ABB REB670 Applications Manual page 166

Section 6
Differential protection
160
× ×
N4 = k 3 N
EQUATION1110 V1 EN
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 25:
é ù é
IA 1 1 1
ê ú ê
=
2
IB a a 1
ê ú ê
ê ú ê
2
IC a a 1
ë û ë
EQUATION1788-ANSI V1 EN
where:
a is complex constant (that is, a=-0.5+j0.866).
By including equation 22, equation 23, equation
the equation for the series-connected, ASCT secondary current (that is, summated
current) can be derived according to equation 26:
1
-j30°
-- - × ( ×
I I e I
= +
SUMM 1 2
k
EQUATION1114 V1 EN
From equation 26 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.
ù é ù
I
1
ú ê ú
×
I
ú ê ú
2
ú ê ú
I
û ë û
0
24
j30°
× × × )
e 2 3 I
+
0
1MRK 505 337-UUS -
and equation 25 into the equation
Application manual
(Equation 24)
(Equation 25)
21
(Equation 26)