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10.5 DISPLAY, METERING, COMMUNICATIONS, ANALOG OUTPUTS
Winding #1: 100 MVA, 220 kV, 250/1 CT ratio (rated current is 262.4 A, hence CT ratio of 250/1)
Winding #2: 100 MVA, 69 kV, 1000/1 CT ratio (rated current is 836.8 A, hence CT ratio of 1000/1)
The 1000/1 CT ratio is not a perfect match for the 250/1 ratio. The high-side CT produces a secondary current
of 262.5/250 = 1.05 A whereas the low-side CT produces a current of 0.837 A. The 745 automatically applies
an amplitude correction factor to the Winding 2 currents to match them to the Winding 1 currents. The following
illustrates how the correction factor is computed:
(
)
CT
ideal
=
2
The mismatch factor is therefore
Winding 2 currents are divided by this factor to obtain balanced conditions for the differential elements.
If this transformer were on line, fully loaded, and protected by a properly set 745 relay, the actual current val-
ues read by the relay would be:
Winding 1: 262.5 A ∠0° (this is the reference winding)
Winding 2: 836.8 A ∠210° (30° lag due to transformer and 180° lag due to CT connections)
Differential current: less than 0.03 × CT as the two winding currents are equal once correctly transformed
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 5–10 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
W 1 a
I
=
---------------------------- - ,
W 1 a '
By injecting a current into phase A of Winding 1 and phase A of Winding 2 only, I
we assume an injected current of 1 × CT, the transformed Y-side currents will be:
I
=
W 1 a '
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 phase A Winding 1, the following values for the differential currents for all three
phases should be obtained:
A-phase differential: 0.57 × CT ∠0° Lag
B-phase differential: 0.57 × CT ∠180° Lag
C-phase: 0 × CT.
b) EFFECTS OF ZERO-SEQUENCE COMPONENT REMOVAL
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 cur-
rent from the delta winding currents.
NOTE
If the zero-sequence component is removed from the Delta-side winding currents, the Winding 2 current values
will change under unbalanced conditions. Consider again the case described above, with the 1 × CT injected
into phase A of Winding 2.
10
10-10
V
250
220 V
×
1
×
CT
=
----- -
--------- -
--------------- -
1
V
1
69 V
2
Ideal CT Ratio
----------------------------------------- -
Actual CT Ratio
inside the relay.
I
I
I
–
–
W 1 c
W 1 b
W 1 a
I
=
---------------------------- -,
W 1 b '
3
3
×
×
1
CT
1 CT
–
I
---------------- -,
=
--------------------,
W 1 b '
3
3
745 Transformer Management Relay
=
797.1
797.1
=
-------------- -
=
0.7971
1000
I
I
–
W 1 c
W 1 b
I
=
---------------------------- -
W 1 c '
3
×
0 CT
I
=
---------------- -
W 1 c '
3
10 COMMISSIONING
= I
= 0 A. Therefore, if
W1b
W1c
GE Power Management
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