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GENERATOR BUSHING MAINTENANCE, DESIGNS & RENOVATION.
BY E.R. PERRY AND ROSALIA TORRES, POLYTECH SERVICES
PRESENTED AT CARILEC ENGINEERS & SUPPLY CHAIN CONFERENCE
JULY 23-25, 2002
There are many new companies and personnel entering the
generation business today. Without some background knowledge in
certain of the technologies (such as bushing design and
maintenance), it is sometimes difficult to determine when a piece
of equipment is in need of maintenance, or near the end of its
life. Generator bushings are a mystery to most operating engineers.
They seldom require maintenance, and each type and design of a
generator bushing has a different set of parameters that must be
evaluated. Yet, a generator bushing can often be the weak link in
the chain that can cause an outage if not properly maintained.
A number of generator bushing types exist, and in each type
category, there is a multitude of design variations. Once you
understand the basics of a generator bushing, the function of each
part, then it is easier to evaluate the bushing to determine its
suitability for continuing duty. In most generator bushings, there
is a commonality of design and function. Learning about these
similarities makes it easier to determine the need for repair or
replacement if it is required.
General Bushing Design
All generator porcelain bushings are spring loaded to compensate
for the variation in thermal expansion of the conductor and
porcelain. The outboard end of the conductor usually has a
nonferrous collar screwed onto the threaded conductor. The collar
is screwed down to compress springs located beneath the threaded
collar. A second loose fitting collar is located between the
springs and the outboard end of the porcelain to hold the springs
in place. Normally, the outboard end of a generator bushing is
actually vented to the atmosphere except when filled with
asphalt.
The inboard end of the bushing has a fixed (usually brazed)
round metal plate approximately the diameter of the porcelain to
anchor that end of the porcelain to the conductor. Gaskets are
placed on both ends of the bushing porcelain to protect the
porcelain from the metal collars and to even the pressure over the
surface of the
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porcelain. On the inboard end, the gasket acts to form a gas
seal between the generator and the bushing. The inboard gasket is
the most important in preventing leakage through the bushing, as
the outboard end is not truly sealed.
With all the pieces in place, the threaded collar is screwed
down on the conductor, placing pressure on the multiple springs and
partially compressing the springs. This allows the conductor to
expand and contract due to heating and cooling. This arrangement
maintains pressure on the porcelain and gaskets to assure a good
seal at the inboard end of the bushing. In larger bushings, the
collar is screwed down on the conductor until contact is made with
the set of springs under the collar. The springs are too strong for
compressing with the threaded collar. There are individual
setscrews in the collar to allow the high pressure springs to be
compressed individually.
BUSHING TYPES
The more common types of generator bushings are:
Simple Hollow Porcelain Bushings- These bushings are insulated
by only an air space between the conductor and the porcelain. Used
primarily on small generating units, not hydrogen cooled. Many of
these are old, dating back for 40-60 years. The most common problem
encountered with these bushings are gaskets that have deteriorated
with time and heat. Almost all of these bushings require
replacement of the neat cement between the flange and the
porcelain. Years of heat and vibration have deteriorated the
strength of the neat cement.
Asphalt Filled Bushings- As generator units became larger; it
was desirable to transfer the heat from the conductor to the
porcelain along the entire length of the outboard porcelain,
thereby reducing the temperature at the connectors. This was
accomplished by filling the bushing with an asphalt material. The
asphalt also became a liquid seal when hot, to prevent hydrogen
leakage Contrary to popular belief, the asphalt does not play a
part in providing dielectric integrity to the bushing. The asphalt
is there primarily for heat transfer.
Most problems with these bushings become apparent by visual
inspection. Asphalt leaking from the bushing is easily observed.
This is not always a danger sign, but should be checked carefully.
Asphalt leakage is usually caused by old gaskets shrinking, or the
bushing has become overheated for various reasons. Asphalt leakage
is a danger signal and should be checked for the cause.
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When overheating has occurred, pressure inside of the bushing
builds up sufficiently to overcome the pressure of the compression
springs and the asphalt leaks from the bushing. The primary concern
here is to determine why the bushing is overheating. The most
common cause of overheating with this type of bushing is a loose
connector, or a connector that is not making sufficient area
contact.
Most of these old bushings also suffer from deteriorated neat
cement beneath the flange.
Forced Hydrogen Cooled Bushings- The hydrogen-cooled bushings
are constructed similar to the Simple and Asphalt Filled bushings.
They are slightly more complex internally, having internal piping
to carry hydrogen around the conductor for cooling. There are three
chambers inside the bushing.
