GE T35 Instruction Manual page 516

DIFFERENTIAL CHARACTERISTIC TEST
If the power transformer phase shift is 0°, make the two currents to be injected to the relay 180° apart. This 180° phase
shift results from the inversion of the field CT, as their positive marks are away from the protected transformer terminals
and are connected to the positively marked terminals on the relay.
8.1.1.2 Minimum pickup
Inject current (I ) into Winding 1 Phase A and monitor the per-unit Phase A differential current until it exceeds the minimum
x
pickup setting. The theoretical injected current for minimum pickup verification can be computed as follows:
where CT is the 1 A or 5 A tap, and M[1] is the calculated magnitude compensation factor (see the Transformer section in
Chapter 5 for details on calculating the M[1] and M[2] factors).
8.1.1.3 Slope 1 / Breakpoint 1
The point of Slope 1 and Breakpoint 1 is tested as follows. See the Differential Restraint Characteristic figure in the next
section for details.
1. Inject current (I
2. At Breakpoint 1, the injected current I
and the differential current is then equal to:
3. Preset the I current to 1.05 x I
x
ratio becomes less than the Slope 1 setting. Switch off the current.
4. Preset the I current to 0.95 x I
x
To test any other point from the Slope 1 section of the curve, inject a per-unit restraint current smaller than the Breakpoint
1 current and repeat the steps above by substituting the Breakpoint 1 value with the new per-unit restraint current value
into the equations.
8.1.1.4 Slope 2 / Breakpoint 2
The point of Slope 2 and Breakpoint 2 is tested as follows. The figure shows details.
1. Preset the I current to a magnitude that results in the restraint current being equal to Breakpoint 2. Use the following
y
calculation to define the magnitude of the injected current:
8
2. At the above current (restraint), the I
3. Preset the I current to1.05 x I
x
ratio becomes less than the Slope 2 setting. Switch off the current.
4. Preset the I current to 0.95 x I
x
To test any point from the Slope 2 portion of the characteristic, inject a per-unit restraint current greater than the
Breakpoint 2 current as restraint and repeat the steps above by substituting the Breakpoint 2 value in the equations with
the new per-unit restraint current value.
8-2
) into Winding 2 Phase A as follows:
y
is determined by:
XOP1
I = Slope 1 (in %) x Breakpoint 1 (in pu)
d
. Switch on the test set. The relay should restraint, as the differential to restraint
XOP1
. Switch on the test set. The relay operates. Switch off the current.
XOP1
current required to operate the element is calculated as:
XOP2
and switch on the test set. The relay should restrain, as the differential to restraint
XOP1
. Switch on the test set and verify relay operation. Switch off the current.
XOP1
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 8: COMMISSIONING
Eq. 8-1
Eq. 8-2
Eq. 8-3
Eq. 8-4
Eq. 8-5
Eq. 8-6