Table of Contents
Advertisement
celerating power and system WR 2 , it is impossible to
generalize. However, based on an inertia constant
(H) equal to 3, and the severe assumption of full load
rejection, a machine will experience (assuming a
uniform acceleration) an angular change in position
of no more than 20° per cycle on the first half slip cy-
cle.
If the inner blinder were set for (0.104Z T) , and the
very severe 20° per cycle swing rate were used, the
outer blinder should be set for approximately:
Z
= 0.5 Z
primary ohms
outer
T
where Z
= Z
+ Z
+ Z
T
A
B
This is the minimum setting of the outer blinder for a
20° per cycle swing rate.
5.1.7 Overcurrent Ground Backup Unit (GB)
The overcurrent ground backup unit GB provides
seven sets of curves, which are similar to the CO
and MCO curves, for backing up the distance ground
on high resistance ground faults. Four settings (GB-
CV, GBPU, GTC and GDIR) should be determined
for applying this unit.
a. GBCV is the ground
Seven sets of familiar CO curves are provided
(C02,5,6,7,8,9 and 11), and are shown in Figures 2-
1 thru 2-7. The selection is based on the application
and coordination time. A selection of "OUT" disables
the ground backup function.
b. GBPU is the current level setting. Its range is 0.5
to 4.0 amperes in 0.5 steps. In general, the current
level setting criterion is:
(I
/2) > Setting level > 2 x (Max.
Fmin
residual load, 3I
)
0
where I
= Minimum ground fault current
Fmin
for a fault two buses away
For better sensitivity, GBPU should be set at 0.5 am-
peres, this would be adequate for most of the appli-
cation.
c. GTC is the time delay setting of the GB unit. As
shown in Figures 2-1 thru 2-7, it has 63 setting selec-
tions, from 1 to 63 in 1.0 steps. In general, the time
delay setting should be coordinated with any protec-
tive device downstream of the line section.
(5/92)
(3)
L
backup curve selection.
d.GDIR is the setting for directional control selection.
The GB unit will become a directional torque control
overcurrent ground unit if GDIR is set at YES. The
following equation can be used to calculate the trip
time for all CO curves from CO-2 thru CO-11 :
K
T (sec) =
T
+
--------------------------
0
(
)
P
3I
–
C
0
R
GTC
×
T (sec) =
---------------------
----------------- -
(
)
,
3I
–
1
24 000
0
I
F
Where 3I
=
----------------- -
0
GBPU
GBPU = Pickup current setting (0.5 to 4.0A).
GTC = Time curve dial setting (1 to 63).
T
, K, C, P and R are constants, and are shown in
0
Table 5-3. (Refer also to the example in Appendix H,
Section 1.1.9.)
Taking the CO-8 curve set as an example (see Fig-
ure 2-5), assuming that the maximum 3Io of unbal-
anced load is 0.2A, the minimum ground fault cur-
rent for a fault two buses away is 10A, and 0.7
seconds is required for coordination with current of
20 times the GBPU setting, then the settings of the
GB function should be as shown below:
10/2 > GB > 2 x 0.2
Using the curve in Figure 2-1, for 0.7 seconds at 20
times the GBPU setting, GTC should be set to 24.
• Set GBCV = C0-8 and
• Set GDIR = YES if directional control is required.
5.1.8 Timer Settings
a. Zone-2 timer (T2) setting should be coordinated
with the Zone-1 and other high-speed trip units on
the adjacent line terminal. Coordination Time Inter-
val (CTI) of 0.3 to 0.5 seconds is recommended. For
example, if T2 of 0.4 seconds is used, then the
phase and ground Zone-2 timers should be set as
follows:
T2P = 0.4 and T2G = 0.4
NOTE: T2P and T2G are separate timers; they
can be set at different time settings.
b. Zone-3 timer (T3) settings would be similar to the
above. For example, if T3 of 0.8 seconds is required,
then the phase and ground Zone-3 timer should be
set as follows:
I.L. 40-385.1B
GTC
(for 3I 0 ≥ 1.5)
×
----------------- -
,
24 000
(for 1<3I
< 1.5)
0
set GBPU = 0.5
5-3
Advertisement
Table of Contents
