GE 745 - Page 103

CHAPTER 5: SETPOINTS AUTO-CONFIGURATION
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 5–7
where CT
1
= winding 1 CT ratio
V
1
= winding 1 nominal voltage
CT
2
= winding 2 CT ratio
V
2
= winding 2 nominal voltage
Thus, for any load, the winding 2 CT secondary current is higher (per unit) than the
winding 1 CT secondary current. The mismatch factor is 1594.2 / 1500 = 1.063.
The transformer type is entered as
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:
S2 SYSTEM SETUP  WINDING 1  WINDING 1 NOM F-F VOLTAGE: “220 kV”
S2 SYSTEM SETUP
 WINDING 1  WINDING 1 RATED LOAD: “100 MVA”
S2 SYSTEM SETUP  WINDING 1  WINDING 1 PHASE CT PRIMARY: “500:1 A”
S2 SYSTEM SETUP
 WINDING 2  WINDING 2 NOM F-F VOLTAGE: “69.0 kV”
S2 SYSTEM SETUP  WINDING 2  WINDING 2 RATED LOAD: “100 MVA”
S2 SYSTEM SETUP
 WINDING 2  WINDING 2 PHASE CT PRIMARY: “1500:1 A”
For a three-winding transformer, the setpoints under the S2 SYSTEM SETUP  WINDING 3
menu must also be set.
5.2.2.2 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
For 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:
(EQ 5.2)
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.
The 745 allows monitoring of the tap changer position via the tap position input. With this
input, the 745 dynamically adjusts the CT ratio mismatch factor based on the actual
transformer voltage ratio set by the tap changer.
Tap changers are operated by means of a motor drive unit mounted on the outside of the
transformer tank. The motor drive is placed in a protective housing containing all devices
necessary for operation, including a tap position indication circuit. This indication circuit
has a terminal for each tap with a fixed resistive increment per tap. A cam from the drive
shaft that provides local tap position indication also controls a wiper terminal in the
indication circuit, as illustrated below.
CT
2
(ideal) CT
1
V
1
V
2min()
V
2tap()
n 1–()+
---------------------------------------------------------
×
500
1
---------
220
61 0.5 n 1–()+
-------------------------------------
×==