Differrential overcurrent protection (87) – Rockwell Automation 865 Differential Protection Relay User Manual
Page 50
3-6 Protection
Functions
865-UM001A-EN-P – July 2009
The differential overcurrent protection comprises two separately
adjustable stages, stage ΔI> and stage ΔI>>.
The differential protection is based on winding currents difference
between IL and I’L side. In transformer applications the current
calculation depends on transformer connection group. E.g. in Yy0
connection measured currents are also winding currents, see Figure
3.5. In generator applications the connection group is always Yy0
and measured currents are also winding currents.
Figure 3.5 – Winding currents in connection group Yy0
In the second example if transformer IL side is connected to open
delta, e.g. Dy11, then winding currents are calculated in delta side
(IL side), see Figure 3.6.
Figure 3.6 – Winding currents in connection group Dy11
Equation 1: Winding current calculation in delta side, Dy11 connection
(
)
)
(
(
)
3
3
3
1
3
3
3
2
2
2
1
1
L
L
W
L
L
L
W
L
L
L
W
L
I
I
I
I
I
I
I
I
I
−
=
−
=
−
=
Differential Overcurrent
Protection ΔI> (87)