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NON-IDEAL BIAS WINDING APPLICATION
FOR LOAD TAP CHANGER TYPE-USB
INTRODUCTION:
In some cases, it is not practical to design
transformers such that the ideal arrangement of tap
sections is obtained. Two such cases will be
addressed here. They are:
1. The case of one turn tap sections.
2. The case of a mixture of differing numbers of
turns per tap section.
In cases where the total number of active turns in the
RV winding is greater than 16 but not evenly
divisible by 16, it is necessary for some tap sections
to have more turns than others. In these cases, the
LTC will produce nonlinear ratio steps. This is true
for all LTC’s (reactance and resistance) but is often
applicable and produces results within the 0.5%
standard allowed by IEEE C57.12.00.
An Example of “mixed” tap sections:
THE CASE OF ONE TURN TAP SECTIONS:
As can be seen from Diagram 1, the bias winding is
an isolated tap section having the same number of
turns and volts as in the tap sections of the regulating
winding (RV). This winding is used to excite a small
2:1 autotransformer which produces a ½ tap voltage
for the bias circuit. As the tap changer operates, the
diverter is connected to the same tap for two
operations, one with the bias circuit and one without
the bias winding. This produces two output voltages
for each tap of the RV winding.
Diagram 1 shows that the bias winding is not in the
circuit when the diverter is on position “v” with load
current through selector contact “b” . Bias circuit
insertion would occur when the diverter moves to
position “x” and current is being drawn through
contact “a”.
THE CASE OF DIFFERING NUMBERS OF
TURNS PER TAP SECTION:
Diagram 1 Sequence Chart
Transformer Rating:
OLTC Applied:
V2:
138/34.5 kV
34.5 kV
19,919 V=34.5/3 kV
If this transformer design were to be optimized at 100
V per turn, let us assume, then the following would
apply:
Desired Volts/Turn:
RV:
RV active turns:
100 V/t
1992 = V2 X 10%
20t = RV/100V
Since 20 turns does not divide evenly into 8 active
sections, the following arrangement could be used
(see Diagram 2):
Turns/Tap:
4 w/ 2 turns
4 w/ 3 turns
Extra Tap for Reversing: 1 w/ 2 turns
Turn Arrangement:
2,3,2,3,2,3,2,3,2 turns
Bias Winding Turns:
1 turn
Max. Calculated Error:
0.368%(Pos 20 or 3R)
Diagram 2 Turn Arrangement
Graph 1
Graph 1 shows the output voltage of this non-ideal
arrangement compared to the ideal values and those
calculated using a reactance LTC. Note, that there
are only (4) positions that have greater error with the
bias winding scheme and (24) positions that produce
the exact same voltage as with reactance LTC. All
positions produce ratios well within the 0.5%
standard allowance (IEEE C57.12.00).
Bias Winding – An isolated tap section used to
produce ½ size steps in voltage.
Glossary:
Effective Sections – Max. number of tap sections that
could be loaded at one time.
Regulating winding (RV) – The total collection of all
tap sections.
Effective Turns – Vector sum of turns carrying load
current at a given moment.
LTC - On Load Tap Changer
Tap - A connection to a sectionalized winding
Tap Section – A group of one or more turns between
taps.
Tap Voltage – Volts per turn times turns per tap
section.
Step Voltage – The change in the output voltage with
each LTC operation.
V2 – Line to ground voltage of transformer
secondary.
Diagram 3 – Single Phase Diagram Applying One Turn Sections
Notes: