Schweitzer Engineering SEL-387-0, Instruction Manual

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H.15
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Protection Application Examples
Transformer Winding and CT Connection Compensation Settings Examples
EXAMPLE H.3 Delta-Wye Transformer With Standard
Phase-to-Bushing Connections, X-Side Non-Standard CT
Connections, and an ABC System Phase Rotation
This example uses standard phase-to-bushing connections as shown
in Figure H.20. Notice the X-side CT connections are non-standard.
This example differs from the previous examples because the wye
winding is now on the high side. The method to solve for the
compensation settings is the same.
Figure H.20 Wye-Delta Transformer With Non-Standard CT Connections on the X-Side
Assume balanced three-phase currents on the wye side of the
transformer. In this example, the wye side is associated with the
H-side of the transformer. The phase currents on the X-side are:
Ia = IA – IC
Ib = IB – IA
Ic = IC – IB
In Figure H.21, IA on the wye side of the transformer is chosen as the
reference to derive the phasor diagram of the delta-side currents. The
system primary current Ia (X-side) lags IA (H-side) by 30 degrees or
leads IA by 330 degrees.
Figure H.21 X- and H-Side Current Phasors for Figure H.20
With the reversed CT polarity on the X-side, the current is leaving
Terminal W2 at the polarity mark instead of entering the polarity
mark, which makes the current seen by the relay 180 degrees out-of-
phase compared to a standard CT connection. In Non-Standard CT
Connections: Reversed CT Polarity and Reversed Connections, two
methods are proposed to correct the phase shift for reverse polarity
CTs. The first method is to rewire the current inputs on the relay so
B (H2)
C (H3)
A (H1)
Winding 1 Winding 2
A
B
C
a (X1)
b (X2)
c (X3)
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
IC
IB
IA
Ic
Ib Ia
–IC
Ia = IA – IC
IA
H-Side Currents X-Side Currents