Phase Differential Trip Test - GE 489 Instruction Manual

7.3 ADDITIONAL FUNCTIONAL TESTING
The 489 phase differential compares the current level at terminal end with the current level at neutral end. The differential
element will trip when:
> ×
I k I
diff restraint
I I I
given = – and
diff A a
where: I
= differential current
diff
I
= restraint current
restraint
k = differential element characteristic slope in percent
(use
DIFFERENTIAL TRIP SLOPE1
I = phase current measured at the output CT
A
I = phase current measured at the neutral end CT
a
See Section 4.6.8: PHASE DIFFERENTIAL on page 4–26 for additional details.
The following is a sample calculation of a trip scenario. Given the following settings and values,
DIFFERENTIAL TRIP SLOPE1:
DIFFERENTIAL TRIP SLOPE2:
= 1.5 × CT at 0°,
I I
A a
The calculations are as follows:
I I I
= – =
diff A a
7
< 2 × CT, the differential trip slope k = 0.1 or 10% and
Since I
restraint
> I
Therefore, since I
diff
The 489 specification for differential phase timing accuracy is ±0.5% of total time. Pickup accuracy is per the output current
inputs (±0.5% of 2 × CT when the injected current is less than 2 × CT and ±1% of 20 × CT when the injected current is equal
to or greater than 2 × CT). Perform the steps below to verify accuracy for phase A.
1. Alter the following setpoints:
S2 SYSTEM SETUP\CURRENT SENSING\PHASE CT PRIMARY:
S5 CURRENT ELEMENT\PHASE DIFFERENTIAL\PHASE DIFFERENTIAL TRIP:
S5 CURRENT ELEMENT\PHASE DIFFERENTIAL\DIFFERENTIAL TRIP MIN. PICKUP:
S5 CURRENT ELEMENT\PHASE DIFFERENTIAL\DIFFERENTIAL TRIP SLOPE1:
S5 CURRENT ELEMENT\PHASE DIFFERENTIAL\DIFFERENTIAL TRIP SLOPE2:
Measured values should be ±5.0 A.
2.
There could be further error due to uncertainty in the phase measurement. It is recommended that the phase
be measured from 489 instead of the current source for the purposes of this test.
NOTE
7-14
I I
+
A a
I
= -------------------- -
restraint
2
if I
restraint
Figure 7–3: PHASE DIFFERENTIAL
10% (user setting)
20% (user setting)
= 1.47 × CT at 190° lag
×
1.5 1.448 j 0.255 0.26 CT
– + =
, the differential TRIP will operate.
trip
489 Generator Management Relay

7.3.8 PHASE DIFFERENTIAL TRIP TEST

< 2 × CT,
DIFFERENTIAL TRIP SLOPE2
I + I
A a
and I
= -------------------- - =
restraint
2
×
I k I
=
trip restraint
1000A
Unlatched
0.10xCT
10%
20%
7 TESTING
≥ 2 × CT)
if I
restraint
×
1.485 CT
× ×
0.1 1.485 0.1485 CT
= =
GE Power Management
.