Download Option B

Doc # 1ZUA003101-AAA
BIAS WINDING APPLICATION FOR
LOAD TAP CHANGER TYPE-USB
INTRODUCTION:
APPLICATION:
The number of transformer tap connections can be
reduced by incorporating an additional isolated tap
section. This isolated tap section, better known as the
bias or tickler winding, adds to, or subtracts from, the
effective turns of the regulating winding. The TypeUSB load tap changer has been designed to take
advantage of this arrangement and requires only nine
tap sections for 33 positions. Additionally, this aids
the manufacturing process as fewer transformer leads
have to be handled for the high current capability of
the Type-USB load tap changer (1,800 amps).
The most convenient application of the bias winding
is when the tap sections of the RV winding are
approximately equal with more than one turn per
section. The ideal application is when all RV tap
sections are exactly equal (i.e. same number of turns)
with an even number of turns. This condition would
result in ideal 5/8% steps. With proper design
consideration, a non-ideal division can be used if
0.5% deviations (IEEE C57.12.00) from the desired
values can be maintained. For non-ideal case, refer
to document 1ZUA003101-AAB.
THEORY OF OPERATION:
For an example of an ideal OLTC application:
One can see from Diagram 1, that the bias winding is
an isolated tap section having one half the number of
turns and volts as in the tap section of the regulating
winding (RV). As the tap changer operates, the
diverter is connected to the same tap for two
operations, one with the bias winding, and one
without the bias winding. This produces two output
voltages for each tap of the RV winding.
Trxfmr Rating:
OLTC Applied:
V2:
Step Voltage:
RV:
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 insertion
would occur when the diverter moves to position “x”
and current being drawn through contact “a”.
Diagram 1: Sequence Chart
115/13.8kV
13.8 kV (V2 x 3)
8 kV = 13.8/3
50 v = V2 x 5/8%
800 v = 50 v/step x 16 step
For the ideal conditions, eight effective sections (nine
actual) having two turns each would be used for the
regulating winding plus a one turn bias winding.
Since the 800v RV winding is divided into 8 two turn
tap sections, the volts/turn will be 50V/t.
Therefore:
N2:
160 turns = 8 kV / 50 volts/turns
LOCATION:
There are various locations for insertion of the bias
winding. Diagram 2 shows two convenient
arrangements which have been successfully used by
transformer manufactures for many years. Option A
shows a single bias turn wound adjacent to turns of
the regulating winding such that only the bias
winding ends after one turn and the rest of the
winding continues for a second turn. This can only
easily be used when two turn tap sections are applied.
Keep in mind, the bias winding must be fully
insulated from the tap sections. Option B applies an
equal number of bias turns at the top and bottom of
the tap section winding which are loaded in parallel
to attain balanced short circuit forces. This
arrangement can be used for designs of more than
two turns per tap section.
GLOSSARY:
LTC - Load Tap Changer
Tap - A connection to a sectionalized winding
Tap Section – A group of one or more turns between
taps.
Option A
Diagram 2 – Possible locations of bias winding
Tap Voltage – Volts per turn times turns per tap
section.
Step Voltage – The change in the output voltage with
each LTC operation.
Bias Winding – An isolated tap section used to
produce ½ size steps in voltage.
Regulating winding (RV) – The total collection of all
tap sections.
Effective Turns – Vector sum of turns carrying load
current at a given moment.
Effective Sections – Max. number of tap sections that
could be loaded at one time.
V2 – Line to ground voltage of transformer
secondary.
Option B
Diagram 3 – Single Phase Diagram
Notes: