GE 745 TRANSFORMER MANAGEMENT RELAY

5-4 745 Transformer Management Relay GE Power Management
5.2 AUTO-CONFIGURATION 5 SETPOINTS
5
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:
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
•CT
2
/ CT
1
= V
1
/ V
2
• Lower sensitivity on differential element
Solution:
• Tap position monitoring
V
2
= V
min
+ ( n – 1) V
incr
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:
where
n = current tap changer position
V
2(min)
= Winding 2 minimum voltage (at n = 1)
V
2(tap)
= Winding 2 voltage increment per 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.
Under
S2 SYSTEM SETUP / WINDING 1:
Under
S2 / SYSTEM SETUP / WINDING 2:
WINDING 1 NOM
Φ
-
Φ
VOLTAGE: 220.0 kV
WINDING 2 NOM
Φ
-
Φ
VOLTAGE: 69.0 kV
WINDING 1 RATED
LOAD: 100 MVA
WINDING 2 RATED
LOAD: 100 MVA
WINDING 1 PHASE CT
PRIMARY: 500:1 A
WINDING 2 PHASE CT
PRIMARY: 1500:1 A
CT
2
(ideal) CT
1
V
1
V
2min
()
V
2tap
()
n 1–
()
+
-----------------------------------------------------------
×
500
1
----------
220
61 0.5
n 1–
()
+
---------------------------------------
×
==