GE 845, Instruction Manual

Page: 209 / 552

CHAPTER 4: SETPOINTS SYSTEM
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–81
M
W3
= (CT
W3
.V
W3
)/(CT
W1
.V
W1
) – magnitude compensation factor for winding 3
currents
c. OLTC installed on Winding 3
M
W1
= 1 – magnitude compensation factor for winding 1 - REFERENCE
M
W2
= (CT
W2
.V
W2
)/ (CT
W1
.V
W1
) – magnitude compensation factor for winding 2
currents
M
W3
= (CT
W3
.(V
min
+ ∆
V
TLC
))/(CT
W1
.V
W1
) – magnitude compensation factor for winding
3 currents
NOTE
NOTE:
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”.
Figure 4-20: Example Transformer
The Example Transformer figure shows the physical connections within the transformer
that produce a phase angle in the delta winding lagging the respective wye winding by
30°. The winding currents are also identified. Note that the total current out of the delta
winding is described by an equation. Now assume that a source, with a sequence of ABC,
is connected to transformer terminals ABC, respectively. The currents that would be
present for a balanced load are shown below.