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6.1.3.2
Fundamental frequency differential currents
The fundamental frequency differential current is a vectorial sum (sum of fundamental
frequency phasors) of the individual phase currents from the different sides of the
protected power transformer.
Before any differential current can be calculated, the power transformer phase shift, and
its transformation ratio, must be accounted for. Conversion of all currents to a common
reference is performed in two steps:
•
all current phasors are phase-shifted to (referred to) the phase-reference side,
(whenever possible the first winding with wye connection)
•
all currents magnitudes are always referred to the first winding of the power
transformer (typically transformer high-voltage side)
The two steps of conversion are made simultaneously on-line by the pre-programmed
coefficient matrices, as shown in equation
for a two-winding power transformer, and in
for a three-winding power transformer.
These are the internal compensation algorithms within the differential
function. The protected power transformer data is always entered per its
nameplate. The Differential function will adapt nameplate data and select
proper reference windings.
_ _ 1
_ _ 2
_ 2
_ _ 1
_ _ 2
_ 1
_ _ 1
_ _ 2
IDA
I A W
I A W
Vn W
IDB
A I B W
B I B W
Vn W
IDC
I C W
I C W
é
ù
é
ù
é
ù
ê
ú
ê
ú
ê
ú
= ×
+
× ×
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
ë
û
ë
û
ë
û
_ _ 1
_ _ 2
_ 2
_ _ 1
_ _ 2
_ 1
_ _ 1
_ _ 2
IDA
I A W
I A W
Vn W
IDB
A I B W
B I B W
Vn W
IDC
I C W
I C W
é
ù
é
ù
é
ù
ê
ú
ê
ú
ê
ú
= ×
+
× ×
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
ë
û
ë
û
ë
û
_ _ 1
_ _ 2
_ 2
_ _ 1
_ _ 2
_ 1
_ _ 1
_ _ 2
IDA
I A W
I A W
Vn W
IDB
A I B W
B I B W
Vn W
IDC
I C W
I C W
é
ù
é
ù
é
ù
ê
ú
ê
ú
ê
ú
= ×
+
× ×
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
ê
ú
ë
û
ë
û
ë
û
1
2
3
EQUATION1879 V1 EN
(Equation 1)
where:
1.
is the resulting Differential Currents
2.
is the current contribution from the W1 side
3.
is the current contribution from the W2 side
Section 6
1MRK 502 048-UUS A
Differential protection
92
Technical manual
Содержание 650 series
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