TAPS IN AUTOTRANSFORMERS
By
Dr. Thomaz Kalicki
Hydro One
Toronto, Canada
V. Sankar P. Eng.
Power Transformer Services
Burlington, Canada
PRESENTED IN IEEE TRANSFORMERS COMMITTEE
MEETINGS IN TORONTO, CANADA ON OCTOBER 25, 2010
-1-
OBJECTIVES
•
Assist users in writing autotransformer specifications that meet system needs
and also in procuring autotransformers at economical prices.
•
Explain the effects of taps on design and manufacture of autotransformers.
•
Show how the electrical location of taps in autotransformers affects cost.
•
Influence organizations like IEEE, IEC and CSA to conduct workshops on how to
determine the tap range, type of taps, and electrical connection of taps;
considering operational requirements, reliability, manufacturing problems and
costs.
•
Reduce energy costs by substituting reversing taps with linear or coarse/fine taps
where economical and technically feasible
•
Highlight the usefulness and limitations of Standards and Guides in preparing
autotransformer procurement specifications.
•
Remove restrictions in specifications (Examples: Load Tap Changer in a
separate compartment, reversing taps only, reactive type tap changers only etc.,)
and give freedom to designers to offer economical and technically sound
autotransformers.
•
Discuss advantages and disadvantages of De-energized Tap changer (DTC) in
the operation. Also to discuss complications of DTC taps in design and
manufacture of autotransformers. Suggest ways to eliminate DTC taps without
foregoing operational requirements.
•
Encourage users to purchase autotransformers contrary to the specifications
when the alternates meet all system requirements and cost less without
compromising the reliability. Some examples are:
---Different electrical location of taps.
---Linear taps in place of reversing taps.
---Extending Load Tap Changer (LTC) tap range to eliminate DTC taps.
•
Pinpoint the importance of interaction between users and manufacturers to their
mutual benefit.
-2-
OUTLINE
This tutorial covers taps in autotransformers only. The authors recommend
referring the tutorial titled “TAPS” presented at IEEE Transformers Committee
Meetings in Las Vegas, Nevada on October 26, 2004 because it contains
fundamentals on taps. “TAPS” tutorial can be accessed at:
http://www.transformerscommittee.org/info/F04/F04-Taps.pdf
The topics covered in this tutorial are listed below:
ƒ
Types of taps.
ƒ
Standards and Specifications.
ƒ
Tap locations and their functions.
ƒ
DTC taps (de-energized taps).
ƒ
LTC taps (on-load taps).
ƒ
DTC and LTC taps in combination.
ƒ
Effects of tap locations on dielectric design.
ƒ
Effects of tap locations on other parameters.
ƒ
Effects of taps in step-up and in step-down operations.
ƒ
Effects of HV or LV taps on tertiary.
ƒ
On-load tap changers.
ƒ
Overloads and life cycle cost.
ƒ
Tap windings.
ƒ
Metal-oxide arresters and static shields.
ƒ
Function of taps stated in specifications versus their operation in the
system.
ƒ
Conclusions.
-3-
AUTOTRANSFORMER DESIGNATIONS
An autotransformer is normally comprised of two circuits: HV and LV. There can
be an additional circuit such as a tertiary.
An autotransformer is different from a two-winding transformer in that one
winding is commonly shared between both the HV and the LV. This shared
winding is known as the common winding. The LV is composed of the common
winding. The HV is composed of the common winding, plus an additional portion
known as the series winding. Figure 1 below shows autotransformer schematic.
Tertiary is not shown in this figure.
Figure 1
The series and common windings are commonly abbreviated as SV and CV,
similar to HV and LV. Tertiary is commonly abbreviated as TV.
-4-
TYPES OF TAPS
1. Based on Function
Core flux is proportional to the volts per turn. Volts per turn is defined as the
voltage applied across the turns of a circuit divided by the turns in that circuit. If a
tapping winding is included in that circuit, then the turns in that circuit change
with each tap position, or the volts per turn changes with each tap position or
both. The taps can be classified into three categories based on their effect on flux
in the core.
A. Constant flux taps
If the voltage changes with each tap position in direct proportion to the turns then
volts per turn is constant throughout the tap range. Such taps are known as
constant flux taps. For constant flux taps core flux density remains constant
throughout the tap range. Two commonly used tap arrangements of constant flux
taps are shown below.
Figure 2
Taps in Series
Figure 3
Taps in LV Line End
Taps in Figure 2 are constant flux taps when they are used to compensate for
input voltage fluctuations in step-down operation or to compensate for regulation
in step-up operation. Taps in Figure 3 are constant flux taps when they are used
-5-
to compensate for regulation in step-down operation or to compensate for input
voltage fluctuations in step-up operation.
B. Variable flux taps
If the volts per turn changes with each tap position, then the taps are known as
variable flux taps. With these taps flux density in the core changes when the taps
are changed. Taps in Figure 2 are variable flux taps when they are used to
compensate for regulation for step-down operation or to compensate for input
voltage fluctuations for step-up operation. Taps in Figure 3 are variable flux taps
when they are used to compensate for input voltage fluctuations for step-down
operation or to compensate for regulation for step-up operation.
A third arrangement of tap connections is shown below.
Figure 4
Neutral End Taps
Taps arrangement in Figure 4 is always of variable flux taps. Number of tap turns
will be determined based on whether they are for LV variation or for HV variation.
C. Mixed regulation taps
A portion of the taps act as constant flux taps and the remaining portion act as
variable flux taps. Many autotransformers are purchased as constant flux taps or
-6-
as variable flux taps but in service they are mostly used as mixed regulation taps.
2. Based on type of tap changer
A. DTC taps (de-energized taps).
B. LTC taps (on-load taps).
3. Based on connection
A. Linear taps.
B. Coarse/fine taps.
C. Reversing taps.
4. Based on electrical connection
A. In series winding (at common end).
B. In common winding (at neutral end).
C. In LV line.
D. In both series and common windings as shown in Figure 12.
-7-
STANDARDS
At present no standard recommends type of taps, the tap range, electrical
connection and the number of taps for autotransformers. The current ANSI
standard C57.12.10 suggests ±5% DTC taps in 2.5% steps in the HV winding
and ±10% LTC taps in 0.625% steps in the LV winding. But, the current
C57.12.10 covers only two winding transformers of certain MVA and voltage
ranges. Current C57.12.10 does not cover autotransformers. The IEEE
Transformers Committee is working on a revision of C57.12.10 to include
autotransformers and recommend, where possible, not to specify DTC taps.
In a considerable number of autotransformer specifications ±5% DTC taps in HV
and ±10% LTC taps in LV (similar to two winding transformers) are being
specified. In most cases, these DTC and LTC taps make the autotransformer’s
design complex and costly compared to two winding transformers. Design
complexities and relative costs are covered later in this tutorial.
Suggestions
1. IEEE Transformers Committee should soon publish Standards for
autotransformers. Since design and manufacturing problems in
autotransformers are more complex compared to two-winding transformers,
separate standards are required for autotransformers rather than including
them in the next version of C57.12.10.
2. Until the National Standards are revised, users should consult with
manufacturers when preparing their autotransformer specifications. Both
users and manufacturers will benefit from this.
3. Users should strongly consider eliminating DTC taps (in particular when the
LTC taps are needed) from their specifications. In many cases, the LTC tap
range can be increased to cover the DTC tap range.
4. Specifications should not restrict location of LTC taps (e.g. in LV line etc.).
Manufacturers should be given freedom to offer reliable and economical
autotransformers with LTC taps in other locations.
5. Some suggested modifications to C57.12.10 are:
(a) Include linear and coarse/fine LTC taps and recommend adopting them
instead of reversing taps where possible. Linear or coarse/fine taps
reduce life-cycle costs compared to reversing taps.
(b) Where possible, eliminate DTC taps to procure reliable and
economical autotransformers.
-8-
TAP LOCATIONS AND THEIR FUNCTIONS
An autotransformer’s output voltage changes depending on input voltage
fluctuations and/or on changes in regulation due to load variations. The function
of the LTC taps is to keep the output voltage constant at all the times by
compensating for these fluctuations. An autotransformer with LTC taps on HV
side and also LTC taps on LV side will compensate for both the above
fluctuations and also keep the volts per turn constant throughout the tap range.
To manufacture and maintain such autotransformers is expensive. So, it is
normal practice to specify LTC taps either on HV side (in series winding) or on
LV side (in common winding or in LV line). Understand from some users that
based on MVA rating, HV voltage, LV voltage etc., that having DTC taps on HV
with no LTC taps on LV or DTC taps on HV and LTC taps on LV will result in
savings in life-cycle costs in their systems.
1. DTC taps
DTC taps are normally located in the HV (series winding). The DTC tap
position is set prior to energizing the autotransformer. During operation of the
autotransformer, the DTC taps will neither compensate for fluctuations of the
HV system voltage nor will they compensate for regulation due to load
fluctuations. This is the main reason to extend the LTC tap range and not to
specify the DTC taps. As DTC taps are seldom specified on LV side, this is
not covered in this tutorial.
2. LTC taps in series winding
(a) Step-down operation
When LTC is varied to compensate for input voltage fluctuations then the
taps will act as constant flux taps. When LTC is varied to compensate for
regulation due to fluctuations in load then the taps will act as variable flux
taps.
(b) Step-up operation
When LTC is varied to compensate for regulation due to fluctuations in
load then the taps will act as constant flux taps. When LTC is varied to
compensate for fluctuations in input voltage then the taps will act as
variable flux taps.
3. LTC in common winding
Either for step-down operation or for step-up operation and also either to
compensate for input voltage fluctuations or to compensate for regulation
due to fluctuations in load, the taps will act as variable flux taps.
-9-
4. LTC in LV line
(a) Step-down operation
When LTC is varied to compensate for regulation due to fluctuations in
load then the taps will act as constant flux taps. When LTC is varied to
compensate for fluctuations in input voltage then the taps will act as
variable flux taps.
(b) Step-up operation
When LTC is varied to compensate for input voltage fluctuations then the
taps will act as constant flux taps. When LTC is varied to compensate for
regulation due to fluctuations in load then the taps will act as variable flux
taps.
Table 1 below summarizes the effects of LTC location on core flux.
LTC
Location
Operation
Series
Step-down
Compensate for Compensate for
Input Voltage Load Regulation
Core
Flux
constant
variable
variable
Step-up
almost
constant
LV Line
Step-down
variable
almost
constant
Step-up
constant
variable
Common
Step-down
variable
variable
Step-up
variable
variable
Table 1
-10-
Suggestions
1. Specifications should clearly state whether the taps are for LV variation
or for HV variation. If the user’s requirement is mixed regulation, which is
usually the case, then the specifications should clearly state the tap range for
constant flux taps and the tap range for variable flux taps. IEC standards give
some guidelines on mixed regulation taps.
2. Operating engineers must clearly understand the behaviors and operating
limitations of the autotransformer when the taps are operated as constant flux
taps and also when operated as variable flux taps. These behaviors effect the
following:
(a) Current division between the series and common windings.
(b) Core flux density.
(c) Impedance.
(d) Fault current.
(e) Sound level.
3. To reduce the severity during short circuit, taps in the common winding at the
neutral end is preferred over the taps in series winding or the taps in LV line.
The reason is, on autotransformers with the taps in series winding or the taps
in LV line, when a line to ground fault occurs from tap leads to ground or from
tap winding to ground or a ground fault in the tap changer then the shortcircuit current is very large as only the system impedance limits the shortcircuit current. Whereas if a similar fault occurs in an autotransformer with
taps in the common winding at neutral end, then the fault current is limited by
both system impedance and the common winding impedance.
4. Most of the specifications state that the system impedance is zero (or infinite
bus). System always has an impedance. With zero system impedance the
autotransformer will be over designed and the cost will be increased
unnecessarily.
-11-
DTC TAPS
Linear DTC taps can be either bridging type or selector type. The names bridging
and selector evolved based on the principle of operation of the tap changer.
Linear or bridging DTC taps can be used in the body of the main winding. When
DTC taps are used as a separate winding, then they will be normally of linear
type. DTC taps of a separate winding can be connected as reversing taps if the
number of turns in the tap winding needs to be reduced. Either two bridging type
or two selector type tap changers can be used to connect DTC taps as reversing
taps. Reversing taps can not be used in the body of the main winding, because
due to flux reversal high voltage could be generated within the winding.
Physical location of DTC taps
Some of the considerations in deciding the location of DTC taps are:
--voltage rating of the HV winding
--ratio of HV and LV voltages
--tap range
--current and voltage ratings of available DTC tap changers.
--Impedance at the extreme tap positions
--winding insulation level (BIL of the winding)
--short circuit forces
--eddy losses and
--autotransformer dimensions and weight.
Preferred locations for bridging and linear DTC taps in the body of main winding
are shown in the Figures 5 and 6. These figures also illustrate the principle of
operation of these types of tap changers with a typical center-fed series winding,
where the top and bottom parts of the winding are symmetrical paralleled
halves. In either case based on tap changer current rating and design
consideration one or two tap changers may be used. If one tap changer is used,
then the top and bottom tap leads are paralleled before connecting them to the
tap changer.
On transformers with low impedance it is difficult to have the DTC taps in body of
the main winding due to high short circuit forces. Additional difficulty for an
autotransformer is that the percent of DTC tap turns to series winding turns will
be large compared to the percent of DTC tap turns to HV winding turns of a twowinding transformer. In a two-winding transformer, when the tap range is 10% of
HV circuit, then the tap turns will be 10% of HV turns. However, in a 2-to-1 ratio
autotransformer, when the tap range is 10% of the HV circuit, then the tap turns
will still be 10% of the HV circuit (HV turns) but 20% of the series winding turns.
This makes it difficult to have DTC taps in the body of the series winding as
compared to having the DTC taps in the body of HV winding of a two-winding
transformer.
-12-
Figure 5: Bridging type DTC taps
Figure 6: Selector type DTC taps
-13-
The most commonly used physical locations of separate DTC winding are shown
in the Figures 7 and 8.
D
CV
SV
T
C
FIGURE 7: Separate DTC Winding between CV and SV
DTC
CV
SV
DTC
Figure 8: Separate Outside DTC Winding
DTC taps in HV
If DTC taps are placed at the ends of the series winding then selector type taps
(Figure 6) are preferred over bridging type taps. If bridging type taps are used at
the top and bottom of the series winding, then the tap gap will be very near to LV
line end. This makes it difficult to design the tap gap to withstand the high
impulse voltages that will appear across this gap.
Impedance on different tap positions of a 60/80/100MVA, 140/46/7.2KV
autotransformer with +/-3.7% DTC taps at the top and bottom of the series
winding and also at the middle of the series winding are shown in Table 2.
Tap 1 Maximum turns tap) =
Tap 3 (Rated tap)
=
Tap 5 (Minimum turns tap) =
Taps at ends
Taps in the middle
7.38
7.23
7.13
7.47
7.23
7.04
Table 2
-14-
Impedance swing across the tap range for taps in a separate winding is not
included as it is extensively covered in a CIGRE paper dated July 1973. When
the ratio of HV and LV voltages is drastically different from those stated in the
paper, then the impedance swing over the tap range will also be different. When
the ratio of HV and LV voltages is approaching 3 or higher, the variation in the
impedance over the tap range becomes similar to that in a two winding
transformer. The disadvantages and limitations of DTC taps are not included
because they are covered in the "TAPS" ”tutorial presented at IEEE
Transformers Committee Meetings in Las Vegas, Nevada on October 26, 2004.
When DTC taps are in the body of a series winding, axial amp-turn balance
between the series and common windings is recommended. If not, the winding
eddy losses due to the radial flux will be increased and the short circuit forces will
also be increased.
DTC taps in tertiary
Occasionally, some specifications call for DTC taps in the tertiary. Depending on
the tap range and short circuit forces, DTC taps can be in the body of the tertiary
winding. However, considering the reliability and cost, it is highly recommended
not to specify taps in the tertiary.
DTC taps in LV line
In 50 years of the authors’ experiences, they have come across only two
specifications for autotransformers (280MVA and 167MVA) where DTC taps
were specified in the LV line. For economic and reliability reasons, it is not
recommended to specify DTC taps in the LV line. These taps will have very few
turns and this makes it difficult to design a reliable autotransformer. High current
in the LV line will cause additional problems in designing of the tap winding and
in selecting a DTC tap changer. If system requirements need DTC taps in LV
line, users will benefit if they discuss with manufacturers at the time of writing the
specifications to achieve the same function by an alternate way of specifying the
taps.
