Table of Contents
Advertisement
88
Time delayed phase and residual
overcurrent protection (TOC1)
ILmax
⋅
<
<
⋅
1.2
---------------- -
Is
0.7 If min
k
Where:
ILmax
is the maximum permissible load current of the protected unit
Ifmin
is the minimum fault current that the relay has to clear
1.2
is a safety factor due to load estimation uncertainty etc
0.7
is a safety factor, due to calculation uncertainty
k
is the reset ratio of the overcurrent function: 0.95.
The settable time delay tP allows the time selectivity of the overcurrent function, ac-
cording to the time grading plan of all the other overcurrent protections in the system.
The time setting value should also consider 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 energized.
Where the time delayed overcurrent function is used as back-up of impedance protec-
tion, normally the time delay is set higher than the time delay of distance zone 2 (or 3)
in order to avoid interferences with the impedance measuring system.
6HWWLQJ LQVWUXFWLRQV IRU UHVLGXDO FXUUHQW IXQFWLRQ ,1!
The residual current protection is very sensitive to the change of zero sequence source
impedance. Since it must operate only in a selective way, it is necessary to check all sys-
tem and transient conditions that can cause unwanted operation.
The settings should be chosen in such a way that it can detect high resistance faults on
the protected line and still be selective to other residual time delayed protections in both
forward and reverse directions. The value of the time setting (tN) should also consider
transients that can cause a high increase of the residual line current during short periods.
A typical example is a transmission line with a power transformer at the remote end,
which can cause high inrush current when being energised.
In well transposed meshed system, the false earth-fault current is normally lower than
5% of the line current. For non transposed lines a considerably higher false residual cur-
rent may be found.
Transposition or not does not influence the false residual current level in two-phase sys-
tem which is radial and where the mutual coupling between parallel lines is negligible.
&KDSWHU
&XUUHQW
(Equation 33)
Advertisement
Table of Contents
