ABB REL 356 Current Differential Protection
3-18
Settings and Application
shown that a phase B-to-C fault or a phase C-to-A fault will produce the highest sequence output of all the low voltage
phase faults. A C-to-G fault will produce the highest filter output current for phase-to-ground faults on the low voltage
side of the transformer. It depends on the values of Z and ZT as to whether the phase-to-phase or phase-to-ground fault
produces the larger output. Therefore both must be considered.
Settings for OTH, C0, C1 and C2 for Tapped Load Applications
The relay setting OTH and the sequence filter constants, C1, C2 and C0, should be first selected according to normal
conditions following the recommendations on page 3-9. Then, the relay setting and the selected constants need to be
checked to make sure they satisfy the special requirements for tap load applications:
if
0
2
=C
CT
LN
C
LN
RZZ
V
COTH
RZ
V
C
⋅+
⋅⋅>>
⋅⋅
⋅⋅
11
3.0
2
3.0
(1)
if
0
2
≠C
CT
LN
j
C
LN
j
RZZ
V
eCCOTH
RZ
V
eCC
⋅+⋅
⋅+⋅>>
⋅⋅
⋅+⋅
−
2
3.0
2
3.0
120
21
60
21
οο
(2)
where
LN
V
is the line-to-neutral voltage,
c
R
is the CT ratio,
Z
is the total equivalent system impedance looking back into the system from the transformer bank, in high side ohms
T
Z
is the transformer impedance, in high side ohms
Figure 3-2. Tapped Load Application
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