Rockwell Automation 865 Differential Protection Relay - Manual (Page 144)

8-2 Applications
865-UM001A-EN-P – July 2009
Figure 8.2 shows and example where three phase current CTs are
connected parallel with each other and then in series with the CT in
the neutral point.
Note: All the CTs have the same ratio and
the nominal secondary current is 1 A. In
through fault the residual secondary currents
of phase CTs and the neutral CT cancel each
other and in inside fault the two residual
secondary currents are summed up and
forced to flow through the I
0 input of the relay
and the voltage limiting VDR. (For 5A CTs,
use appropriate relay connection points and
setpoint scaling.)
R
S
VDR
865
I
L1
I
L2
I
L3
1 A
1 A
1 A
1 A
1 A
5 A
I
0
X1-1
X1-2
X1-3
X1-4
X1-5
X1-6
X1-7
X1-8
X1-9
X1-10
Restricted Earth Fault 3
Figure 8.2 – Overcurrent and restricted earth fault protection of transformer's wye winding
CT Requirements
Any difference between the CTs will give a misleading residual
current signal to the relay. Especially during heavy through faults
(i.e. the fault is outside the protected zone) the dissimilar saturation
of the CTs should not yield to a REF trip. On the other hand a very
high fault current causing an unselective earth fault start or trip is not
a fatal error.
Class X CT
In restricted earth fault protection the high and low side CTs should
give similar responses even for high over currents.
Class X CTs will fulfill this requirement. Their performance is
defined in terms of a knee-point voltage (V
KP
), the magnetizing
current at the knee point voltage and the resistance of the secondary
winding at +75 °C.
Knee point voltage (V
KP
) is the secondary voltage at which a 50 %
increase of primary current is needed to increase the secondary
voltage by 10 %.
Restricted Earth Fault
Protection for a Transformer
with Neutral Connection