GE 745 Instruction Manual page 103

CHAPTER 5: SETPOINTS
5.2.2.2 Onload Tap Changer
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
where CT = winding 1 CT ratio
1
V = winding 1 nominal voltage
1
CT = winding 2 CT ratio
2
V = winding 2 nominal voltage
2
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
. The 745 calculates and automatically corrects for CT mismatch to a maximum
TYPE
mismatch factor of 16. The following information is entered as setpoints:
S2 SYSTEM SETUP

S2 SYSTEM SETUP

S2 SYSTEM SETUP

S2 SYSTEM SETUP

S2 SYSTEM SETUP

S2 SYSTEM SETUP

For a three-winding transformer, the setpoints under the
menu must also be set.
• Onload tap changer
• Variable voltage ratio
• CT / CT = V / V
2 1 1 2
• Lower sensitivity on differential element
Solution:
• Tap position monitoring: V
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:
CT (ideal) CT ×
=
2 1
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.
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.
S2 SYSTEM SETUP
WINDING 1 WINDING 1 NOM F-F

WINDING 1 WINDING 1 RATED

WINDING 1 WINDING 1 PHASE CT

WINDING 2 WINDING 2 NOM F-F

WINDING 2 WINDING 2 RATED

WINDING 2 WINDING 2 PHASE CT

= V + (n – 1)V
2 min incr
V
1
-------------------------------------------------------- -
=
V V ( n 1 )
+ –
( ) ( )
2 min 2 tap
AUTO-CONFIGURATION
TRANSFORMER TRANSFORMER
 
VOLTAGE: "220 kV"
LOAD: "100 MVA"
PRIMARY: "500:1 A"
VOLTAGE: "69.0 kV"
LOAD: "100 MVA"
PRIMARY: "1500:1 A"
S2 SYSTEM SETUP WINDING 3

500 220
-------- - × -------------------------------------
1 61 0.5 n 1 ( )
+ –
(EQ 5.2)
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