Micom P721, Technical Manual

Page: 291 / 334

Commissioning and Maintenance P72x/EN CM/C21
MiCOM P721/P723 Page 21/28
4.8 Busbar Primary Injection Tests
First choose any of the CTs as the reference CT. The polarity and ratio of the reference CT
should be checked first. Afterwards the polarity and ratio of the other CTs in the high
impedance differential busbar scheme should be verified against the reference CT.
As shown in Figure 7, primary current should be injected through the reference CT to verify
the CT ratio. It is important to short circuit the stabilizing resistor since it may not be rated to
continuously withstand the injected current. Since the P723 has a low burden, the varistor is
effectively short circuited when the stabilizing resistor is short circuited. If the P79x is de-
energized, the stabilizing resistor and varistor will be short circuited.
A
B
C
Primary Injection
Test Set
A2
AC Supply
P72x
49
50
R.
St .
P72x
51
52
R.
St .
P72x
53
54
R.
St .
A
B
C
P79x
Auxiliary
relay
Protection
trip contact
Protection N . O.
watchdog contact
+V
-V
Protection Bus -wire
Supervision contact
N
V
P4507ENa
FIGURE 7: BUSBAR APPLICATION - PRIMARY INJECTION – RATIO CHECK OF REFERENCE CT
The ratio of A2 to A1 should approximate closely to the actual CT ratio.
To check the polarity of reference current transformers, current injection should be carried
out through the primaries of two current transformers in the group as shown in Figure 8.
During this test keep the P79x de-energized so that the stabilizing resistor and the varistor
are kept short circuited. The polarity is correct if the reading on A1 is a few milliamperes.
Phases A and B can be tested first and then phases B and C. If the current transformers are
switched between main and reserve bus wires by isolator auxiliary switches, it is
recommended to perform the tests with the busbar isolators in both positions.