Suggestion
Eliminating DTC taps will result in economical and more reliable
autotransformers. When the autotransformer has an LTC, then the LTC tap
range can be increased to cover the DTC tap range and DTC taps can be
eliminated.
-15-
LTC TAPS
In autotransformers LTC taps can be connected electrically in four different ways
as described on pages 16 and 17. Figures 9 to 18 show these arrangements for
linear, reversing and coarse/fine taps.
In an autotransformer whether the requirement is a three-phase tap changer or a
set of three single-phase tap changers is determined by several factors. Mainly
these factors are phase to phase voltage and current rating. Based on HV and
LV voltage ratings and MVA the cost difference between an autotransformer with
a three phase tap changer and an autotransformer with a set of three single
phase tap changers can vary approximately 3% to 10%. Therefore users should
be aware that specifying a particular electrical location of taps could significantly
increase the price of the autotransformer.
Electrical location of the tap changer together with the physical location of the tap
winding and the voltage ratio determine the impedance variation (impedance
swing) across the tap range. The CIGRE paper of July 1973 shows the variation
in impedance over the tap range for different LTC winding locations, electrical
connections and types of taps. When ratio of HV and LV voltages is different
from those shown in the paper then patterns of variations will change.
Figures 9 to 12, Linear taps
With presently available linear type tap changers a maximum of 21 steps can be
obtained. Advantages and disadvantages of linear taps in series winding or in
common winding or in LV line are same as that of reversing or coarse/fine taps in
these locations. Arrangement of taps in Figure12 can only be linear taps, in this
arrangement it is not possible to use reversing or coarse/fine taps.
Figure 12
During LTC operation HV total number of turns remains same. During step-down
operation when input voltage changes core flux density will change i.e. taps will
be variable flux taps. During step-down operation with constant input voltage
(constant HV voltage) when the tap changer is used to compensate for regulation
then the taps will be constant flux taps. Whereas during step-up operation when
LTC is used for variation in input voltage then the taps will be constant flux taps.
But during step-up operation with constant input voltage (constant LV voltage)
when LTC is used to compensate for regulation then the taps will be variable flux
taps.
In this arrangement except on extreme tap positions, in other tap positions some
taps will be in series winding and some taps will be in common winding. As such,
taps should be designed for maximum series current and also for maximum
common current. Often phase to phase voltage requires using three single
-16-
Series End
LV Line End
Neutral End
Series/Common
Figure 9
Figure 10
Figure 11
Figure 12
phase tap changers and this will increase the cost compared to a design with a
three-phase tap changer.
For core form autotransformers with this type of taps based on dielectric and
short circuit considerations, the tap winding will be mostly either between
common and series windings or over the series winding (outer most winding).
When LTC winding is between common and series windings, eddy losses in LTC
winding will be high as it is in the main leakage flux path. To reduce the eddy
losses in LTC winding, winding conductor dimensions or strand dimensions in
case of continuously transposed cable (CTC) should be carefully chosen. When
LTC is over series winding, during maximum turns position, series winding will be
in fairly high leakage flux path and attention should be given to reduce series
winding eddy losses and also to limit the hot spots temperatures in series
winding to safe limits.
In this arrangement LTC winding being at LV line end, unless LTC winding series
capacitance is high, connecting metal oxide arresters across LTC winding will be
a very economical solution. Use of shields between windings is another method
to improve impulse voltage distribution in LTC winding. With shields between
windings, unless the shield is brought-out, it is not possible to measure power
factor between windings for maintenance purpose. Special LTC winding designs
can be used to obtain a fairly uniform impulse voltage distribution in LTC winding.
Figures 13 & 16
Taps in series winding at LV line end or at the “auto” point is most widely used
arrangement. As these taps are at LV line end, many comments made for
Figure 12 are applicable to Figures 13 & 16 also. When LTC winding is between
common and series windings and when taps are for HV variation then impedance
over the tap range will be almost constant when ratio of HV and LV voltages is
around 2.
-17-
Figure 13
Figure 16
Figure 14
Figure 17
Figure 15
Figure 18
Note: -Reversing and coarse/fine arrangements are not suitable for the
arrangement of Figure 12 i.e. taps in both series and common windings.
-18-
Advantages of these taps are, current will be not normally high and there will be
enough tap turns to design a reliable tap winding.
With this arrangement either linear or coarse/fine or reversing taps can be used.
For step-down operation if the taps are used for HV variation then they will be
constant flux taps. For step-down operation with constant input voltage, if the
taps are used to compensate for regulation then they are variable flux taps. For
step-up operation if the taps are used for input voltage variation then they are
variable flux taps. For step-up operation with constant input voltage if the taps
are used to compensate for regulation then they are variable flux taps.
Figures 14 & 17
These taps are similar to the taps on the LV side of a two winding transformer.
As such, all operating limitations, advantages, disadvantages etc., applicable to
the taps on LV side of a two winding transformer are also applicable to
autotransformer with taps in LV line. Main disadvantages of taps in LV line are
high current and often not enough turns in tap winding to obtain an economical
design. Another disadvantage is often three single phase tap changers are
needed.
Figures 15 & 18
Main advantage of these arrangements is, a 3-phase Y connected tap changer
can be used. As common winding current will be flowing in the taps, normally a
high current LTC is not required. However, LTC current is determined based on
MVA, HV & LV voltages and their ratio. BIL level from taps to ground and also
between phases will be small. Main disadvantage of this arrangement is that the
autotransformer will be of variable flux design under all operating conditions.
User should consider variations in parameters of the autotransformer from tap to
tap in operating the autotransformer. In this arrangement linear, coarse/fine or
reversing taps can be used.
Compared to the arrangements of taps in series winding or in LV line, economics
depend on savings in tap changer cost, reduction in oil quantity due to less
clearances and increase in core and winding materials due to variable flux
design. Being a variable flux design, this could pose a problem if the tertiary is
brought-out and loaded. Because the tertiary voltage will be changing when LTC
taps are changing. One solution to keep the tertiary voltage constant is by
connecting a compensating transformer in the tertiary.
Suggestion
Considering the autotransformer cost and reliability, tap arrangements of
Figures 9,11,12,13,15,16 and 18 are preferable over the tap arrangement of
Figures 10,14 and 17. Users can obtain less costly and more reliable
autotransformers by specifying only the operating requirements and by giving the
freedom to manufacturers to decide the electrical location and type of taps.
-19-
DTC & LTC TAPS
The problems in autotransformers with DTC taps only and the problems in
autotransformers with LTC taps only were described in earlier sections. In
autotransformers with both DTC and LTC taps, the problems are compounded as
the problems of both DTC and LTC taps are added.
A considerable numbers of autotransformer specifications call for both DTC and
LTC taps. Typically DTC tap range is 65% of HV in HV for HV variation and LTC
tap range is 610% of LV in LV for LV variation. Understand from the users that
these taps are based on ANSI C57.12.10 standard. This standard is for two
winding transformers of certain range and not for autotransformers. Design and
manufacture of an autotransformer with DTC and LTC taps is more difficult and
expensive than a two-winding transformer with the same taps.
Different variations in DTC and LTC taps are given below.
(A) Based on electrical connections
(1) DTC taps in series winding for HV variation and LTC taps in series winding for
HV variation.
(2) DTC taps in series winding for HV variation and LTC taps in LV line for LV
variation.
(3) DTC taps in series winding for HV variation and LTC taps in common winding
for LV variation.
(B) Based on physical location
(1) DTC taps in the body of series winding and LTC taps between tertiary and
common winding.
(2) DTC taps in the body of series winding and LTC taps between common and
series windings.
(3) DTC taps in the body of series winding and LTC taps over the series winding.
(4) DTC taps between common and series windings and LTC taps between
tertiary and common windings.
(5) DTC taps over series winding and LTC taps between tertiary and common
windings.
(6) Both DTC and LTC taps between common and series windings.
(7) DTC taps over series winding and LTC taps between common and series
windings.
(C)
(1)
(2)
(3)
(4)
Based on type of taps
Linear DTC taps and linear LTC taps.
Linear DTC taps and reversing LTC taps.
Reversing DTC taps and linear LTC taps.
Reversing DTC taps and reversing LTC taps.
-20-
Typical DTC and LTC taps arrangements are shown in Figures 19A to F. In
arrangements of Figures 19A, B and C DTC taps should be of linear (bridging or
selector) type only; in these arrangements reversing type DTC taps can not be
used. In arrangement on Figure 19F LTC taps can be of linear or coarse/fine type
only; in this arrangement reversing type LTC taps can not be used.
Figure 19: Typical DTC and LTC taps arrangements
It is very important for users to know that even when there is no change in DTC
tap range and/or no change in LTC tap range; based on electrical connection,
physical location and type of taps (linear or reversing) the impedance at extreme
tap positions could change considerably.
Calculated impedances for different tap positions and for different tap windings
locations of a 90/150MVA 230/115/13.8KV autotransformer with 65% DTC taps
in 4 steps in HV for HV variation and 610% reversing LTC taps in 616 steps in LV
line for LV variation are shown in Table 3. DTC1 & LTC1 are maximum turns
positions, DTC3 and LTC17 are rated tap positions and DTC5 & LTC 33 are
minimum turns positions.
-21-
Calculated percent impedances on 90MVA base:
Figure 20 A
Figure 20 B
Figure 20 C
Figure 20 D
Figure 20 E
7.08
5.75
4.89
5.38
6.52
6.08
5.75
5.76
6.12
5.72
5.97
5.75
5.84
6.21
5.66
5.36
5.75
6.46
5.03
7.02
5.72
5.75
6.24
4.34
7.81
TV to LV
LTC1
LTC17
LTC33
21.38
23.99
27.63
17.06
19.11
21.97
15.68
17.54
20.16
4.87
4.68
4.72
6.17
5.95
5.99
TV to HV
DTC1
DTC3
DTC5
32.19
31.26
30.42
25.91
25.83
25.77
24.11
24.12
24.16
11.35
11.11
10.86
13.42
12.61
11.87
HV to LV
DTC1 – LTC1
DTC3 – LTC17
DTC5 – LTC33
DTC5 – LTC1
DTC1 – LTC33
Table 3
Due to the volume of work, impedances for all other combinations of DTC and
LTC taps are not shown. Aim is to bring to the attention of users that if the
impedance at rated tap is only stated in the specifications then most likely the
offers will be with the least cost combination of DTC and LTC taps. This could
give rude surprises of short circuit currents on extreme taps exceeding station
design limit and inadequate breaker capacity. If the impedance is high on an
extreme tap position then regulation suffers. In the specifications users must
clearly specify how they will use taps, maximum and minimum impedances on
any combination of DTC and LTC taps positions. Users should also aware of the
changes in impedances for step-up operation and for step-down operation.
During the design, manufactures must determine on which combination of DTC
and LTC taps the forces will be maximum and calculate the forces for that
combination. In some cases one combination of taps position may give maximum
forces in one winding and another combination of taps position give maximum
forces in another winding.
TV to LV and TV to HV impedances for winding arrangements in Figures 20A,
20B and 20C are much higher than those in Figures 20D and 20E. For winding
arrangements 20D and 20E most likely a tertiary reactor or some special design
method is required to design TV for a three phase fault on its terminals.
-22-
-23-
At the time of evaluating the tenders users should know the losses at rated tap
and also the maximum losses for any combination of taps position. This is very
important because variation in losses at different taps positions could be very
large. Only a few users are asking for this information with the tenders and using
it in evaluating the tenders. To give an example, when tap winding is the outer
most winding then on Tap1 eddy losses will be high as main windings will be in
the path of considerable leakage flux. Maximum oil, winding average and hot
spot rises may not occur at one combination of DTC and LTC taps position. If this
occurs at different combination of taps positions then manufacturer should
determine the maximum oil, winding and hotspot rises considering different
combination of taps positions.
Bringing-out the leads
Which side (LV side or HV side) the main windings leads (CV and SV windings
leads) and the tap winding leads are brought-out has to be carefully selected to
avoid well known half-turn problem. This problem is very serious in core type five
legged cores compared to core type three legged cores.
Suggestion:
Users should avoid specifying both DTC and LTC taps for autotransformers. If
required LTC tap range can be increased to cover DTC tap range. Users should
know the changes in impedance over the tap range between an autotransformer
with DTC and LTC taps and an autotransformer with extended LTC tap range
with no DTC taps. User should obtain cost difference between an
autotransformer with DTC & LTC taps and an autotransformer with extended
LTC tap range.
-24-
EFFECTS OF TAPS LOCATION ON DIELECTRIC DESIGN
In autotransformers taps can be located in any one of the following
arrangements.
1. In the body of series winding at the ends or in the middle.
2. Physically in a separate winding and electrically connected to the series
winding at the LV line end (at the auto point).
3. Physically in a separate winding and electrically at the neutral end.
4. Physically in a separate winding and electrically in the LV line.
1. Taps in the body of series winding at the ends or in the middle
Fig. 21A: Taps at series winding ends
Fig. 21B: Taps at series winding center
-25-
When the taps are at the top and bottom of the series winding as shown in
Figure 21A, the series winding will be a center-fed winding. This is by far, the
most widely used arrangement. In this arrangement the impulse voltage that
appears at the taps will be lower as the taps are at the series winding ends (away
from the HV line end). In this arrangement short-circuit forces are normally at
manageable limits as the taps are placed symmetrical to the center of the coil.
The taps will be electrically near to the LV line. The insulation level of the taps is
higher than the LV insulation level at the minimum turns position shown in Figure
21A. This is due to the overhang of the taps that effectively allows the ends of the
series winding to float. Normally these taps will be linear type of taps. If bridging
type taps are used in this arrangement then it is very difficult to insulate the axial
gap (tap gap) in the middle of the taps.
When the taps are in the middle of the series winding, as shown in Figure 22B, a
tap gap is required, whether the taps are linear type or bridging type. Since
these taps are closer to HV line end compared to the taps at top and bottom of
the series winding, they have to be designed for a higher insulation level than the
taps at the coil ends. Based on the type of winding, care is required in
determining the insulation level across the tap gap, across the tap range and
from the taps to the ground. Often RSG (Recurrent Surge Generator) test is done
to know the impulse voltages that appear across different points in the winding
and from winding to ground before the core & coils assembly is placed in the
tank.
For both the arrangements in Figures 21A & 21B, connecting metal-oxide
arresters (zinc oxide discs) across the taps provide a simple and effective
solution to control transient surges. Many autotransformers with metal-oxide
arresters across the taps are in trouble free operation in many utilities around the
world for many years. CIGRE had discussions on metal-oxide arresters and have
concluded that these devices increase reliability. Benefits in using metal-oxide
arresters are discussed in detail in a later section.
Reversing type of taps should not be used in the body of the main winding
because high voltages will be developed due to the magnetic field reversal.
2. Physically in a separate winding and electrically connected to the series
winding at the LV line end (at the auto point)
In this type of arrangement, in most cases, three single-phase tap changers are
required rather than one three-phase tap changer. This is because of the high
phase to phase voltage required in the tap changer.
-26-
Figure 22: Series Tap Winding
(a) Tap winding between the tertiary and the common windings
Figure 23: Tap Winding between TV and CV
Dielectrically, this arrangement is not preferred. The electrical stress to ground
will be high because the grounded neutral end of the common winding (HoXo) is
-27-
located between the two high potential coils (series and LTC windings). In this
arrangement special care is required in designing the end insulation on the
windings.
Another difficulty with this arrangement is that the high potential tap leads are
brought-out very close to the core clamps unless large end clearances are
provided. Often, specially molded insulation snouts are required for the leads.
Special care is also required to avoid undesirable hot spots on the tap leads.
Creep stress must be considered in the insulation design because the tap leads
are close to the core and the core clamps.
If this arrangement is used for reversing or coarse/fine taps, then the recovery
voltage when the tap changer goes through the neutral tap must be calculated. If
this recovery voltage exceeds tap changer’s limit then tie in resistors must be
connected to ensure that the recovery voltage is within the value of the tap
changer’s limit.
(b) Tap winding between the common and the series windings
Figure 24: Taps between CV and SV
This arrangement is more widely used. When HV line is placed at the center of
the series winding; then difference in potential among LV line lead, LTC leads
and the ends of the series winding will not be high. This simplifies the insulation
design of these leads when they are brought-out through the end blocks (leads
exiting above and below the coils).
By using coarse/fine taps instead of reversing taps, the radial inward forces on
the tap winding are eliminated. Modern vacuum type tap changers simplify the
design with coarse/fine taps by removing the historical requirement of minimum
mutual inductance between the coarse and the fine tap windings.
-28-
(c) Tap winding over the series winding (outer most winding)
Figure 25: Taps on outside
This arrangement is the simplest to bring-out the tap leads because they can be
accessed directly from the winding surface. However, this arrangement increases
the complexity for the HV line lead exit (bring-out) insulation because it has to
pass through the LTC winding. To reduce electrical stresses between HV line
lead and the tap winding, often stress rings and special collars are used in the
LTC winding at HV lead exit. A typical HV lead exit arrangement is shown in the
Figure 26. Due to the capacitance currents flowing from HV winding to LTC
winding, transient voltage distribution in the series winding could be less linear
when compared to other physical locations of the LTC winding.
The tap winding is almost always arranged in two parallel halves to reduce the
short-circuit forces in the windings. This allows HV line lead to exit between the
two halves of the tap winding. Due to this, there will be a large axial gap between
two halves of the tap winding. Either pressboard blocks or pressboard cylinder is
used to maintain this large gap. These blocks or the cylinder provides the needed
support to the HV line lead and avoids any movement during the transportation
and the life of the autotransformer. With this arrangement no additional support
like clamps are needed for the HV line lead. The tap winding should have
sufficient radial build so that it will have good mechanical stability.
In this arrangement, the recovery voltage when the tap changer passes through
the neutral tap is usually higher that the tap changer’s limit because of the high
series winding voltage and the small capacitance between the tap winding and
the tank. So, in this arrangement tie-in resistors are mostly provided in the LTC to
limit the recovery voltage to be below the tap changer’s limit.
-29-
Figure 26: Outside Tap Winding Insulation
3. Physically in a separate winding but electrically at the neutral end
Dielectrically, this arrangement is simple because the taps and tap-changer are
at the lowest possible insulation level. This makes it possible to use one threephase tap changer. The biggest disadvantage of this arrangement is that the taps
are variable flux taps. Cost reduction obtained by using one three phase tap
changer has to be compared with the increase in cost due to the variable flux
taps. The tap winding is normally located next to the core or between the tertiary
and the common windings. These are the most economical arrangements
because of the low insulation levels between the windings. However, bringing-out
-30-
the tap leads is not easy compared to when the tap winding is the outer most
winding. Large potential difference between the tap leads and the LV & HV line
leads and the tap leads and the CV & SV coils require increased clearances
when the tap leads are routed to the tap changer.
Figure 27
Figure 28
Tap winding next to core
Figure 29
Tap winding between TV and CV
-31-
The type of tap winding and the method to bring-out the tap leads should be
carefully chosen to avoid undesirable hot spots to occur in the core.
4. Physically in a separate winding but electrically in LV line
Many users specifies +/-10% LV taps but are not clear whether they accept LTC
electrically at neutral end in the common winding (Variable flux design) or their
requirement is only with LTC electrically connected in LV line. Compared to the
LTC in series winding at LV line end, LTC in LV line has increased complexity in
both dielectric and short circuit designs.
Figure 30: Taps in LV Line End
For taps in LV line, most usual locations for the LTC winding are as shown in
Figures 31 and 32. Arrangement in Figure 31, LTC winding between TV and CV
windings, is preferred as it makes design of the coils to withstand forces simpler.
This is because the impedance between TV & LV and between TV and HV will
be much higher than other winding arrangements and this lowers the short circuit
-32-
currents in the windings. However, dielectrically this arrangement is not preferred
as the electrical stress to ground will be high. This is because the grounded
neutral end of the common winding is located between the two high potential
coils (series and LTC windings). This arrangement poses two more difficulties
also. The first difficulty is that the high potential tap leads are brought-out very
close to the core clamps. The second difficulty is that the large current (LV line
current) flowing in the tap leads could produce undesirable hot spots in the core
and the core clamps.
Figure 31
Taps between TV & CV
Figure 32
Taps between CV & SV
Arrangement in Figure 32 makes dielectric design simpler. But this arrangement
makes short circuit design of coils very difficult. In this arrangement, often a
current limiting reactor is needed in the TV to obtain an economical design.
Due to the high currents in the LTC leads their area of cross-section is normally
large. This poses manufacturing difficulties, especially on large autotransformers.
Due to the high voltage between LTC leads and the ground, thicker insulation is
needed on these leads. This thick insulation makes it difficult to bend and in
routing them to the tap changer.
The use of metal-oxide arresters across the LV line LTC taps has a huge benefit
in controlling the surge voltage distribution in the LTC winding.
Suggestion
Life cycle costs of autotransformers can be minimized if the users consult
manufacturers while writing purchasing specifications. When there are no system
restrictions then user should only specify which circuit is regulated, tap range and
number of taps. The manufacturer can then best choose where the taps are to be
electrically connected and where the tap winding is to be physically located.
-33-
OTHER CONSIDERATIONS OF TAPS LOCATIONS
1. Taps in the Body of the Series Winding
This arrangement is cost effective because a separate winding can be
eliminated. In this arrangement axial short-circuit forces are high. As such, this
arrangement is limited to autotransformers with small ratings or high impedance.
Short circuit forces and eddy losses are high without the proper amp-turn
balance between the common and series windings. This arrangement is not
suitable for reversing taps. This arrangement is usually not adopted when the tap
range is large because the short-circuit forces become unmanageable. When
metal-oxide arresters are not permitted by the user, this arrangement is not
practical unless the HV BIL level is low. Sometimes it is difficult to bring-out the
tap leads from outside the coil. When LV to HV voltage ratio is more than 0.5 this
arrangement is mostly not possible to adopt because tap turns as percent of
series turns will be large.
2(a) Separate Tap Winding and electrically connected to the Series Winding
Location of the tap winding and ratio of LV to HV voltages determine the
impedance swing over the tap range. When the tap winding is between the
common and series windings, the gap between these windings is large. To obtain
needed impedance, either the volts per turn or windings heights has to be
increased. Usually, a multi-start type winding is used. Special winding designs
are often adopted to increase the dielectric strength within the tap winding. Since
the tap winding is in the main leakage flux path, eddy loss in the winding will be
high. Winding conductor must be carefully chosen to reduce eddy loss and hot
spots temperature. Normally the hoop stress in tap winding is high and often
requires expensive work-hardened conductor.
For a typical multi-start winding special attention is required on the winding pitch.
A large pitch makes it difficult to bring-out the tap leads. A multi-start winding with
a large pitch also causes the short-circuit forces to increase due to the large
difference in electrical heights of the windings. Large edge strips used to square
up the pitch are weak spots for short-circuit forces. Unless proper type of
winding is chosen for the taps, large amp-turns in the leads will produce
undesirable hot spots in other metal parts.
When the tap winding is the outer most winding, the coil may be a multi-start
winding, tapped helical winding or a disc winding depending on factors such as
the number of taps, turns per tap and the maximum current in the winding. Eddy
losses are low because the winding is outside of the main leakage flux field. This
arrangement makes it difficult to bring the HV line lead out through the outer tap
winding. Insulation design within the tap winding is complicated and costly
especially when the HV line bushing lead is connected at the center of the series
winding. In that case, a large axial gap is required in the tap winding and special
care is needed to maintain the mechanical stability of the tap winding.
-34-
Arrangement of tap winding between core and common winding or between
tertiary and common winding is not discussed as this arrangement is mostly not
used due to many design problems. This arrangement will be used only when a
particular swing in impedance over the tap-range in required by the user.
(b) Taps in Common Winding
Taps are variable flux taps whether they are for LV variation or for HV variation.
Variable flux taps may or may not make the design costly compared to that with
constant flux taps. It depends on whether the savings on the insulation and the
tap-changer can compensate for the increased core size and increased CV and
SV coils diameters. With variable flux taps, it is not possible to obtain tap steps
with equal voltages; in some cases the difference can be minor. In an
autotransformer with a tertiary, a compensating transformer is required to
maintain a constant tertiary voltage.
