GE 845 Instruction Manual page 196

SYSTEM
NOTE:
4–92
computation for CT mismatch based on the initially entered CT and ph-ph voltage per
winding, will be different, than the actual CT mismatch. This mismatch will appear as
differential current.
To compensate for the differential current, in cases of installed tap changer causing the
winding voltage to either go higher or lower than the nominal winding voltage entered in
the setpoints, the 845 provides dynamic CT mismatch compensation based on detection of
tap position.
The algorithm checks if the Tap Changer function is "Enabled", and on which winding it is
installed. To determine the winding, the algorithm checks which winding has the same
selection of Signal input as the one selected for the tap changer.
If the Signal input selected for the OLTC (Onload Tap Changer) is the same as the one for
the reference winding, the magnitude compensation factors for each non-reference
winding will be computed using the following equation.
a. OLTC installed on Winding 1
M = 1 – magnitude compensation factor for winding 1 - REFERENCE
W1
M = (CT .V )/ (CT
W2 W2 W2
winding 2 currents
M = (CT .V )/(CT
W3 W3 W3
3 currents
b. OLTC installed on Winding 2
M = 1 – magnitude compensation factor for winding 1 - REFERENCE
W1
+∆V
M = (CT .(V
W2 W2 min
winding 2 currents
M = (CT .V )/(CT
W3 W3 W3
currents
c. OLTC installed on Winding 3
M = 1 – magnitude compensation factor for winding 1 - REFERENCE
W1
M = (CT .V )/ (CT
W2 W2 W2
currents
+∆V
M = (CT .(V
W3 W3 min
3 currents
The maximum allowed magnitude compensation factor (and hence the maximum allowed
CT ratio mismatch) is 20.
Phase Shift Compensation
Phase Compensation Reference: The percent differential protection from the 845 relay
uses either the Delta, or the Zig-Zag winding (depending on the transformer setup) as a
reference to perform phase shift compensation. If the transformer has only "Wye"
connected windings, the phase currents from the first Wye winding in the setup are used
as a reference.
The power system phase sequence must be set when describing the winding phase
relationships, since these relationships change, when the phase sequence changes. The
example below shows why this happens, using a transformer described in IEC
nomenclature as "Yd1".
+∆V
.(V )) – magnitude compensation factor for
W1 min TLC
+∆V
.(V )) – magnitude compensation factor for winding
W1 min TLC
))/ (CT .V ) – magnitude compensation factor for
TLC W1 W1
.V ) – magnitude compensation factor for winding 3
W1 W1
.V ) – magnitude compensation factor for winding 2
W1 W1
))/(CT .V ) – magnitude compensation factor for winding
TLC W1 W1
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS