7.2 Restricted earth fault protection
for a transformer with neutral
connection
7 Applications
Technical description
154
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VM265.EN011
7.2.
Restricted earth fault protection for a
transformer with neutral connection
Figure 7.2-1 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.
Figure 7.2-1 Overcurrent and restricted earth fault protection of
transformer's wye winding. 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.
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