Advertisement
element using scheme switches. We shall see the scheme switches later.
DT characteristic
(i)
The operate time in the DT characteristic will be a constant time; it is not affected by the level
of the negative-sequence current. The reset time is set to zero so that resetting is performed
immediately. The user can set the operate time using a setting; it is discussed in section (iv).
IDMT characteristic
(ii)
The operate time in the IDMT characteristic is inversely proportional to the level of the fault
current. The user can choose the operate time from a preset curve. Ten characteristic curves
are available; i.e. nine pre-installed curves covering IEC, IEEE, US and UK standard
characteristic curves and one original (user programmable) characteristic curve for which the
user can set the equation parameters. The IDMT curve equation and parameters will be
explained in section (iii).
The reset time of the IDMT is also user-selectable; either inverse-time-reset or definite-
time-reset is selected. For the inverse-time-rest, the user can choose the reset time for the
preset characteristic from six characteristic curves; i.e. IEEE and US standard five pre-
installed characteristic curves and one user-original characteristic carver where the user can
set the equation parameters. The inverse-time-reset curve equation and parameters will be
discussed in section (v).
Operate time of IDMT
(iii)
The dependent time in the IDMT is defined by Equation (2.5-4). (ref IEC 60255-151):
where:
t = operating time for constant current I (seconds),
I = energizing current (amperes),
Is = earth fault current setting (amperes),
TMS = time multiplier setting,
k, α, c = constants defining curve.
The nine pre-programmed standard characteristic curve are tabulated in Table 2.5-1;
additionally, one original (user-programmable) curve can be applied. One curve can be chosen
for each OCN stage by setting. Figure 2.5-3 illustrates these characteristic curves. For example,
the user should set one for the scheme switch [OCN1-Timer]: "IEC-NI" for IEC Normal Inverse,
"IEC-VI" for IEC Very Inverse, "IEC-EI" for IEC Extremely Inverse, "UK-LTI" for UK Long Time
k
�� ( �� ) = ������ × {[
α
( I I
⁄ )
s
- 93 -
] + c}
− 1
6F2T0207 (0.01)
(2.5-4)
GRE200 (5,6)
Advertisement
