure between line and earth ) which may cause
overvoltages in the system.
www.jntuworld.com
A i ground:
Arcing
ground
d: The
h phenomenon
h
off iintermittent
i
arc taking
ki
place in line to ground fault of a 3phase system with
consequent
q
pproduction of transients is known as arcingg
ground.
Resonance:
Resonance: It occurs in an electrical system when inductive
reactance of the circuit becomes equal to capacitive reactance.
under resonance , the impedance of the circuit is equal to
y
resistance of the circuit and the pp.f is unity.
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010-6-21
What is lightning?
www.jntuworld.com
www.jntuworldupdates.org
It is an electric discharge between cloud &
earth, between clouds or between the charge
centers of the same cloud
cloud.
www.jntuworld.com
www.specworld.in
Ramchander.N
www.smartzworld.com
www.jwjobs.net
2010
2010--6-21
Some Facts about Lightning
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
A strike can average 100 million volts of
electricity
Current of up to 100,000
100 000 amperes
Can generate 54,000 oF
Lightning strikes somewhere on the Earth
every second
Kills 100 US residents per year
www.specworld.in
Ramchander.N
www.smartzworld.com
10
www.jwjobs.net
2010
2010--6-21
10
Lightning Doesnt Go Straight
Down
Down
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
www.specworld.in
Ramchander.N
www.smartzworld.com
11
www.jwjobs.net
2010
2010--6-21
11
What Does This Mean?
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
Lightning can strike ground up to ten
miles from a storm (Lightning
(Lightning out of the
blue
blue))
There is an average of 2-3 miles between
strikes
So how can we tell how far away lightning
has struck?
www.specworld.in
Ramchander.N
www.smartzworld.com
12
www.jwjobs.net
2010
2010--6-21
12
Use The Five Second Rule
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
Light travels at about 186,291
186 291
miles/second
Sound travels at only 1
1,088
088 feet/second
You will see the flash of lightning almost
immediately
5280/1088= 4.9
About 5 seconds for sound to travel 1 mile
www.specworld.in
Ramchander.N
www.smartzworld.com
13
www.jwjobs.net
2010
2010--6-21
13
Fig.1 Typical Lightning
Overvoltages
O
l
Wave [2]
www.jntuworld.com
www.jntuworld.com
www.jntuworldupdates.org
Tr 0.1
Tr=
0.1--20s,
Th<300s
Tr is the time
time--to
to--crest
value
Th is the timetime-to half
value)
www.specworld.in
Ramchander.N
www.smartzworld.com
14
www.jwjobs.net
2010
2010--6-21
14
Fig.2 Typical
Switching
S i hi Overvoltages
O
l
Wave [2]
www.jntuworld.com
www.jntuworld.com
www.jntuworldupdates.org
Tr
Tr=20
Tr=2020-5000s,
Th<20 000s
Tr is the time
time--to
to--crest
value
Th is the timetime-to half
value
www.specworld.in
Ramchander.N
www.smartzworld.com
15
www.jwjobs.net
2010
2010--6-21
15
Types of lightning strokes
www.jntuworld.com
Direct stroke
I di t stroke
Indirect
t k
www.specworld.in
Ramchander.N
www.smartzworld.com
16
www.jwjobs.net
2010
2010--6-21
16
www.jntuworld.com
www.jntuworldupdates.org
Direct stroke
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
In direct stroke
stroke, the lightning discharge is
directly from the cloud to the subject
equipment From the line
equipment.
line, the current
path may be over the insulator down the
pole to the ground
ground.
www.specworld.in
Ramchander.N
www.smartzworld.com
17
www.jwjobs.net
2010
2010--6-21
17
Indirect stroke
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
Indirect stroke results from the
electrostatically induced charges on the
conductors due to the presence of charge
clouds.
www.specworld.in
Ramchander.N
www.smartzworld.com
18
www.jwjobs.net
2010
2010--6-21
18
Harmful effects of lightning
www.jntuworld.com
The traveling
g waves produced
p
due to lightning
g
g will
shatter the insulators.
If the traveling waves hit the windings of a transformer
or generator it may cause considerable damage.
Causes flashover fault sags and interruptions
Raises local ground voltage by several kV
Induces voltages and currents in nearby equipment and
systems
May cause transients in power and communication
systems through conduction (resistive coupling),
inductive coupling, or capacitive coupling
Resistive coupling (conduction) between devices or
www.specworld.in
www.smartzworld.com
buildings due to change in 19ground voltage
www.jntuworld.com
www.jntuworldupdates.org
Ramchander.N
19
www.jwjobs.net
2010
2010--6-21
Protection against lightning
www.jntuworld.com
www.jntuworldupdates.org
Different types of protective devices are:are:-
www.jntuworld.com
E
Earthing
thi screen
Overhead ground wires
Lightning arresters
www.specworld.in
Ramchander.N
www.smartzworld.com
20
www.jwjobs.net
2010
2010--6-21
20
The Earthing screen
www.jntuworld.com
www.jntuworld.com
www.jntuworldupdates.org
The p
power station & subsub-station can be
protected against direct lightning strokes by
providing earthing screens.
On occurrence of direct stroke on the station
,screen provides a low resistance path by which
lightning surges are conducted to ground.
Limitation:
It does not provide protection against the
traveling waves which may reach the
equipments in the station.
www.specworld.in
Ramchander.N
www.smartzworld.com
21
www.jwjobs.net
2010
2010--6-21
21
Overhead ground wires
www.jntuworld.com
It is the most effective way of providing protection to
t
transmission
i i lilines against
i t di
directt lilightning
ht i strokes.
t k
It p
provides damping
p g effect on anyy disturbance traveling
g
along the lines as it acts as a shortshort-circuited secondary.
www.jntuworld.com
www.jntuworldupdates.org
Limitation:
It requires additional cost.
There is a possibility of its breaking and falling across
the line conductors, thereby causing a shortshort-circuit fault.
www.specworld.in
Ramchander.N
www.smartzworld.com
22
www.jwjobs.net
2010
2010--6-21
22
Lightning Arresters (Surge Arrester)
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
It is a protective device which conducts the high
voltage surge on the power system to ground.
The earthing screen and ground wires fail to
provide protection against traveling waves. The
lightning arrester provides protection against
surges.
Arrester
www.specworld.in
Ramchander.N
www.smartzworld.com
23
www.jwjobs.net
2010
2010--6-21
23
The figure shows the basic form of the
surge arrester.
www.jntuworld.com
www.jntuworldupdates.org
OPERATION :
i. Under normal operation,
the lightning arrester is off
line.
www.jntuworld.com
ii On occurrence
ii.
occ ence of o
over
e
voltage the air insulation
breaks down and an arc is
formed. In this way the
excess charge on the line
due to surge is harmlessly
conducted through the
arrester to the ground
instead of being sent back
www.specworld.in
24
over the line.
Ramchander.N
Lightning arrester
24
www.jwjobs.net
2010
2010--6-21
www.smartzworld.com
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
www.specworld.in
25
Ramchander.N
25
www.jwjobs.net
Grading
resistor
2010
2010--6-21
www.smartzworld.com
Types of lightning arrester
www.jntuworld.com
Rod
od gap arrester
a este
Horn g
gap
p arrester
Multigap arrester
Expulsion type lightning arrester
Valve type lightning arrester
www.specworld.in
Ramchander.N
www.smartzworld.com
26
www.jwjobs.net
2010
2010--6-21
26
www.jntuworld.com
www.jntuworldupdates.org
Installation
www.jntuworld.com
The arrester should be connected to ground to a
low resistance for effective discharge of the
surge current.
The arrester should be mounted close to the
equipment to be protected & connected with
shortest possible lead on both the line & ground
side
d to reduce
d
the
h inductive
d
effects
ff
off the
h lleads
d
while discharging large surge current.
www.specworld.in
Ramchander.N
www.smartzworld.com
27
www.jwjobs.net
2010
2010--6-21
27
www.jntuworld.com
www.jntuworldupdates.org
Maintenance
www.jntuworld.com
Cleaning the outside of the arrester housing.
The line should be de
de--energized before handling the
arrester.
The earth connection should be checked periodically.
To record the readings of the surge counter.
Th liline lead
The
l d is
i securely
l fastened
f t
d to
t the
th line
li conductor
d t
and arrester.
The ground lead is securely fastened to the arrester
www.specworld.in
28
www.smartzworld.com
terminal and ground.
www.jntuworld.com
www.jntuworldupdates.org
Ramchander.N
28
www.jwjobs.net
2010
2010--6-21
Uses
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
www.specworld.in
Ramchander.N
www.smartzworld.com
29
www.jwjobs.net
2010
2010--6-21
29
www.jntuworld.com
www.jntuworldupdates.org
www.jntuworld.com
www.specworld.in
Ramchander.N
www.smartzworld.com
30
www.jwjobs.net
2010
2010--6-21
30