The primary advantage of this arrangement is that the taps require only an
insulation level slightly higher than the neutral, rather than the insulation level of
the HV or LV as in other LTC arrangements. The voltage between the taps of
each phase is small; so a three-phase tap-changer is normally used, provided
the current rating is adequate. The insulation level to ground is also low
resulting in a much less expensive tap-changer. The current in the tap leads and
the tap-changer is much lower compared to taps in LV line.
When the taps are for LV variation, placing the tap winding between the tertiary
and the common windings minimizes the impedance swing across the tap range.
This arrangement results in low eddy losses in the tap winding. A simple tapped
helical winding is often used to reduce cost and complexity. TV to LV and TV to
HV impedances are normally large resulting in much lower fault currents.
When the tap winding is placed between the common and the series windings,
the LV to TV impedance is small; resulting in much higher short-circuit currents
specially in TV. This arrangement often requires a current limiting reactor in the
TV circuit to limit the short-circuit currents in TV. This will increase TV to LV and
TV to HV impedances resulting in much lower fault currents. A tapped helical
winding is not practical because of unacceptable hotspot temperatures on the tap
leads as they are in the main leakage field.
Placing the tap winding as the outermost winding is not practical because of cost
and complexity.
(c) Taps in LV line
When the taps in LV line are used for LV voltage variation in step-up operation or
to compensate for regulation in step-down operation then they are constant flux
taps. These taps pose many design problems and a few are listed below.
-35-
--Current in the taps will be large compared to the taps in series winding or taps
in common winding.
--The number of taps and turns per tap often forces the use of an uneconomical
design.
--Winding arrangements which give least variation in impedance over the tap
range is not preferred due to the complications in dielectric design.
--Winding arrangements, which are simple in dielectric design, are more
expensive and pose higher eddy losses.
--Taps being at LV line BIL, in majority of designs single phase tap changers are
to be used to meet clearances from phase to phase.
-36-
EFFECTS OF TAPS ON STEP-DOWN AND STEP-UP OPERATIONS
Table 4 below summaries when the taps are constant flux taps or variable flux
taps based on operation. This is extremely important to users because the
autotransformer parameters change drastically based on operation. The users
should not forget that the autotransformer will be manufactured to meet the
specifications only and users are solely responsible how it is used.
Tap Location
Operation
Constant
Voltage
Varying
Voltage
Core
Flux
Series
Step-down
HV
LV
variable
LV
HV
constant
HV
LV
constant
LV
HV
variable
HV
LV
constant
LV
HV
variable
HV
LV
variable
LV
HV
constant
HV
LV
variable
LV
HV
variable
HV
LV
variable
LV
HV
variable
Step-up
LV Line
Step-down
Step-up
Common
Step-down
Step-up
Table 4
During operation of the autotransformer when the taps become variable flux taps,
users should be aware of the ramifications on impedance, sound level and
tertiary voltage. This is due to the changing volts per turn with each tap position.
--Flux density in the core is directly proportional to volts per turn.
--Impedance across the tap range shifts, with a greater effect towards one
extreme tap position.
-37-
--The sound level generated by the core varies greatly from one end of the tap
range to the other end.
--Tertiary voltage at no-load will change for each tap position.
--Figures 35 to 36 show that during step-up operation of an autotransformer with
arithmetic loading the common current increases when the tertiary loading is
increased from zero. For an autotransformer with taps in common winding this
has to be considered in determining the maximum current in the taps. For an
autotransformer with vectorial loading the common current magnitude also
depends on power factors of HV and TV loadings.
--Figures 33 and 34 show that during step-down operation of an autotransformer
with arithmetic loading the common current decreases when the tertiary loading
is increased from zero.
Figure 33
Figure 35
Figure 34
Figure 36
-38-
Suggestion
Autotransformers are designed by manufacturers to meet the specifications.
They are not designed to meet system requirements. Autotransformer design
engineers are not system engineers. As such, when preparing a specification, a
meeting among system planners, system operators, user maintenance personnel
and autotransformer manufacturing designers is beneficial to all.
-39-
EFFECTS OF HV OR LV TAPS ON TERTIATY
Very rarely taps are specified in tertiary. Authors have not come across any
specification calling LTC taps in tertiary. Authors have designed a couple of
autotransformers with DTC taps in tertiary. As these taps do not keep the TV
voltage constant during operation, it is suggested not to specify taps in tertiary.
Cost of an autotransformer with taps in TV is expensive due to complications in
tertiary winding design, costly tap changers or link board and much higher than
the normal short-circuit currents.
When an autotransformer with taps in HV or LV is operated as variable flux taps,
the tertiary voltage changes. Tertiary voltage is based on volts per turn and the
changes in volts per turn on each tap position. Fault current in the tertiary varies
based on the HV to TV, LV to TV and HV to LV impedances at each tap position.
1. Buried tertiary
A buried tertiary is one in which the terminals are not brought-out but buried
inside the tank. Some specifications call for one corner of TV delta to be broughtout. A few specifications call that two terminals of the tertiary have to be broughtout and connected together outside the tank. As the effects of HV or LV taps are
same on all the above cases, all these are considered as buried tertiary. By
definition a single-phase autotransformer can not have a buried tertiary because
the delta must be made externally. But when this TV is not loaded effects of HV
or LV taps will be same as in buried tertiary of a three-phase autotransformer.
Whether the taps are in series winding or in common winding or in LV line, when
they are variable flux taps then tertiary voltage will be changing from tap position
to tap position. Normally this does not pose much of a problem in buried tertiary
winding design. Tertiary voltage will be different at different tap positions based
on volts/turn on that tap position. If a current limiting reactor is used in the buried
tertiary then for short circuit current calculations, current limiting reactor
impedance has to be recalculated based on the tertiary current at that tap
position.
Though the tertiary is buried, for different tap positions over the complete tap
range fault currents in tertiary should be calculated to determine the maximum
fault current and then the tertiary should be designed for this maximum fault
current. For three phase autotransformers tertiary fault currents have to be
calculated for both single line to ground fault on HV and also for single line to
ground fault on LV. In single-phase autotransformers the tertiary fault current has
to be calculated for three-phase fault on tertiary also.
-40-
2. Tertiary brought-out
Variable flux taps cause the tertiary no-load voltage to vary with each tap
position. Tertiary no-load voltage can be maintained constant to some extent by
connecting a compensating transformer in the tertiary. Even in an
autotransformer with constant flux taps tertiary voltage will be changing due to
in-put voltage fluctuations and/or due to regulation (change in voltage the core
sees due to voltage drop because of the autotransformer HV to LV impedance
and the load on the autotransformer).
Reactors or capacitors connected to the tertiary bus, must be designed to
withstand the fluctuations in the tertiary voltage.
3. Effects of physical location of HV or LV tap winding on TV impedances
Normal arrangement of windings in an autotransformer from core is tertiary,
common and series windings. If the tap winding is located between the tertiary
and common windings, then the impedance between the TV and LV is normally
large. This helps to limit the fault current in the tertiary to an economically
manageable level. If the tap winding is between the common and series windings
or outside the series winding, then the impedance between TV and LV is
normally small. This causes large fault currents to flow in the tertiary. A
commonly used method to control the fault currents is to connect a current
limiting reactor in the tertiary winding to increase TV to LV and TV to HV
impedances.
-41-
ON_LOAD TAP CHANGERS
1. Taps in the Common Winding (Figure 11)
This arrangement allows the selection of a less costly tap changer compared to
any other location of taps. Since the taps are at the neutral end, a three-phase
wye-connected tap changer can be used as opposed to three single-phase tap
changers. Current in the taps is lower compared to the taps in LV line. Insulation
levels of the tap changer between phases and also to ground are lower
compared to all other tap arrangements.
2. Taps in the LV Line (Figure 10)
Usually, the use of three single-phase tap changers is required due to the
insulation levels between phases. The current in taps is much higher compared
to taps in series or common windings. The above increases the autotransformer
cost considerably compared to taps in the common or series windings. Tank size
and oil quantity are also larger increasing the cost further. This arrangement has
no redeeming characteristics and should be used when taps in other locations do
not meet system requirements.
3. Taps in the Series Winding (Figure 9)
This has all the disadvantages as taps in LV line, except that the current rating of
tap changer required is much lower. Currents in tap leads are also much lower
compared to taps in LV line. Due to these reasons this makes a much
economical design compared to taps in LV line.
4. Taps in Series and Common Windings (Figure 12)
Only a linear tap changer can be used in this arrangement. Thus, the number of
taps is limited to a much lower value compared to reversing taps or coarse/fine
taps. All the disadvantages of taps in series winding are also applicable to this
arrangement. Current rating of the tap changer should be selected considering
the maximum current in the series winding and also the maximum current in the
common winding for all tap positions.
5. Specifications
Most specifications from users in North America call for reversing taps. In many
cases, the same performance can be achieved by using coarse/fine taps. A
design with coarse/fine taps often yields lower life-cycle costs because of lower
load loss for taps below rated voltage. Presently available vacuum type tap
changers make the coarse and fine tap windings design simple, as there is no
requirement of designing these windings with needed impedance between them.
Most specifications from users in North America also specify that the main bid
must have no exceptions and that any alternate bid must be accompanied with
explanations of how it will benefit the user. In general, the design of an
-42-
autotransformer takes much more time than the design of a two-winding
transformer. Usually, the designer does not have enough time to do a main
design and an alternative nor can the manufacturer afford the cost of quoting two
complex designs. When an alternate design is quoted, users seldom consider it.
Users should give financial incentives to manufacturers to come up with
economical alternative designs.
Functional specification brings greater rewards to users than a rigid specification.
Users should consider different bids with different designs based on performance
and evaluated cost. For economic benefits to both users and manufacturers,
specifications should allow alternate bids without a main bid. North American
specifications should give a free hand to manufacturers to adopt existing on load
tap changers to achieve overall performance and economics. Consultants of
North American users should accept and encourage the designs successfully
used in other countries.
-43-
OVERLOADS AND LIFE CYCLE COSTS
Most specifications have some form of the following statement:
All transformers, including accessories and load tap-changing
equipment shall be capable of overloading in accordance with
ANSI C57.91. Overloading shall be limited only by the core and
coils, not by any accessories such as bushings, tap-changers, tank or
clamping structure. Leads should be designed to not be a limiting factor.
The above statement is inadequate to design the autotransformer for overloads.
Specifications must include the information per C57.91, clause 9.7, Loading
Information for Specifications. This includes overload profiles (magnitudes and
durations), ambient temperatures and limiting temperatures. Without this
information it is not possible for the manufacturer to select ancillary equipment,
and also design the autotransformer to meet the overloads required by the user.
In many specifications, there is no correlation among overloads, impedance and
the tap range. Often the tap range is too small to compensate for regulation and
maintain the output voltage during the overloads. Users must realize that when
the tap range is specified, the autotransformer is designed for that tap range only
and the users bare the responsibility for its adequacy. A few specifications do not
specify the tap range, location of taps etc, but instead give all of the operating
conditions. The autotransformer designer determines the tap range, location of
taps etc.
In some specifications, the overloads and ambient temperatures specified are so
high that the autotransformer is actually designed as if it has a higher capacity
and this reflects in the cost, but not on the nameplate rating. Users should be
sure that what they are specifying is really what is needed, and be aware of how
much extra it is costing them. It is a good idea for users to ask for a price adder
to meet the overloads. Before specifying the overloads, users should determine
the cost implications.
A considerable number of specifications ask for RCBN (Reduced Capacity Below
Normal) taps, which is a good cost saving idea. Unfortunately, they do not state
that the overloads are to be pro-rated accordingly for these taps, which then
negates the savings.
Often, an ambient temperature of +40ºC is specified throughout the overloads
duration. This makes the autotransformer very expensive. Users should
investigate the statistical maximum temperatures expected for the region and, if
applicable, for the time of the day. To obtain savings users should specify
ambient temperatures at different times of the day (say, each hour of the day).
-44-
TAP WINDINGS
Most of the tap windings in autotransformers are of three types,
--tapped helix.
-- multi-start.
-- disc.
1. Tapped helix
A tapped helix winding can be located either inside of the common winding or as
the outer most winding (outside of the series winding), where the leakage flux is
low. To minimize the short circuit forces, this winding is built in two axial halves.
This ensures the loaded portion of the tap winding remains symmetrical about
the electrical center of the main windings. A tapped helix winding is not used
between the common and series windings because of the large leakage flux
between these windings. This causes dangerously high hot spots at the joints
where the taps are formed on the coil’s face.
One advantage of a tapped helix winding compared to a disc winding is that taps
can be accessed at every turn (one turn between the taps) also, allowing greater
design flexibility. Another advantage of this winding compared to a multi-start
winding is that good tension on the winding can be maintained by anchoring the
taps and also the turns at which the taps are made.
When tapped helix winding is used as the inside winding nearer or next to the
core, some radial space is required between this winding and the next outer
winding to bring-out the tap leads. This is because the tap leads come-out from
face of the coil. Often an eccentric duct is used to bring the tap leads out. By
using the eccentric duct radial space can be reduced and this results in cost
savings. This was described in detail with a sketch in “TAPS” tutorial presented
at IEEE Transformer Committee Meetings in Las Vegas on October 26, 2004.
In tapped helix winding only two tap leads carry current at any given time. As
these currents flow in opposite directions as shown in Figure 37, this winding can
be safely placed next to the core without any risk of hot-spots being generated in
the core or in the clamping structure.
Tapped helix winding is mostly used when the taps are at the neutral end or
when the insulation level at the taps is low. This is because the series
capacitance of the winding is low, resulting in a non-uniform voltage distribution
in the winding. This winding is handy to adopt when the taps are in LV line,
because for taps in LV line often the number of turns per tap is small. When this
winding is used for taps in LV line, special care is required in dielectric design of
the autotransformer. This winding does not produce significant end thrust during
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Figure 37
faults because the pitch of the winding is small compared to that in a multi-start
winding.
2. Multi-start
This winding can be placed either,
--between the tertiary and the common windings,
--between the common and series windings, or
--outside of the series winding.
This winding is very useful if the tap winding has to be placed between the
common and series windings, where the leakage flux is high. This is possible
because there are no joints in the winding to cause dangerous hot-spots. When
this winding is used as the outer most winding, some special method is required
to keep the winding tight.
There are many variations of multi-start winding; each type is suitable to a given
application. Near extreme tap positions, (when most of the taps are carrying the
current) total amp-turns of the leads will induce high currents in the core and
other parts that could generate unacceptable hot-spots. Based on the physical
location of the multi-start, by adopting a variation of multi-start winding the ill
effects of unacceptable hot-spots can be eliminated.
Since the series capacitance of this winding is very high, it can be used at high
insulation levels such as in series winding or in LV line. The turn pitch is usually
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large in this type of winding; special precaution is needed to reduce the end
thrust during the faults.
3. Disc
A disc winding is normally used when the number of turns per tap is large and
also when the tap winding is the outer most winding. To reduce axial force during
faults this winding is built in two halves equally placed from the electrical center
of the main windings, similar to the tapped helix winding. This type of winding is
usually not placed as an inside winding because there is almost no benefit to do
so. Placing this type of winding as outer most winding usually brings-in economic
advantages as common and series winding diameters are reduced. Since there
are many turns in the winding, it can be comfortably clamped to withstand forces
during faults.
When the taps are in the body of the main winding and the main winding is the
outer most winding then by adopting the main winding as disc winding it is easy
to bring out the taps. Main advantage of this winding is that the taps can be
placed at ends on the main winding or at the middle of the main winding or at
other parts of the main winding.
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METAL-OXIDE ARRESTERS AND STATIC SHIELDS
1. Metal-oxide arresters
Metal-oxide arresters fall under the class of non-linear resistors. They have the
same technology used in modern day lightning arresters. When the taps exposed
to the high dielectric stresses (taps in series winding or taps in LV line) the use of
metal-oxide arresters across the taps provides an economical and simple method
of transient voltage control. Large numbers of autotransformers with metal-oxide
arresters connected across the tap windings are in trouble-free service
throughout the world.
Metal-oxide arresters allow the use of less expensive tap changers. One
example is given here. Presently available reactor type tap changer is with a
maximum rating of 400KV BIL. A considerable numbers of specifications with
taps in LV line have a voltage rating of 69KV with 350KV BIL insulation level.
Connection of metal-oxide arresters across the taps allows the use of this tap
changer. If the user does not allow use of metal-oxide arresters across the taps,
then less reliable type of windings or complicated arrangements are needed to
use this tap changer.
Some specifications state that internal non-linear resistors are not allowed. The
user needs to understand that this is based on one or two bad experiences and
likely from the old technology of silicon carbide arresters. If the application and
method of connection on non-linear resistors are done correctly, then they do not
cause any problems in service and survive the life of the autotransformer. In
tender and design review meetings users and their consultants should
investigate these practices and the design methodology including operating
voltage, switching surge and impulse voltages criteria.
When metal-oxide arresters are not allowed, special types of windings or special
arrangements must be used to control the impulse voltage both across the tap
winding and also from the tap winding to ground. Users and their consultants
should investigate the reliability and cost of these special tap windings and
special arrangements verses the use of metal-oxide arresters.
A few years back, CIGRE concluded that the use of zinc oxide discs (metal-oxide
arresters) improves the reliability of the transformers. Authors have experience
with these devices as users and also as designers. Many autotransformers with
zinc oxide discs inside the tank designed by the authors and also purchased by
the authors are operating satisfactorily for many years in USA and in Canada.
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2. Static shields
Static shields are conducting cylinders that are placed either inside the winding,
outside the winding or both inside and outside the winding. The shields are
connected to the tap winding. These shields prevent ground currents flowing in to
the tap winding. This reduces the tap winding to ground capacitance almost to
zero, making the impulse voltage distribution in tap winding almost linear. Use of
static shields will increase the cost of the autotransformers. Special precautions
are needed in the manufacture of static shields to avoid partial discharges,
especially from the top and bottom ends of the static shields and also at the
connection of the braid. An external connection (outside the tank) to the shield is
required if condition-based monitoring of the autotransformer based on power
factor is desired. To the knowledge of the authors there have been more failures
in the field due to the static shields than there have been with metal-oxide
arresters.
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FUNCTION OF TAPS STATED IN SPECIFICATIONS
VERSES THEIR OPERATION IN THE SYSTEM
Users want that autotransformer output voltage to be constant all the time in the
service. If no corrective action is taken, the output voltage varies whenever the
input voltage fluctuates. The output voltage also varies due to regulation when
the load varies.
1. Taps in series
Almost 100% of the specifications state that taps in series are constant flux taps
i.e. keeps LV voltage constant for HV voltage variation when the autotransformer
is operated as a step-down unit. During step-up operation these taps are
constant flux taps when they are used to keep HV voltage constant for load
variations i.e. to compensate for regulation.
In reality in any station input voltage will be varying and also the load will also be
varying. It is a must for users to know that autotransformers are designed to meet
the specifications only. Users must not take it for granted that the
autotransformers are designed to meet all operating situations even when they
are not specified in the specifications. It is users’ responsibility to write the
specifications of autotransformers such that they meet all the operating
conditions. When users have consultants, then the consultants should ensure
that the specifications cover all operating conditions. It is essential to have a
detailed tender review meeting between the manufacturer and the user with the
consultant present. It is authors experience as users that a tender review
meeting is much more important than a design review meeting.
Many specifications state that autotransformers with taps in series winding
should be suitable to operate for both step-down and step-up operations. But
these specifications do not say that the taps in series winding will be used for HV
voltage fluctuation during step-down operation and to compensate for regulation
during step-up operation. When users purchase autotransformers with constant
flux taps and before using them as variable flux taps they must be fully aware
what effects it will have on the autotransformers. At the time of calling for tenders
users must ensure that their specifications meet their operating requirements.
Users must have a tender review meeting to examine that the tendered
autotransformer meets their requirements. Design review meetings are
conducted after completing the production design. Often it is very difficult to
make changes at this stage without affecting cost and delivery. So, a tender
review meeting is much more essential than a design review meeting.
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2. Taps in LV line
These taps will be constant flux taps when they are used to compensate for
regulation during step-down operation. During step-up operation these taps will
be constant flux taps when they are used for LV voltage fluctuations. Other
discussions for taps in series stated above are applicable to taps in LV line also.
3. Taps in common
Consider an autotransformer specification with taps in common for HV voltage
variation during step-down operation. Though these taps are variable flux taps
when they are used to compensate for regulation during step-down operation
then changes in autotransformer parameters will be different at different tap
positions compared to the parameters on these tap positions when they are used
for HV voltage variation. Users should be aware of these changes in the
parameters to operate the autotransformers properly.
Suggestions
---A tender review meeting is much more important than a design review
meeting. Users with their consultants present should have a detailed tender
review meetings with manufacturers before placing orders.
---There are many different ways to solve a problem. Consultants employed
by users should not impose changes to design and manufacturing methods of
a manufacturer simply because they are not coinciding to the consultants’
experiences. All parties should encourage innovations and developments, or
else advancement in design and manufacture will cease.
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CONCLUSIONS
1. Taps in autotransformers will have more effect on design complications
compared to taps in two winding transformers.
2. Impedance swing over the tap range depends on ratio of HV to LV voltages,
physical location of the tap winding and its electrical connection to the main
winding.
3. It is strongly recommended not to specify DTC taps in autotransformers for
the following reasons.
(a) During operation DTC taps neither regulates for input voltage
fluctuation nor for regulation.
(b) ± 5% of HV DTC taps are ±10% of the series winding turns in a 2 to 1
ratio autotransformer. As such, there are more design complications
compared to a two winding transformer.
(c) Generally, DTC taps are near the LV line end and this makes the
dielectric design of the DTC winding (or DTC discs when the DTC taps
are in the body of the main winding) difficult and costly.
(d) In many cases, it is not possible to use a bridging type tap changer.
A linear type tap changer is more expensive compared to a bridging
type tap changer.
4. For LTC taps in the series winding or in the LV line, single-phase tap
changers are usually required. These tap changers are costly compared to
neutral end tap changers.
5. For taps in the series winding or in the LV line use of metal-oxide arresters
provide an effective, reliable and economical solution.
6. Taps in the common winding permit the use of a three-phase tap changer but
the autotransformer is of a variable flux design.
7. The IEEE Transformers Committee should conduct a work shop on how to
determine the requirement of taps considering the characteristics such as type
of taps (DTC or LTC), the tap range, the location of the taps (in series winding,
in common winding or in LV line) etc.
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8. To avoid problems due to large currents and small number of turns per tap,
taps in the series winding or taps at neutral end (in common winding) is
recommended over the taps in LV line.
9. To avoid design problems and a higher cost specifying both DTC and LTC
taps is not recommended. It is preferable to avoid DTC taps. If required, LTC
tap range can be extended to eliminate the DTC taps.
10. A tender review meeting is more essential than a design review meeting.
11. Specifications should be functional. All parties in the transformer field (users,
consultants and manufacturers) should encourage innovations and
developments.
12. During preparation of purchasing specifications, both users and
manufacturers benefit from the discussions to finalize the details of taps.
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Acknowledgements:
1. Mr. Frank David of FD Consulting Services, Winnipeg, Manitoba, Canada
for his guidance and for valuable suggestions.
2. Mr. Peter Franzen of Manitoba Hydro, Winnipeg, Manitoba, Canada for
drawing all the figures in the tutorial and also for correcting the draft to
reflect all the technical discussions frankly without any bias.
3. Mr. Bernhard Kurth of Reinhausen Manufacturing Inc., Humboldt,
Tennessee, USA for steering to the correct usage of tap-changers and for
constructive suggestions.
4. Mr. Shivananda Prabhu, Retired Professor of Electrical Engineering,
Ryerson University, Toronto, Canada for pain takingly rewriting the draft
such that a person with a little exposure to autotransformers can follow the
tutorial.
5. Hydro One Networks, Toronto, Ontario, Canada.
6. CG Power Systems Canada Inc, Winnipeg, Manitoba, Canada.
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For comments/clarifications on this tutorial contact
Dr. Tomasz Kalicki
E-mail: tomasz.kalicki@HydroOne.com
Tel: 416-345-6111
Or
Vallamkonda Sankar
E-mail: powertransformer@hotmail.com
Tel: 905-634-5926