ABB REB670 Series, Applications Manual

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6.1.6.6 SLCE 8/ASCT characteristics for end-connection
M12141-3 v4
Typical ASCT end-connection is shown in Figure 84 . For this ASCT connection type, the
ampere-turn balance equation has the form according to Equation 12 :
N4 I
SUMM
×
N1 IL1
×
N2 IL1 IL2 +
( ) ×
N3 IL1 IL2 IL3 + +
( ) ×
+ + =
EQUATION1107 V1 EN-US
(Equation 12)
The relationships between number of turns for this SLCE 8, ASCT for REB670, is shown in
Equation 13 , Equation 14 and Equation 15 :
N1 N2 N; = =
EQUATION1108 V1 EN-US
(Equation 13)
N3 2 N
×
=
EQUATION1109 V1 EN-US
(Equation 14)
N4 k 3 N
× ×
=
EQUATION1110 V1 EN-US
(Equation 15)
where:
k 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 16 :
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 16)
where:
a complex constant (that is, a=-0.5+j0.866).
By including Equation 13 , Equation 14 , Equation 15 and Equation 16 into the Equation 12 the
equation for the end-connected, ASCT secondary current (that is, summated current) can be
derived according to equation 17 :
I
SUMM
1
k
---
I
1
e
-j30°
×
I
2
e
j30°
×
3 3 I
0
× ×
+ +
( ) ×
=
EQUATION1112 V1 EN-US
(Equation 17)
From Equation 17 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.
1MRK 505 370-UEN D Section 6
Differential protection
Busbar protection REB670 141
Application manual