1
Dimensional Standards for Bushings Applied to
Liquid Filled Power Transformers and Reactors
and why they are Important to your Electric
Power System
Keith P. Ellis, Member, IEEE/PES
Abstract:--This paper will outline the history of IEEE
Standard Performance Characteristics and Dimensions for
Outdoor Apparatus Bushing, C57.19.01-2000 and how the input
from US utility groups brought these standards to their present
form. This will include how these efforts to standardize bushing
ratings and dimensions are having an impact on increasing the
reliability of electrical systems in the United States. It will include
comparisons between the requirements of the IEC Bushing
Standard 60137 Insulated Bushings for Alternating Voltages
Above 1000 V, used in Latin America, and the IEEE C57.19
bushing standards. It will offer scenarios that can be considered
for harmonizing existing requirements for IEC bushings and the
IEEE dimensional bushings. Recommendations will be offered to
insure that your new power transformers or reactors will
conform to your requirement for bushing interchangeability. In
the conclusions, the impact of adding dimensional requirements
to your bushing requirements will discussed in detail.
The only other alternative the Utility had was to purchase
spare bushings with each new liquid filled power transformer
or reactor. This action increased the purchase price of new
equipment, increased their physical inventory of spare
bushings, increased warehousing space and increased
maintenance requirements for this inventory. After all of this
extra effort many Utilities discovered that when the need for a
specific spare bushing arose, locating the correct bushing style
was a challenge and many times, when located, the spare
bushing was not suitable for service.
II. HISTORY OF DIMENSIONAL STANDARDS
The first bushing dimensional standard was developed by
the American National Standards Instituted (ANSI) in the late
1950s and added these requirements to ANSI C76.1 – 1943.
In 1968 the ANSI C76.1 Committee decided to divide the
Index Terms--Bushings, Dimensions, IEC, IEEE, Power bushing standard into three parts. The ratings and dimensions
standard became ANSI 76.2. In 1977 ANSI C76.2 – 1977 /
Transformers, Reactors.
IEEE Std. 24 added duel ratings (Example 1200/1600 amps)
for transformer (1200 A) and circuit breakers (1600 A), By
I. INTRODUCTION
1991 the bushing standards were purely IEEE Standards with
ITH nearly 50 years of real life experience using
the Ratings and Dimensions standards becoming IEEE Std.
ANSI/IEEE bushing interchangeable standards, utilities
C57.19.01 – 1991.
in the United States have found that following these
standards have reduced their spare bushing inventories,
In 1991 the IEEE, PES, Transformers Committee, Bushing
shortened the lead times for replacement bushings and lowered
Subcommittee began work to revise the 1991 document. When
the overall cost of supporting the reliability of their liquid
this work began feedback from EEI (Edison Electrical
filled power transformers and reactors.
Institute), OEMs (original equipment manufacturers) and other
users indicated that a greater degree of standardization was
Those Utilities around the world that utilize the IEC
needs in bushing ratings. With this mandate from the Industry,
bushing standards, which do not provide dimensional
the PC57.19.01 Working Group began a nine year effort to
requirements, have struggled with the issue of bushing
meet the goals set by this feedback. The result of this work is
interchangeability. Each bushing supplier has their own unique
IEEE Standard Performance Characteristics and Dimensions
dimensional profiles for their bushings, preventing
for Outdoor Apparatus Bushing, C57.19.01 – 2000.
interchangeability with other brands. If the Utility adopted a
unique dimensional standard for their bushings they soon
found the lead-times for those bushings increasing and in
many cases, the cost escalating.
W
Keith P. Ellis is the Bushing Product Manager, Americas for the Trench
Bushing Group (e-mail: Keithcota@aol.com)
978-1-4244-2218-0/08/$25.00 ©IEEE
2
The most significant and sometimes most controversial
As you review the Routine, Design and Special tests
change to C57.19.01 was the reduction of the “standard” required for each standard you will note that they are very
bushing voltage class ratings. 15, 25, 46, 115 and 161 kV close regarding the required tests.
classes were removed. This left 34.5, 69, 138, 230, 345, 500
and 765 kV classes. The most controversial of these reductions
TABLE II: ROUTINE TESTS REQUIREMENTS
was removing the 25 and 115 kV classes. The Working Group Test
IEEE
IEC
agreed to include the electrical characteristics of those bushing
Internal Pressure
Yes
Yes
deleted from the revised standard in the annex of C57.19.01 –
Capacitance
Yes
Yes
2000. The table A.1 in Annex A lists electrical ratings for
Power
Factor
Yes
Yes
those bushing classes removed from the standard.
Tao Withstand (Test or Voltage
Yes
Yes
CT pocket lengths were standardized with two lengths, 533
mm for bushings though 69 kV class and 584 mm for all
bushings above 69 kV class This action greatly reduced the
number of bushing designs,
Dry Power-Frequency Withstand
Yes
Yes
Partial Discharge Measurement
Yes
Yes
Tightness Test of the Flange
No
Yes
Dry Lightning Impulse > 850 kV BIL
No
Yes
Another major change to this document was removing
bushings for oil circuit breakers. In 1993 the last new oil
circuit breaker was produced in the United States. Since
standards are for new equipment only, there was no longer a
requirement to include this type of bushing application in
C57.19.01.
Verify Nameplate Markings
Yes
No
III. BUSHING STANDARDS COMPARISON
It is clear that totally adopting all IEEE bushing standards
may not be a practical approach to adding “standardization” to
your bushing requirements. However, before incorporating the
dimensional requirements of the IEEE bushing standards into
your liquid filled power transformers and reactors
specifications it is important to compare the IEEE bushing
group of standards to the IEC bushing standard you now use.
The following tables will highlight the similarities and
differences for your consideration:
TABLE III: DESIGN TESTS REQUIREMENTS
Test
Internal Pressure
Mounting Flange
IEC
Yes
Yes
Draw-Lead Cap Pressure
Yes
No
Cantilever
Yes
Yes
Capacitance
Yes
Yes
Power Factor
Yes
Yes
Tap Withstand (test or Voltage)
Yes
No
Full Wave Lightning Impulse
Yes
Yes
Wet Withstand Voltage 230 kV & Below
Yes
Yes
Wet Switching Impulse Voltage > 300 kV
Yes
Yes
Temperature
Yes
Yes
Thermal Stability
No
Yes
Verify Nameplate Markings
Yes
No
TABLE IV: SPECIAL TEST REQUIREMENTS
TABLE I: MAJOR DIMENSIONS CONTROLLED
Dimensions
IEEE
IEEE
IEC
Test
IEEE
IEC
Yes
No
Thermal Stability
Yes
No
Yes
No
Length Below the Flange
Yes
No
Front-of-Wave Impulse
CT Pocket Length
Yes
No
Seismic Withstand
Yes
Yes
Top Terminal
Yes
No
Artificial Pollution
No
Yes
Bottom Terminal
Yes
No
Voltage Tap
Yes
Yes
Test Tap
No
No
Special Tests are agreed on between the purchaser and
manufacturer at time of order.
Optional bushing accessories are not listed as available
The dimensions controlled by IEEE C57.19.01 are the within the IEEE standards. The bushing manufacturers do
critical dimensions in providing bushing interchangeability.
offer options such as different top terminal plating, none,
silver or tin. Within the IEC standards a few optional
accessories are available. Besides the accessories listed below
items such as arcing horns are offered for IEC bushings. For
the most part, manufacturers of either IEEE or IEC bushings
can offer accessories to meet most customer requirements.
3
TABLE VII: VOLTAGE CLASSES & BIL RATINGS
TABLE V: ACCESSORIES
Standard
IEEE
IEC
Yes
Yes
Test Tap Above 72 kV
No
Yes
Voltage Tap Above 72 kV
Yes
No
No
Yes
Test Tap 72 kV and Below
Optional
Voltage Tap Above 72 kV
An option that is very common for all bushings is the altitude
rating and the creep distance on the outdoor insulator.
All bushings are designed to operate at rated voltage up to
1,000 meters. For operation above 1,000 meters the bushings
design must electrically de-rated or be modified by increasing
the arcing distance from live part (Metal top part of the
bushing) and ground (The bushing’s mounting flange). The
actual arcing distance required to increase the altitude rating is
subject to the specific characteristics of a particular design.
Today, most IEEE bushing manufacturers in North America
are standardizing on ratings up to 3,000 meters for new
designs.
The creep level is arrived at differently between the IEEE
and IEC standards. IEC levels are based on the highest system
voltage (Um), while IEEE levels are based on the nominal line
to ground voltage of the bushing. The result of using either
method is very small, usually only a few mm in favor of the
IEC method. Today many new IEEE bushing designs being
developed are with creep levels that comply with the Heavy
level of creep listed in C57.19.100, IEEE Guide for
Application of Power Apparatus Bushings.
If high pollution levels are a concern consider selecting a
Silicone Rubber Insulator (SRI) instead of porcelain. SRI is
proven to perform much better in heavy pollution then
porcelain.
Table VI provides the formulas for calculating both IEEE
and IEC standard creepage levels for bushings.
115 kV (IEEE)
123 kV
138 kV (IEEE)
145 kV
161 kV (IEEE)
170 kV
230 kV (IEEE)
245 kV
300 kV
300/362 kV
345 kV (IEEE)
362/420 kV
420/550 kV
500 kV (IEEE)
550 kV
TABLE VI: CREEP DISTANCES
Contamination Level
kV Class (Um)
3.6 kV
7.2 kV
12 kV
15 kV (IEEE)
17.5 kV
24 kV
25 kV (IEEE)
34.5 kV (IEEE)
36 kV
46 kV (IEEE)
52 kV
69 kV (IEEE)
72.5 kV
100 kV
IEEE
IEC
Light
28 mm/kV
16 mm/kV
Medium
35 mm/kV
20 mm/kV
Heavy
44 mm/kV
25 mm/kV
Extra Heavy
54 mm/kV
31 mm/kV
Basic insulation levels are one of the reasons bushing
interchangeability with IEC designed bushings can be
frustrating. The following table may show why this is a
possible issue:
765 kV (IEEE)
800 kV
kV BIL
40
60
75
110
95
125
150
200
170
250
250
350
325
380
450
550
450
550
650
450
550
650
750
550
650
750
900
650
750
850
950
1050
850
950
950
1050
1175
1050
1175
1175
1300
1425
1675
1300
1425
1550
2050
1425
1550
1800
1950
2100
2400
IEEE
No
No
No
Yes
No
No
Yes
Yes
No
Yes
No
Yes
Yes
No
No
Yes
No
No
Yes
No
No
No
Yes
No
No
No
Yes
No
No
No
No
Yes
No
No
No
No
Yes
No
No
No
No
No
Yes
No
No
No
Yes
No
No
No
No
No
No
IEC
Yes
Yes
Yes
No
Yes
Yes
No
No
Yes
No
Yes
No
No
Yes
Yes
No
Yes
Yes
No
Yes
Yes
Yes
No
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
4
The following rules of thumb that are used in applying
bushings to liquid filled power transformers and reactors may
assist in reducing the number of different bushing design that
will be required for each specific utility system:
1.
2.
3.
4.
5.
The bushing’s BIL must be equal to or greater than the
BIL of the winding.
Higher BIL bushings will not necessarily increase the
altitude capability of a specific bushing.
Higher creep levels will not allow bushings to operation at
higher altitudes.
For the best pollution abatement performance, select
bushings with SRI.
For the highest Seismic requirements always specify
bushings with SRI.
TABLE VIII: IEEE VOLTAGE CLASSES & BIL RATINGS
Max. Line to
Max.
Ground
System
1 min. dry
Voltage Class & BIL
Rating kV
Voltage kV
rms kV
34.5 kV - 200 kV BIL
22
38
80
69 kV - 350 kV BIL
44
76
160
138 kV - 650 kV BIL
88
152
310
138 kV - 650 kV BIL
102*
176
310
230 kV - 900 kV BIL
146
252
425
230 kV - 1050 kV BIL
156*
270
460
345 kV - 1175 kV BIL
220
381
520
500 kV - 1675 kV BIL
318
550
750
765 kV - 2050 kV BIL
485
840
920
* These rating are available upon request
The information contained in this section is not intended to
IEC 60137 does not standardize on nor recommend current
promote the use of either IEC or IEEE standards for your ratings for bushing. IEC does provide current rating limits for
bushings but rather to allow you to evaluate applying IEEE the cantilever test requirements and most IEC bushing
dimensional standards to IEC standard bushings.
manufacturers have designed bushings with current ratings
inline with those listed in IEC 60137 for the cantilever test. .
IV. HARMONIZING IEC AND IEEE
BUSHING STANDARDS
IEEE C57.19.01 – 2000 has specific standard current
5000 A
3000 A
2000 A
1200 A
When selecting an IEEE Standard bushing rating for your
system you need to select a bushing with a maximum line to
ground (phase-to-earth) rating that will meet or exceed your
maximum system voltage (Um) as defined in IEC 60137,
clause 4. Once this is determined you can select an IEEE
standard bushing rating from Table VIII.
800 A
400 A
ratings and therefore, all the IEEE standard bushing
In section III it was made clear that, for the most part, IEC manufacturers have designed their standard bushings with
and IEEE standards are fairly compatible with each other. The those current ratings found in Table IX:
largest difference is in the large number of available BIL
levels for IEC standard bushings. This wide range of BIL
TABLE IX: IEEE STANDARD CURRENT RATINGS
levels is not readily available on IEEE standard bushings.
Fortunately, the IEEE standard BIL ratings are generally equal
to the highest BIL level for a given IEC voltage class.
Voltage Class
34.5 kV
X
X
X
X
69 kV
X
X
X
X
138 kV
X
X
X
X
230 kV
X
X
X
X
345 kV
X
X
X
X
500 kV
X
X
X
X
765 kV
X
X
X
X
X
Table VIII contains two ratings that are not listed in IEEE
C57.19.01 – 2000. These are the 138 kV Class bushing with a
Current ratings of 400 and 800 amps are for draw lead
102 kV line to ground rating (located in Annex A) and a 1050
cable applications. These rating can be higher if the liquid
kV BIL, 230 kV bushing with a 156 kV line to ground rating.
filled power transformer or reactor manufacturer insulates the
These ratings are available from most IEEE standard bushing
cable with Class F, high temperature insulation. Bushings with
manufacturers in North America.
current ratings of 1200, 2000 and 3000 amps can be supplied
with removable draw lead conductors for ease of installation
and removal. Consult with the bushing manufacturer for
available deigns.
Bushings that require current ratings higher then those
listed above are available from all IEEE bushing
manufacturers with a high degree of interchangeability
between brands.
5
V. BENEFITS OF ACHIEVING
INTERCHANGEABILILTY
Utilities in the United States have been enjoying the
benefits of bushing standardization for more than 40 years. In
the early days the immediate benefit was bushing
interchangeability between liquid filled power transformers
and reactors and bulk oil circuit breakers. By specifying a
Transformer/Breaker Interchangeable (TBI) bushing they
could apply the same spare bushing inventory to support liquid
filled power transformers and reactors as well as their bulk oil
circuit breakers.
It is important to note that the above cost information
applies when comparing standard IEEE standard designed
bushings with special IEEE standard designed bushings. IEEE
standard bushings have specific dimensional requirements and
mandated features. These requirements dictates a higher cost
then a typical IEC standard bushing, which does not mandate
those specific dimensions or required features. This difference
could be between 10% and 30% per bushing. When you
consider that bushings represent less then 5% of the value of
the liquid filled power transformers or reactors, adding 10 to
30% to price of each bushing will hardly be noticed in a
competitive market when purchasing new liquid filled power
With bushing standardization the overall spare bushing transformers or reactors.
inventory is reduced. In the United States the bushing
inventory incurs a larger tax burden from accruing bushing VI. ACHIEVING BUSHING INTERCHANGEABILITY
costs in years 2 to 5. This real cost was a major reason
C57.19.01 – 2000 reduced the number of standard bushings
The easiest way to achieve bushing interchangeability
being offered from 56 to 21 designs while increasing the would be to just specify IEEE C57.19.01 – 2000 for the
current ratings from 1200 amperes to 3,000 amperes across the dimensional requirements for bushings in your liquid filled
board and adding a 5,000 ampere design at 34.5 kV. If your power transformers or reactors specification. This would
utility does not experience this type of taxation there are still certainly accomplish this objective. However, before you do
other tangible savings from standardization.
this you need to consider some of the minor items that could
present issues for your operation.
With fewer spare bushings in inventory the costs of
warehousing these bushings is reduced. These savings extend
The first item would be the bushing’s top terminal stud.
to lowering the expense of periodic inventory testing as well The IEEE standards specify the use of a top terminal stud that
as the size of the warehouse itself.
utilizes threads. In most of the rest of the World threads are
not used for the bushing top terminal stud, they use a smooth
The real benefits come when the spare bushing is required. terminal stud. This difference may present a logistic issue in
With a standardized bushings inventory, that is well obtaining line terminal in your region that utilizes threads.
maintained, the outage time for your liquid filled power Most bushing manufacturers that produce IEEE standard
transformers or reactors is assured to be as short as possible. bushings can easily supply the IEEE standard bushing with a
Compare this with your own experience trying to locate a smooth top terminal stud.
serviceable spare for many of your IEC standard bushings.
Dimensionally interchangeable bushings increases system
The next item would be the color of the outdoor
reliability and minimize customer irritation.
insulator. Since the late 1960s the United States government
has required that insulators used by Utilities be sky gray. This
IEEE standard bushings have the shortest standard lead- was the results of beatification efforts inspired by Lady Bird
times in the World. Manufacturers of IEEE standard bushings Johnson, wife of then President Lyndon B. Johnson. Since
can generally ship these bushings in eight weeks or less on then all IEEE standard bushings have been delivered with sky
average. Many maintain small revolving inventories that can gray porcelain as the standard offering. Brown porcelain has
ship in less then one week, even at 500 kV. This is certainly always been available but at a small added cost and a long
not the case with IEC standard bushings. Considering that lead-time. Those Utilities in Latin America and Mexico that
these bushings have thousands of possible dimensional the one have adopted IEEE standard dimensional requirements have
you may require could take six months or more to deliver.
accepted sky gray porcelain or sky gray SRI for their new
liquid filled power transformers and reactors.
Additional benefits include lower liquid filled power
transformer and reactor costs. (Standard bushings designs cost
much less then special bushing designs) The cost of spare
bushings is reduced as well when replacing the spare bushing
once it is used to replace an old bushing.
6
Since there is not a great deal of difference between the
between IEC and IEEE bushing testing this area should not
present an issue. However, one item under tests that should be
discussed is the lightning impulse testing. IEC requires that all
bushings with a Basic Insulation Level (BIL) greater then 850
kV must receive a routine impulse test. This is not a
requirement within IEEE. This issue has been discussed by
IEEE, PES, Transformers Committee, Bushing Subcommittee
and the consciences of the Group was that since the bushings
would receive the impulse tests on the new liquid filled power
transformer or reactor, that adding this test to the routine tests
were not needed. In addition, the frequency of actual impulse
test failures of the bushings at transformer OEM’s was
extremely rare. The conclusion was that the added cost of
100% impulse testing at the bushing factory could not be
justified by the extremely low rate of failures during
transformer factory testing.
The following scenarios for specifying dimensional
bushing requirements for your liquid filled power transformers
and reactors specification are offered:
A. All bushings shall be capacitance graded and shall be
designed to incorporate the dimensional requirements as
stated in IEEE C57.19.01 – 2000 for the specific voltage
class specified. The outdoor insulator shall be sky gray.
The top terminal of the bushing shall be a smooth copper
alloy stud of amble metric diameter to carry the full rated
current as stated on the bushing’s nameplate. The top
terminal shall be silver plated. All bushings rated above
345 kV class shall be subjected to a routine impulse test
by the bushing manufacturer.
B. Another scenario that could be considered is to make it
clear that it is desired that the electrical performance of
the bushing shall comply with the requirements of IEC
However, this author offers the following commentary on
60137 and then state the requirements in scenario number
this issue for your consideration:
1. To address the 1050 kV BIL rating for the 230 kV class
IEEE bushing you could state that for your 245 kV system
A. At voltage levels above 345 kV class, given the high cost
a BIL rating of 1050 kV is required at 156 kV line-toof these bushings and potential delays caused by a test
ground.
failure at the transformer OEM, it is recommended that
the bushings receive a routine impulse test by the bushing C. Many Utilities in the United States are very specific
manufacturer.
regarding the bushings they will accept on their new
B. Bushings purchased as a spare do not receive a factory
liquid filled power transformers and reactors by listing the
impulse test. It is recommended that when purchasing
specific catalog numbers of each bushing class and brand
spare bushings with BIL ratings above 850 kV BIL, that
they have approved. Table X is an example of what is
they receive a routine impulse test at the bushing factory.
found in some specifications.
