L&T Electrical & Automation OMEGA, Manual

Page: 43 / 135

41
Matrix
Protection Curves
Overload - SI
(UW-MTX3.5/3.5EC/3.5H ) /4.5
Overload - LI/VI
(UW-MTX3.5/3.5EC/3.5H ) /4.5
0.1
1
10
100
1000
10000
1 2 3 4 5 6 7 8 9 10 1.2
0.5s
1s
2s
4s
6s
12s
18s
24s
30s
Current Multiples (x Ir)
Tolerance Trip Time: ±20%
Tr
ip
T
im
e
(s
)
0.5s
1s
2s
4s
6s
12s
18s
24s
30s
0.1
1
10
100
1000
10000
1.2
Current Multiples (x Ir)
Tolerance Trip Time: ±20%
Tr
ip
T
im
e
(s
)
1 2 3 4 5 6 7 8 9 10
O v e r v o l t a g e c a u s e s a n d e f f e c t s
Over voltage (or surges) in the power system are the abrupt rise in the voltage level in the system. There could be
several reasons for over voltage. The normal operating voltages of the system do not stress the insulation severely.
But the voltage stresses due to over voltages can be so high that they may become dangerous to both the cables and
the connected equipment and may cause damage, unless some protective measure against the over-voltages are
taken. Over-voltages occurrence in the system can be categorized by reasons:
1 . E X T E R N A L O V E R V O L T A G E S :
These over-voltages originate from the atmospheric disturbances, mainly due to lightning. These over-voltages
take the form of a unidirectional impulse whose maximum possible amplitude has no direct relation with the
operating voltage of the system. They may be due to any one of the following causes:
a. Direct lightning strokes
b. Electromagnetically induced voltages due to the lightning discharge near the line (sidestroke)
c. Voltage induced due to the changing atmospheric condition along the transmission line
d. Electrostatically induced over voltages due to the presence of the charged clouds
e. Due to the friction of the charged particles like dust, snow in the atmosphere or due to change in the altitude
of the line
42
Matrix
Protection and Control Units
Direct
Lightning
Stroke
Phase
Conductor
Towers
Cloud