Schweitzer Engineering SEL-387-0, Instruction Manual

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H.18
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Protection Application Examples
Transformer Winding and CT Connection Compensation Settings Examples
Figure H.25 Delta-Wye Transformer With Standard Phase-to-Bushing Connections With an ACB Phase Rotation
The H-side currents are:
IA = Ia – Ib
IB = Ib – Ic
IC = Ic – Ia
Figure H.26 uses a balanced set of three-phase currents with an ACB
phase sequence on the wye winding as a reference to derive the delta
winding (H-side) currents. When compared to Example H.1, even with
the same transformer and the same connections, Figure H.26 shows
that Ia (X-side) now leads IA (H-side) by 30 degrees because of the
system phase rotation. Note that in Example H.1, Ia (X-side) lags IA
(H-side) by 30 degrees. The CT connections are standard, which
results in IAW2 lagging IAW1 by 150 degrees. Figure H.28 shows the
phase relationship of both the primary system phase currents and the
secondary phase currents as seen by the relay.
Figure H.26 X- and H-Side Current Phasors for Figure H.25
A common misconception is that a different compensation setting
pair is required depending on the system phase rotation. However,
closer inspection of the compensation matrices and the direction of
correction in Table H.1 (also shown in Figure H.27) indicate that the
matrix causes the compensated currents to rotate in the opposite
direction depending on the system phase rotation.
B (H2)
C (H3) A (H1)
Winding 1 Winding 2
A
B
C
a
b
c
SEL-387
(Partial)
IAW1
IBW1
ICW1
IAW2
IBW2
ICW2
H3
H2
H1
X3
X2
X1
IA System Ia System
IA Relay Ia Relay
c (X3)
b (X2)
a (X1)
Ib
Ic
Ia
IB
IC IA
–Ib
IA = Ia – Ib
Ia
X-Side Currents H-Side Currents