43
Matrix
Protection and Control Units
2
.
I
N
T
E
R
N
A
L
O
V
E
R
V
O
L
T
A
G
E
S
:
Caused due to changes in the operating condition of the network. Can be further classified in two groups:
a
.
S
w
i
t
c
h
i
n
g
o
r
t
r
a
n
s
i
e
n
t
o
v
e
r
v
o
l
t
a
g
e
s
These over voltages are generally of oscillatory nature caused by the transient phenomena which appears
when the state of the network is changed by switching operation or a fault condition.
The frequency of the oscillations is governed by the inherent inductance and capacitance of the system & may
vary from few hundred Hz to few kHz
For e.g.
i. Switching On & OFF of equipment, like switching of high voltage reactors or switching of
transformer on no load
ii. If fault occurs in any one phase the voltage of other phases with respect to the ground increases
until the faults get cleared
iii. Because of the re-striking voltage across the contacts of the breaker, when breaker has tripped
on fault, voltage of amplitude nearly twice the system voltage and relatively high frequency gets
generated
b
.
S
t
e
a
d
y
-
s
t
a
t
e
o
r
t
e
m
p
o
r
a
r
y
o
v
e
r
v
o
l
t
a
g
e
s
These are over voltages developed due to the disconnection of loads at power frequency.
44
Matrix
Protection and Control Units
D
I
R
E
C
T
I
O
N
A
L
S
H
O
R
T
C
I
R
C
U
I
T
P
R
O
T
E
C
T
I
O
N
Consider a single-end-fed system of parallel feeders. in the normal condition Circuit Breakers 1, 2, 3 and 4 will be
closed and will feed the load through two parallel feeders (Fig1). when the fault occurs at point A Breaker 4 will see
the fault in its zone and trip as per its time delay settings. Breaker 3 will remain closed as fault is not in its zone. To
keep the load bus charged Breaker 1 and 2 should be closed and feed the load.
The direction of current flow at Breaker 3 will get reversed and allow to feed fault at point A as shown in fig 2.
reverse short circuit protection.To prevent such situation Breaker 3 should sense this reversal of power flow and
should trip immediatily to provide complete isolation of the fault at A. while the supply to the load remain intact
through the Breaker 1 and Breaker2. this funtion of Breaker 3 is called reverse short circuit protection.
Load
4
1
3
2
Load
4
1
3
2
A
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