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Time delayed phase and residual
overcurrent protection (TOC1)
In case of extremely short or not fully transposed parallel lines, the false residual current
must be measured or calculated when maximum sensitivity is desired. The residual cur-
rent is proportional to the load current.
General criteria for the primary current setting value of the time delayed residual over-
current protection is given in the formula below:
⋅
<
<
⋅
1.3 IRmax
Is
0.7 If min
Where:
IRmax
is the maximum permissive residual current flowing in the protection unit
during any service conditions and
Ifmin
is the minimum residual fault current that the relay has to clear.
1.3
and
0.7
are safety factor values.
The residual protection must not operate under any high load conditions. If the system
is in a stressed situation due to loss of some parallel line the observed line must not be
tripped due to increased residual current caused by load current increase.
6HWWLQJ RI UHOD\ RSHUDWLQJ FXUUHQWV ,! DQG ,1!
If Is is the primary setting operating value of the function, then the secondary setting
current (Is
) is:
SEC
I
S EC
⋅
Is
------------- - Is
=
SEC
I
PRIM
where I
is the secondary rated current of the main CT and I
SEC
current of the main CT.
The relay setting value is given in percentage of the secondary base current value, I
associated to the current transformer input I1. The value for IP> and IN> is given from
the formula:
&KDSWHU
&XUUHQW
(Equation 34)
(Equation 35)
is the primary rated
PRIM
1b
89
,
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