GE 745 Instruction Manual page 250

METERING
7.4.4.3 Effects of Zero-sequence Compensation Removal
Note
NOTE
7–12
• Differential current: < 0.03 × CT as the two winding currents are equal once
correctly transformed inside the relay.
• The loading of each winding would be 100% of rated.
The above results can be verified with two adjustable sources of three-phase current. With
a single current source, how the relay performs the necessary phase angle corrections
must be taken into account. Table 5–1: Transformer types on page 5–13 shows that the Y-
side currents are shifted by 30° to match the Delta secondary side. The 30° phase shift is
obtained from the equations below:
I I
–
W1a W1c
I --------------------------- , I
=
W1a'
3
By injecting a current into Phase A of Winding 1 and Phase A of Winding 2 only, I
= 0 A. Therefore, if we assume an injected current of 1 × CT, the transformed Y-side currents
will be:
1 CT
I --------------- , I
=
W1a'
For the purposes of the differential elements only, the transformation has reduced the
current to 0.57 times its original value into Phase A, and created an apparent current into
Phase B, for the described injection condition. If a 1 × CT is now injected into Winding 1
Phase A, the following values for the differential currents for all three phases should be
obtained:
Phase A differential: 0.57 × CT ∠0° Lag
Phase B differential: 0.57 × CT ∠180° Lag
Phase C: 0 × CT.
The transformation used to obtain the 30° phase shift on the Y-side automatically
removes the zero-sequence current from those signals. The 745 always removes the
zero-sequence current from the delta winding currents.
If the zero-sequence component is removed from the Delta-side winding currents, the
Winding 2 current values will change under unbalanced conditions. Consider the case
described above, with the 1 × CT injected into Phase A of Winding 2.
For the 1 × CT current, the zero-sequence value is 1/3 of 1.0 × CT or 0.333 × CT A. The value
for I is therefore (1.0 – 0.333) × CT = 0.6667 × CT A. This value must be divided by the CT
W2a'
error correction factor of 0.797 as described above.
Therefore, the value of differential current for Phase A, when injecting 1 × CT in Winding 2
only, is:
I
A differential (
The action of removing the zero-sequence current results in a current equal to the zero-
sequence value introduced into phases B and C. Hence, the differential current for these
two elements is:
I
=
B differential ( )
I I
–
W1b W1a
--------------------------- - , I
=
W1b'
3
× 1 CT ×
–
------------------ - , I
=
W1b' W1c'
3 3
0.667 CT A ×
------------------------------ - 0.84 CT A
= =
)
0.797
0.333 × CT A
I ------------------------------ -
=
C differential ( )
0.797
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: COMMISSIONING
I I
–
W1c W1b
---------------------------
=
W1c'
3
W1b
0 CT ×
---------------
=
3
×
0.42 × CT A
=
(EQ 7.7)
= I
W1c
(EQ 7.8)
(EQ 7.9)
(EQ 7.10)