Schweitzer Engineering SEL-387-0, Instruction Manual

Page: 525 / 556

H.7
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Protection Application Examples
Transformer Winding and CT Connection Compensation Settings Examples
Figure H.7 Non-Standard System Phase-to-Transformer Bushing Connections
Assume balanced X-side (wye-winding) three-phase currents Ia, Ib, and Ic.
Kirchhoff’s Current Law is used at each H-node to determine the primary
phase currents on the H-side of the system:
IA = Ia – Ic
IB = Ib – Ia
IC = Ic – Ib
Figure H.8 shows that system primary current Ia (X-side) leads the system
primary current IA (H-side) by 30 degrees.
Figure H.8 X- and H-Side Current Phasors for Figure H.7
The primary load phase shift determined in Figure H.8 applies for the
phase-to-bushing connections shown in Table H.3 . In each of these
phase-to-bushing connections, the order of the phase connections (A, C, B) is
opposite the order of the bushings (H1, H2, H3).
The system phase-to-transformer bushing connection diagrams in Figure H.5
and Figure H.7 are on the same transformer, but with a different order of the
phases connected to the transformer bushings. As a result, the X-side primary
current shifts 30 degrees in opposite directions in the two systems.
A (H3)
B (H2)
C (H1)
Winding 1 Winding 2
A
B
C
a
b
c
H3
H2
H1
X3
X2
X1
b (X2)
a (X3)
c (X1)
IA
IB
IC
Ia/N
Ib/N
Ic/N
Ia
Ib
Ic
Table H.3 (A, C, B) to (H1, H2, H3) Phase-to-Bushing Connections
Bushing
H1 H2 H3 X1 X2 X3
System
Phase
C B A c b a
B A C b a c
A C B a c b
Ic
Ib
Ia
IC
IB IA
–Ic
IA = Ia–Ic
Ia
X-Side Currents H-Side Currents