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⎯ 53 ⎯
6 F 2 S 0 8 5 7
I
r
= max.( 1kct1⋅|I
1a
| , 1kct1⋅|I
1b
| , 1kct1⋅|I
1c
| , 1kct2⋅|I
2a
| , 1kct2⋅|I
2b
| , 1kct2⋅|I
2c
| ,1kct3⋅|I
3a
| ,
1kct3⋅|I
3b
| , 1kct3⋅|I
3c
|
, |I
N
|
)
where,
I
1o
,I
2o
,I
3o
: residual current of primary, secondary and tertiary winding
I
1a
,I
1b
,I
1c
,I
2a
,I
2b
,I
2c
,I
3a
,I
3b
,I
3c
: phase current of primary, secondary and tertiary winding
I
N
: residual current of neutral circuit
1kct1, 1kct2, 1kct3: CT ratio matching of primary, secondary and tertiary line CT to neutral
CT
Characteristic DF2 is expressed by the following equation:
I
d
≥ p2
(I
r
−kp)
where
p2 : slope of DF2
kp : break point of DF1 characteristic
The characteristic of REF_DEF is composed of a directional characteristic and a non-directional
characteristic as shown in Figure 2.10.3 (a) and (b). This characteristic is employed so that the
REF is not blocked at one-end infeed current I
N
.
0
90°
180
°
270°
0°
I
N
3I
0
Operating
Ik1
Ik1, ik2: Current sensitivity (0.01pu, 0.025pu fixed)
I
N
: Neutral current of transformer
3I
0
: Zero sequence current (calculated from Ia, Ib, Ic)
*1: MAX(Ia,Ib,Ic) ≤ 2×MAX(kct1,kct2,kct3)
(*1)
Ik2
3I
0
(a) (b)
Figure 2.10.3 REF_DEF Characteristic
The REF_DEF detects an internal fault by checking the direction between transformer neutral
current I
N
and zero-sequence current 3I
0
calculated from phase currents I
a
, I
b
and I
c
. The REF_DEF
is blocked when the maximum phase current is larger than 2 times of Max-kct as follows:
Max.(1kct1•I
1a
, ….1kct3•I
3c
) ≥ I
BLK
=Max.(1kct1,1kct2,1kct3)×2