C. Bushings below 900 kV BIL have an inherently high
TABLE X: TYPICAL US UTILITY BUSHING SPECIFICATION
reliability record, therefore paying the high cost of a
routine impulse test is not justified.
Nominal
One item that we have not discussed yet is bushing
technologies. Today, there are two major bushing technologies
being utilized around the World. The oldest of these is Oil
Impregnated Paper (OIP) and the newer Epoxy-Resin
Impregnated Paper (ERIP). Both of these technologies are
available in IEEE standard dimensions according to C57.19.01
– 2000. This will allow each Utility to select the technology
that best meets their needs.
System
Voltage
kV
BIL
kV
Current
Rating
Amps
ABB
34.5
200
1200
034W0412AP
3000
034W3000BC
1200
069W0412AN
2000
069W2000UH
800
138W0800AA
3000
138W3000UA
2000
550W2000UT
69
138
500
350
650
1675
Trench
200-H015-21-AG3-01AYS
200-F030-21-AG3-01AYS
350-H014-21-AG3-01AYS
350-F020-21-AG3-01AYS
650-H014-23-AG3-01A
650-F030-23-AG3-01AEP
1675-F020-23-AG3-01AFP
Each Utility must evaluate if other requirements are
required to meet the needs of their electric power system.
7
X. BIOGRAPHEY
VII. CONCLUSIONS
Experience has proven that standardizing the dimensions
and features of bushings for liquid filled power transformers
and reactors has a positive impact on the performance of
electric power systems. It reduces overall operating expenses,
decreases system outage time and improves customer
satisfaction. Dealing with fewer bushing styles increases
reliability as employees rapidly gain a high degree expertise in
dealing with the fewer styles of bushings with standard
dimensions and features.
Implementing bushing standardization within a utility will
require support from Management and it is hoped that this
paper will provide the support to accomplish this cost saving
program for the utilities in Latin America.
VIII. REFERENCES
Standards
[1] IEEE Standard General Requirements and Test Procedure for Outdoor
Power Apparatus Bushings, IEEE Standard C57.19.00 – 2004
[2] IEEE Standard Performance Characteristics and Dimensions for
Outdoor Apparatus Bushings, IEEE Standard C57.19.01 – 2000
[3] IEEE Guide for Application of Power Apparatus Bushings, IEEE
Standard C57.19.100 – 1995 R2000
[4] International Electrotechnical Commission, Insulated Bushings for
Alternating Voltages Above 1000 V, IEC Standard 60137
Papers Presented at Conferences
[5]
Revision of IEEE Standard C57.19.01, Doble Paper, 2003 BIIT4, by
Mark Rivers, Doble Engineering Company
IX. ACKNOWLEDGEMENTS
I wish to thank my employer, Trench Ltd for supporting my
efforts in writing this paper.
Keith P. Ellis is a 16 year member of IEEE, PES,
Transformers Committee, past Working Group
Chairman for C57.19.00, active in all Bushing
Subcommittee Tasks Forces and Working
Groups and is a Working Group member of
IEEE 693, Seismic Substations Standards. He
graduated from Mare Island Navel Shipyard with
a
journeyman
certificate
in
Machine
Technology. Attended the University of
California, where he majored in Mechanical
Engineering.
After serving with distinctions in the US Navy during the Vietnam War, he
continued his education at the University of Wisconsin, Milwaukee. He
joined RTE-ASEA’s engineering group at the onset of this new joint venture.
While with RTE-ASEA, he also worked as an application engineer in
Marketing and was then promoted to field sales for RTE and RTE-ASEA in
Upstate New York. After nine successful years in field sales he returned to
Waukesha to start up the OEM Components sales operation for ASEA, as
Sales and Marketing Manager. When the Waukesha transformer operation
was sold he was promoted to Sales Manager for the new ABB Power T & D
Company’s Components Division. He was then hired by Trench Limited to
develop a line of IEEE/ANSI Standard bushings for the US market. Once the
new bushings went into production in Canada he was promoted to Bushing
Product Manager, Americas, for the worldwide Trench Bushing Group. He
takes particular interest in component applications to power transformers
with special interest in high voltage bushings and on-load tap changers.