ABB RELION REX640 - Page 735

87T3
3dI>T
100 MVA
YNyn0d1
4000/1 A
400/1 A
14.4 kV
154 kV
4000/1 A
4.37 kV
Figure 419: Example of three-winding power transformer differential
protection
154 kV side: CT ratio correction factor = 400 A / 375 A = 1.07
The rated current 375 A corresponds to a 100 MVA power
but on the MV sides 1.732 · 14.4 kV · 3007 A = 75 MVA and
1.732 · 4.37 kV · 3303 A = 25 MVA. Thus, the MV side rated
currents cannot be used for calculations.
14.4kV side: CT ratio correction factor = 4000 A / (375 A · 154 kV / 14.4 kV) =
1.00
4.37kV side: CT ratio correction factor = 4000 A / (375 A · 154 kV / 4.37 kV) =
0.30
Assume for example a 50 MVA load on the 14.4 kV side and zero on the 4.37
kV side.
• 154 kV side current = 50 MVA / (1.732 · 154 kV) = 187.45 A
TR3PTDF sees this as 185.45 A / 400 A · 1.07 = 0.50 xIr
• 14.4 kV side current = 50 MVA / (1.732 · 14.4 kV) = 2004.7 A
TR3PDF seez this as 2004.7 A / 4000 A · 1.00 = 0.50 xIr
Assume for example a 20 MVA load on the 4.37 kV side and zero on the 14.4
kV side.
• 154kV side current = 20 MVA / (1.732 · 154 kV) = 75.0 A
TR3PTDF sees this as 75 A / 400 A · 1.07 = 0.20 xIr
• 4.37kV side current = 20 MVA / (1.732 · 4.37 kV) = 2642.3 A
TR3PTDF sees this as 2642.3 A / 4000 A · 0.30 = 0.20 xIr
Vector group matching and elimination of the zero-sequence component
The vector group of the power transformer is numerically matched on the windings
on different sides of the transformer by the Phase shift Wnd 1-2 and Phase shift
Wnd 1-3 settings. Thus no interposing CTs are needed if there is only a power
transformer inside the protected zone. The matching is based on phase shifting and
a numerical delta connection in the function.
1MRS759142 F
Protection functions
REX640
Technical Manual
735