Characteristics Of Measuring Elements - Toshiba GRT100 Series Instruction Manual

2.11 Characteristics of Measuring Elements

2.11.1 Percentage Current Differential Element DIF
The segregated-phase current differential element DIF has dual percentage restraining
characteristics. Figure 2.11.1 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.
Characteristic DF1 is expressed by the following equation:
I d  p1  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 = |kct1I
d
I =(kct1|I
r
where,
kct1 ,kct2 ,kct3 : CT ratio matching settings of primary, secondary and tertiary winding
I ,I ,I
1 2 3
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  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
I d
ik
Figure 2.11.1 Current Differential Element
+ kct2I + kct3I |
1 2 3
|+ kct2|I |+ kct3|I
1 2 3
: currents of primary, secondary and tertiary winding
I d = 2I r (one-end infeed)
DF2
DF1
kp
|)/2
 38 
6 F 2 S 0 7 8 9
I r