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Two step definite and inverse time
delayed phase overcurrent protection
(TOC2)
The pick up current setting (inverse time relays) or the lowest current step (constant
time relays) must be given a current setting so that the highest possible load current does
not cause relay operation. Here consideration also has to be taken to the relay reset cur-
rent, so that a short peak of overcurrent does not cause operation of the relay even when
the overcurrent has ceased.
The lowest setting value can be written:
I
max
≥
⋅
I
1.2
---------- -
s
k
Where:
1.2
is a safety factor due to load estimation uncertainty etc.,
k
the resetting ratio of the relay (0.95) and
I
the maximum load current.
max
The maximum load current on the line has to be estimated. From operation statistics the
load current up to the present situation can be found. Also emergency situations must
be considered.
There is also a demand that all faults, within the zone that the protection shall cover,
must be detected by the phase overcurrent relay. The minimum fault current Iscmin, to
be detected by the relay, must be calculated. Taking this value as a base, the highest pick
up current setting can be written:
≤
⋅
I
0.7
I
s
s c min
Where:
0.7
is a safety factor, due to calculation uncertainty and
I
the smallest fault current to be detected by the overcurrent protection.
scmin
As a summary the pick up current shall be chosen within the interval:
&KDSWHU
&XUUHQW
(Equation 37)
(Equation 38)
95
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