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96
Two step definite and inverse time
delayed phase overcurrent protection
(TOC2)
I
max
⋅
≤
≤
⋅
1.2
---------- -
I
0.7
I
s
s cmin
k
The high current function of the overcurrent relay, which only has a short or no delay
of the operation, must be given a current setting so that the relay is selective to other
relays in the power system. It is desirable to have a rapid tripping of faults within as
large portion as possible of the part of the power system to be protected by the relay
(primary protected zone). A fault current calculation gives the largest current of faults,
Iscmax, at the most remote part of the primary protected zone. Considerations have to
be made to the risk of transient overreach, due to a possible dc component of the short
circuit current. The lowest current setting of the most rapid stage, of the phase overcur-
rent relay, can be written:
≥
⋅
⋅
I
1.2
k
I
high
t
sc max
Where:
1.2
is a safety factor, due to calculation uncertainty
k
is a factor that takes care of the transient overreach due to the DC component of
t
the fault current. kt is less than 1.05 if the power system time constant is less
than 100 ms.
I
is the largest fault current at a fault at the most remote point of the primary pro-
scmax
tection zone.
The operate times of the phase-overcurrent protection have to be chosen so that the fault
time is so short that equipment will not be damaged due to thermal overload, at the same
time selectivity is assured. For overcurrent protection, in a radial fed network, the time
setting can be chosen in a graphical way. This is mostly used in the case of inverse time
overcurrent protections.
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The current setting can be made in the same way as for radial networks but observe the
possibility to get high fault currents in the reverse direction if the adjacent station have
low source impedance.
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(Equation 39)
(Equation 40)
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