5-6
745 Transformer Management Relay
GE Multilin
5.2 AUTO-CONFIGURATION
5 SETPOINTS
5
b) 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.
Figure 5–1: TAP POSITION INPUT
The “zero position” terminal and the “wiper” terminal of the tap position circuit are connected to the positive and negative
745 tap position terminals. Polarity is not consequential. The following setpoints configure the 745 to determine tap posi-
tion. In the
S2 SYSTEM SETUP
ÖØ
ONLOAD TAP CHANGER
setpoint menu, make the following settings:
WINIDNG WITH TAP CHANGER
: "Winding 2"
NUMBER OF TAP POSITIONS
: "33"
MINIMUM TAP POSITION VOLTAGE
: "61.0 kV"
VOLTAGE INCREMENT PER TAP
: "0.50 kV"
RESISTANCE INCREMENT PER TAP
: "33
Ω
"
The maximum value resistance on the top tap is 5 k
Ω
.
CT
2
(ideal)
CT
1
V
1
V
2 min
(
)
V
2 tap
(
)
n
1
–
(
)
+
-----------------------------------------------------------
×
500
1
----------
220
61 0.5
n
1
–
(
)
+
---------------------------------------
×
=
=
ZERO
POSITION
TAP 1
TAP 2
TAP 3
TAP 4
TAP 6
TAP 7
TAP 8
TAP 9
etc.
WIPER
AT TAP 5
R
R
R
R
R
R
R
R
R
ZERO POSITION
TERMINAL
(to TAP POSITION -)
WIPER
TERMINAL
(to TAP PO)
