ABB REB670 Series, Applications Manual

Page: 148 / 476

6.1.6.7 SLCE 8/ASCT characteristics for series-connection
M12143-3 v4
Typical ASCT series-connection is shown in Figure 85 . For this ASCT connection type, the
ampere-turn balance equation has the form according to equation 18 :
N4 I
SUMM
×
N1 IL1 N2 – IL3 N3 – IL1 IL2 IL3 + +
( ) × × ×
=
EQUATION1113 V1 EN-US
(Equation 18)
The relationships between the number of turns for this SLCE 8 ASCT for REB670, is shown in
Equation 19 , Equation 20 , Equation 21 :
N1 N2 N; = =
EQUATION1108 V1 EN-US
(Equation 19)
N3 2 N
×
=
EQUATION1109 V1 EN-US
(Equation 20)
N4 k 3 N
× ×
=
EQUATION1110 V1 EN-US
(Equation 21)
where:
k is a constant, which depends on the type of ASCT
(that is, k=1, for 1/1A ASCT or k=5 for 5/1A ASCT or k=2 for 2/1A ASCT).
The well-known relationship, between positive, negative and zero sequence current
components and individual phase current quantities is shown in Equation 22 :
IL1
IL2
IL3
1 1 1
a
2
a 1
a a
2
1
I
1
I
2
I
0
×
=
EQUATION1111 V1 EN-US
(Equation 22)
where:
a is complex constant (that is, a=-0.5+j0.866).
By including Equation 19 , Equation 20 , Equation 21 and Equation 22 into the Equation 18 the
equation for the series-connected, ASCT secondary current (that is, summated current) can be
derived according to equation 23 :
I
SUMM
1
k
---
I
1
e
-j30°
×
I
2
e
j30°
×
2 3 I
0
× ×
+ +
( ) ×
=
EQUATION1114 V1 EN-US
(Equation 23)
From Equation 23 it is obvious that the ASCT rated ratio is declared for balanced three phase
current system, when only positive sequence current component exist. For any unbalanced
condition (that is, external or internal fault), both negative and zero sequence current
components will give their own contribution to the summated current.
Section 6 1MRK 505 370-UEN D
Differential protection
142 Busbar protection REB670
Application manual