Schweitzer Engineering Laboratories SEL-787 - Page 92

4.14
SEL-787 Relay Instruction Manual Date Code 20081022
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 currents, as do all of the matrices having non-
zero 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.)
The Complete List of Compensation Matrices (m = 1 to 12)
IAWnC
IAWn ICWn–()
3
------------------------------------------=
IBWnC
IBWn IAWn–()
3
------------------------------------------=
ICWnC
ICWn IBWn–()
3
------------------------------------------=
CTC 12()[]
1
3
---
2 –1 –1
–1 2 –1
–1 –1 2
•=
IAWnC
+2 IAWn IBWn ICWn––•()
3
--------------------------------------------------------------------------=
IBWnC
–IAWn 2IBWn• ICWn–+()
3
--------------------------------------------------------------------------=
ICWnC
–IAWn I–BWn 2ICWn•+()
3
--------------------------------------------------------------------------=
CTC 1()[]
1
3
-------
1 –1 0
0 1 –1
–1 0 1
•=CTC2()[]
1
3
---
1 –2 1
1 1 –2
–2 1 1
•=
CTC 3()[]
1
3
-------
0 –1 1
1 0 –1
–1 1 0
•=CTC4()[]
1
3
---
–1 –1 2
2 –1 –1
–1 2 –1
•=