Metering Transformer; Percent Differential Current - GE 845 Instruction Manual

CHAPTER 6: METERING

Percent Differential Current

845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
3. Physically, the transformer windings are connected in Delta, Wye or Zig-Zag
configuration, and they introduce a phase shift.
For the correct performance of the percent differential protection, it is necessary to correct
the magnitude and phase relationships of the CT secondary currents for each winding, in
order to obtain near zero differential currents under normal transformer operating
conditions. Traditionally, the phase shift between the currents from the transformer
windings has been corrected by connecting the CTs from the Wye winding in Delta
connection, and the CTs from the Delta winding in Wye connection. In the past, the
magnitude correction has been accomplished using interposing CTs, or tapped relay
windings. This however is not required any more when installing the 845 relay.
The 845 relay simplifies the process by performing the magnitude and phase shift
compensations automatically (internally). Upon entering settings for the protected
transformer and winding CT ratings, the relay automatically calculates and applies the
correct magnitude scaling to the winding currents as well as applying the correct phase
shift in order to prepare the currents for summation. To perform the correct currents
compensation, all winding CTs need to be connected in Wye (polarity markings pointing
away from the transformer). When the Tap Changer detection feature is enabled on the
relay, the algorithm automatically compensates the currents for the differential
computation based on the new voltage ratio corresponding to the detected tap.
The phasors of differential and restraint currents are displayed in primary amperes.
Path:
Metering > Transformer > Percent Differential
Metering\Transformer\Percent Differential 1
Item Name
Ia Diff
Ib Diff
Ic Diff
Ia Restr
Ib Restr
Ic Restr
Ia Diff 2nd Harm
Ib Diff 2nd Harm
Ic Diff 2nd Harm
Ia Diff 5th Harm
Ib Diff 5th Harm
Ic Diff 5th Harm
PcntDiff1
la Differential (Ia Diff)
Range: 0.000 to 1000.000 x CT in steps of 0.001
Default: 0.000 x CT
lb Differential (Ib Diff)
Range: 0.000 to 1000.000 x CT in steps of 0.001
Default: 0.000 x CT
lc Differential (Ic Diff)
Range: 0.000 to 1000.000 x CT in steps of 0.001
Default: 0.000 x CT
la Restraint (Ia Restr)
Range: 0.000 to 1000.000 x CT in steps of 0.001
Default: 0.000 x CT
TRANSFORMER
Value Unit
0.000 x CT
0.000 x CT
0.000 x CT
0.000 x CT
0.000 x CT
0.000 x CT
0.0 %
0.0 %
0.0 %
0.0 %
0.0 %
0.0 %
6–3