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2.10 Characteristics of Measuring Elements
2.10.1 Percentage Current Differential Element DIF
The segregated-phase current differential element DIF has dual percentage restraining
characteristics. Figure 2.10.1 shows the characteristics of DF1 and DF2 on the differential current
(I
d
) and restraining current (I
r
) plane. I
d
is a vector summation of phase current of all windings
and I
r
is a scalar summation of phase current of all windings.
I
d
ik
I
r
I
d
= 2I
r
(one-end infeed)
DF2
DF1
kp
ik
2
Figure 2.10.1 Current Differential Element
Characteristic DF1 is expressed by the following equation:
I
d
≥ p1 x
I
r
+
(1 −p1/2)ik
where,
p1
: slope of DF1
ik : minimum operating current
Id and Ir are defined as follows for a three-winding transformer.
I
d
= | kct1⋅I
1
+ kct2⋅I
2
+ kct3⋅I
3
|
I
r
=(kct1⋅|I
1
|+ kct2⋅|I
2
|+ kct3⋅|I
3
|)/2
where,
kct1 ,kct2 ,kct3
: CT ratio matching settings of primary, secondary and tertiary winding
I
1
,I
2
,I
3
: currents of primary, secondary and tertiary winding
This characteristic has weaker restraint in the small current region and ensures sensitivity to low
level faults.
Characteristic DF2 is expressed by the following equation:
I
d
≥ p2 x
I
r
+ (p1 −p2)kp + (1 −p1/2)ik
where,
p2 : slope of DF2
kp : break point of DF1 characteristic
This characteristic has stronger restraint in the large current region and ensures stability against
CT saturation during through faults.