CHAPTER 7: COMMISSIONING PROTECTION SCHEMES
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 7–17
For the phase shift for the particular set of vectors, determine the
processing applied to the current vectors for that winding from Table 5–
2: Phase shifts on page 5–24.
Calculate the “dashed” current values using the equations in Table 5–2:
Phase shifts on page 5–24.
If applicable, use the zero-sequence removal computation. This is
applicable for all Delta windings and for both windings of a wye-wye
transformer. Compute the processed current vectors to obtain the
“dashed” values for that winding.
Calculate the CT correction factor for windings 2 (and 3 if applicable)
and apply as necessary.
Turn the equations around to compute the threshold differential currents
in terms of the applied currents.
Note
To check the threshold without performing computations, inject
balanced 3-phase currents into any winding.
With balanced conditions, there is no effect on magnitude due to phase
shifting and zero-sequence removal has no effect. However, the CT ratio
mismatch is still applicable.
Repeat the minimum pickup level measurements for the Phase B (inputs
H2 and G2) and the Phase C element (inputs H3 and G3).
The above tests have effectively verified the minimum operating level of the three
harmonic restrained differential elements. If desired the above measurements may be
repeated for the phase inputs for the other winding(s). The results should be identical.
7.5.2.3 Verification of Local Reset Mode
Set the differential element with a latched target.
Apply enough current to cause the relay to operate, then remove the
current.
The Trip LED and the Phase LED should be latched on.
Set
S1 745 SETUP RESETTING LOCAL RESET BLOCK to Disabled.
Press the RESET key. The target should reset.
Set
S1 745 SETUP RESETTING LOCAL RESET BLOCK to Logic
Input 1(16).
Press the RESET key and verify that the target does not reset if the
logic input is not asserted.
Verify the status of selected logic input through the A1 STATUS
LOGIC INPUTS
LOGIC INPUT 1(16) STATE actual value.
Assert the selected logic input, apply the current to cause the target to
latch and verify that pressing the RESET button does not reset the LED.
The following message should appear:
INVALID KEY: MUST BE IN LOCAL
MODE
.
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