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5.2 AUTO-CONFIGURATION
745 Solution:
The transformer type is entered as the setpoint S2 SYSTEM SETUP/TRANSFORMER/TRANSFORMER TYPE. The 745
calculates and automatically corrects for CT mismatch to a maximum mismatch factor of 16. The following
information is entered as setpoints:
Under S2 SYSTEM SETUP / WINDING 1:
WINDING 1 NOM Φ-Φ
VOLTAGE: 220.0 kV
WINDING 1 RATED
LOAD: 100 MVA
WINDING 1 PHASE CT
PRIMARY: 500:1 A
For a 3-winding transformer, the setpoints under S2 SYSTEM SETUP/WINDING 3 must also be set.
b) PROBLEM 2: ONLOAD TAP CHANGER
•
Onload tap changer
•
Variable voltage ratio
5
•
CT
/ CT
= V
/ V
2
1
1
•
Lower sensitivity on differential element
Solution:
•
Tap position monitoring
V
= V
+ ( n – 1) V
2
min
Example:
The onload tap changer changes the Winding 2 voltage, resulting in an even greater CT mismatch. A perfectly
matched Winding 2 CT ratio (based on the tap changer position) is calculated as follows:
CT
(ideal)
=
CT
2
where n = current tap changer position
V
= Winding 2 minimum voltage (at n = 1)
2(min)
V
= Winding 2 voltage increment per tap
2(tap)
Thus, with the tap changer at position 33, the Winding 2 CT ratio must be 1428.6 / 1 to be perfectly matched. In
this case, the mismatch factor is 1428.6 / 1500 = 0.952.
745 Solution:
The 745 allows monitoring of the tap changer position via the tap position input. With this input, the 745 dynam-
ically adjusts the CT ratio mismatch factor based on the actual transformer voltage ratio set by the tap changer.
5-4
2
incr
V
×
1
---------------------------------------------------------- -
1
(
)
V
V
n 1
+
–
(
)
(
)
2 min
2 tap
745 Transformer Management Relay
Under S2 / SYSTEM SETUP / WINDING 2:
WINDING 2 NOM Φ-Φ
VOLTAGE: 69.0 kV
WINDING 2 RATED
LOAD: 100 MVA
WINDING 2 PHASE CT
PRIMARY: 1500:1 A
500
220
×
=
--------- -
-------------------------------------- -
(
)
1
61
0.5 n 1
+
–
5 SETPOINTS
GE Power Management
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