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Section 6
Differential protection
6.1.6.7
SLCE 8/ASCT characteristics for series-connection
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
EQUATION1113 V1 EN-US
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
=
EQUATION1108 V1 EN-US
N3
=
2 N
EQUATION1109 V1 EN-US
N4
=
k
EQUATION1110 V1 EN-US
where:
k
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
EQUATION1111 V1 EN-US
where:
a
By including Equation 19, Equation 20, Equation
equation for the series-connected, ASCT secondary current (that is, summated current) can be
derived according to equation 23:
I
SUMM
EQUATION1114 V1 EN-US
From Equation
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.
142
×
×
=
N1 IL1 N2
–
IL3
–
N;
=
×
×
×
3 N
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).
1
1
1
I
1
2
×
I
a
a
1
2
2
I
a
a
1
0
is complex constant (that is, a=-0.5+j0.866).
1
-j30°
j30°
-- -
×
(
×
×
=
I
e
+
I
e
+
1
2
k
23
it is obvious that the ASCT rated ratio is declared for balanced three phase
×
(
)
N3
IL1
+
IL2
+
IL3
21
and Equation
×
×
)
2
3 I
0
1MRK 505 370-UEN D
(Equation 18)
(Equation 19)
(Equation 20)
(Equation 21)
(Equation 22)
22
into the Equation
18
(Equation 23)
Busbar protection REB670
Application manual
M12143-3 v4
the
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