Specifications

7SR242 Duobias Applications Guide
Table 2-1 summarises the implications of using Yd or Yy interposing CTs.
Table 2-1 The Effect of ICT Selection on Protection Settings
CT
Secondary
Current
HV
( ICT: Yd11, x 1.02)
W1 ICT
OUT
=
LV
( ICT: Yy0, x 1.07)
W2 ICT
OUT
=
3-Phase
A = 1A
B = 1A
C = 1A
A = 1.02A
B = 1.02A
C = 1.02A
A = 1.07A
B = 1.07A
C = 1.07A
B – C
A = 0
B = 1A
C = 1A
A = 0.589A
B = 1.178A
C = 0.589A
A = 0
B = 1.07A
C = 1.07A
A – E
A = 1A
B = 0
C = 0
A = 0.589A
B = 0
C = 0.589A
A = 1.07A
B = 0
C = 0
The above analysis covers current distributions for internal faults. The table illustrates that the Yd ICT has the
effect of:
Modifying the amplitude of the ICT
OUT
currents
Changing current distribution
The above factors must be considered during any analysis of protection operations and indications.
A similar analysis can be carried for external (through) fault conditions. However as the protection settings already
ensured stability for the maximum through fault condition (3-phase fault) this is not necessary.
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