GE 750, Instruction Manual

Page: 434 / 504

17-
28
750/760 Feeder Management Relay GE Power Management
17.7 PROTECTION SCHEMES 17 COMMISSIONING
17
17.7.12 NEUTRAL DIRECTIONAL OVERCURRENT
If dual polarizing is required, check the operation of the voltage and current polarized elements individually as
outlined below, then check the overall dual polarized response as outlined at the end of this section.
a) FORWARD TRIPPING WITH VOLTAGE POLARIZATION
Test Connections per Figure 17–1: RELAY TEST WIRING – WYE CONNECTION on page 17–4.
Figure 17–4: NEUTRAL DIRECTIONAL – VOLTAGE POLARIZED FOWARD OPERATING REGIONS below is
a plot of the operating characteristic of the voltage polarized neutral directional feature for various settings of
MTA. Other characteristics for specific MTA settings can be deduced from this diagram. Note that the diagram
is plotted for the residual current referred to the system faulted-phase phase-to-neutral voltage.
Figure 17–4: NEUTRAL DIRECTIONAL – VOLTAGE POLARIZED FOWARD OPERATING REGIONS
The following descriptions present angles with reference to the faulted phase voltage and
assume an MTA setting of 180
°
. For an MTA setting other than 180
°
alter the noted angles to
those established by the programmed MTA.
Characteristics
1. Set V
an
= V
bn
= V
cn
= nominal voltage at 0
°
, 120
°
, and 240
°
respectively. Set a current of 0.2 of nominal to
lag V
an
by 180
°
(this is in the tripping direction) and inject into phase A. Note that – V
o
is less than the MIN-
IMUM POLARIZING VOLTAGE.
2. Reduce V
an
until – V
o
is greater than the MINIMUM POLARIZING VOLTAGE.
3. Slowly increase the angle of the phase A current in the lagging direction. At an angle lagging V
an
by 270
±
2
°
, any output relays programmed for this condition should operate if the function is set to “Control”. If the
function is set to Alarm , check that the ALARM and MESSAGE indicators are flashing, the Neutral Direc-
tional Reverse alarm message is displayed, and any output relays programmed for this condition are oper-
ated.
4. Continue to increase the lagging angle through 0
°
, until the block message disappears at an angle lagging
V
an
by 90
±
2
°
. Turn the current off.
5. If desired, repeat the above steps 1 through 4 for phases B and C using I
b
with faulted phase voltage V
bn
and I
c
with faulted phase voltage V
cn
.
NOTE