ABB REG670 Technical Manual page 132

Section 6
Differential protection
126
é ù é
-
IDL 1 _ NS 2 1
ê ú ê
1
×
= -
IDL 2 _ NS 1 2
ê ú ê
3
ê ú ê
ë û ë
- -
IDL 3 _ NS 1 1
1
EQUATION1247 V1 EN
where:
1. is the Negative Sequence Differential Currents
2. is the Negative Sequence current contribution from the W1 side
3. is the Negative Sequence current contribution from the W2 side
and where:
IDL1_NS is the negative sequence differential current in phase L1 (in
W1 side primary amperes)
IDL2_NS is the negative sequence differential current in phase L2 (in
W1 side primary amperes)
IDL3_NS is the negative sequence differential current in phase L3 (in
W1 side primary amperes)
INS_W1 is the negative sequence current on the W1 side in primary
amperes (phase L1 reference)
INS_W2 is the negative sequence current on the W2 side in primary
amperes (phase L1 reference)
Ur_W1 is the transformer rated phase-to-phase voltage on the W1
side (setting parameter)
Ur_W2 is the transformer rated phase-to-phase voltage on W2 side
(setting parameter)
a is the complex operator for sequence quantities,
o
= × = - + ×
j 120
a e
EQUATION1248 V1 EN
Because the negative sequence currents always form the symmetrical three phase
current system on each transformer side (that is, negative sequence currents in every
phase will always have the same magnitude and be phase displaced for 120 electrical
degrees from each other), it is only necessary to calculate the first negative sequence
differential current that is, IDL1_NS.
As marked in equation 23, the first term on the right hand side of the equation,
represents the total contribution of the negative sequence current from the W1 side
compensated for eventual power transformer phase shift. The second term on the right
hand side of the equation, represents the total contribution of the negative sequence
ù é ù
-
1 INS W _ 1
ú ê ú
Ur W _
×
- × +
1 a INS W _ 1
ú ê ú
Ur W _
ú ê ú
2 ×
û ë û
2 a INS W _ 1
2
1 3
j
2 2
1MRK502052-UEN B
é ù é
-
1 0 1
ê ú ê
2 1
× × ×
-
1 1 0 a INS W
ê ú ê
1
3
ê ú ê
2
ë û ë
-
0 1 1 a
3
(Equation 24)
Technical manual
ù
INS W _ 2
ú
×
_ 2
ú
ú
×
û
INS W _ 2
(Equation 23)