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High speed and instantaneous phase
overcurrent protection (HSOC, IOC)
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Only detailed network studies can determine the operating conditions under which the
highest possible fault current is expected on the line.
Also study transients that could cause a high increase of the line current for short times.
A typical example is a transmission line with a power transformer at the remote end,
which can cause high inrush current when connected to the network and can thus also
cause the operation of the instantaneous, overcurrent protection.
Calculate the maximum current I
at the relay site for faults at the end of the protected
fB
line, see figure 42. The calculation should be done using the minimum source imped-
ance values for Z
and in the case of meshed systems, the maximum source impedance
A
values for Z
in order to get the maximum through fault current from A to B.
B
I
fB
A
B
Z
Z
Z
A
L
B
~
~
Relay
Fault
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For meshed systems with strong infeed from station B, also calculate the maximum
through fault current I
for faults at station A. In order to get the maximum through
fA
fault current, the minimum value for Z
and the maximum value for Z
have to be con-
B
A
sidered.
The relay must not trip for any of the two through fault currents. Hence the minimum
theoretical current setting (Imin) will be:
≥
,
Imin
MAX I
(
I
)
fA
fB
(Equation 28)
A safety margin of 30% is recommended. Hence the minimum primary setting (Is) for
the instantaneous phase overcurrent IOC is:
83
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