Sel SEL-787 - Page 101

4.15
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
that is,
The effect of each compensation on balanced three-phase currents is to rotate
them m • 30° without a magnitude change.
The compensation matrix [CTC(12)] is similar to [CTC(0)], in that it produces
no phase shift (or, more correctly, 360 degrees of shift) in a balanced set of
phasors separated by 120 degrees. However, it removes zero-sequence
components from the winding current, as do all of the matrices having
nonzero values of m.
that is,
We could use this type of compensation in applications having wye-connected
transformer windings (no phase shift) with wye CT connections for each
winding. Using WnCTC = 12 for each winding removes zero-sequence
components, just as connection of the CTs in delta would do, but without
producing a phase shift. (One might also use WnCTC = 1 or 11 for this same
application, yielding compensation similar to that from connection of the CTs
on both sides in DAB or DAC.)
I1WnC
IAWn ICWn–
3
------------------------------------------=
I2WnC
IBWn IAWn–
3
------------------------------------------=
I3WnC
ICWn IBWn–
3
------------------------------------------=
CTC 12
1
3
---
2 –1 –1
–1 2 –1
–1 –1 2
=
I1WnC
+2 IAWn IBWn ICWn––
3
--------------------------------------------------------------------------=
I2WnC
–IAWn 2IBWn ICWn–+
3
--------------------------------------------------------------------------=
I3WnC
–IAWn I– BWn 2ICWn+
3
--------------------------------------------------------------------------=