The inner chamber is a hollow pipe in the center of the bushing,
inside of the hollow conductor. The hydrogen is directed under
pressure inside of the inboard end of the hollow pipe. The pipe has
exit holes near the outboard end of the bushing, but still internal
of the bushing, where the hydrogen flow is reversed. The hydrogen
flows back down along the outside of the pipe and in contact with
the internal diameter of the copper conductor, where it is
extracted at the inboard end of the bushing and cooled outside of
the bushing assembly.
As the hydrogen passes along the inside of the conductor, it is
extracting a majority of the heat generated in the bushing
conductor. Some of the remaining heat generated by the conductor is
conducted by the outside diameter of the conductor in contact with
a thin layer of asphalt between the conductor and the inside of the
porcelain housing. This heat is then dissipated through the
porcelain to the outside air.
In this design, there can be more causes for asphalt leakage due
to over heating of the bushing than in the previously discussed
designs. As in the simpler designs, the asphalt is still being used
primarily as a heat conductor and not as a dielectric. Overheating
and the resultant asphalt leakage can still be caused by loose
connectors.
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There is an additional possibility of trouble in this design.
Since the bushing is mounted nearly vertical and the outboard end
of the bushing is pointed down, there is a possibility that oil or
oil vapor can accumulate in the internal outboard end of the
bushing. If sufficient oil accumulates in the bushing, it will plug
the holes in the pipe inside the bushing and stop the cooling
hydrogen flow. The external symptom of this happening is
overheating and asphalt running out the end of the bushing.
Sometimes hydrogen cooled bushing with a clogged hydrogen
passage can be repaired without removing the bushing from the
generator. This is accomplished by removing a plug in the end of
the outboard end of the bushing end cap and drilling a hole through
the end cap to the inside chamber of the bushing. This will allow
the oil that has accumulated in the hydrogen-cooling chamber to
drain out of the bushing, and allow the free flow of the hydrogen
to resume. The small pipe plug can be reinstalled, sealing the
bushing. This procedure only works if the bushing has not been
overheated long enough to solidify the oil in the cooling
passageway. If the oil has solidified, the bushing should be
removed and renovated by trained personnel.
Solid Cast Epoxy Bushings-These bushings are simplicity in
design. The solid cast epoxy bushings utilize a cast epoxy flange
and insulating body as one continuous material.
It has the advantage of placing the body in direct contact with
the conductor for better heat dissipation. It also eliminates the
compression springs necessary with porcelain, and the need for
gaskets to seal around the conductor. There is no neat cement
required to attach the flange to the bushing insulating body. While
many of the problems associated with porcelain, insulated bushings
have been eliminated, other problems have occurred.
There are plasticisers in epoxy to give it flexibility. The
plasticicers tends to dissipate with time and temperature. This
results in the epoxy becoming brittle. The epoxy can no longer
follow the thermal expansion and contraction of the copper
conductor, resulting in either leakage between the conductor
and
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epoxy or cracks appearing in the epoxy. Some problems have been
encountered when tightening down the epoxy flanges onto uneven
surfaces, resulting in cracking the epoxy flange.
The epoxy dielectrics are adequate, but the heat transfer in
epoxy is marginal, resulting in rapid aging of the epoxy.
GASKETS AND NEAT CEMENT
There are a large number of variations in the details of the
above bushing designs. Some designs replace the gasketing on the
inboard end of the bushing with a thin shell of copper, soldered to
the conductor and than to a metalized area on the porcelain. This
has proven to be an excellent seal, but the thin copper shell is
susceptible to damage in handling, as the thin copper is kept thin
by design in order to follow the difference in thermal expansion
and contraction between the copper and the porcelain.
Older generator bushings utilized cork gasketing that ages
rapidly and has a tendency to shrink and crack with age. The cork
was later replaced with a combination of cork and neoprene referred
to as a corkprene gasket. The advantage of this material as a
gasket was less prone to cracking with age, and could still be used
without containment of the gasket, such as on a flat surface. The
material has been improved with time and is still often used
today.
More recently, and especially with hydrogen cooled machines, the
tendency has been to use a Buna-N material as it has less porosity
to hydrogen than the other gasket materials. The Buna-N material
has good resilience for proper conforming under pressure to match
the adjoining surfaces. This results in an excellent seal. It is
necessary to contain the Buna-N gasket as it will cold flow under
pressure unless contained by placing it in a groove or contained by
multiple bolts, such as found with a flat mounting flange.
The neat cement located between the bushing flange and the
porcelain is a gasket of sorts. The purpose of the cement is to
form a leak proof seal, but it also forms a mechanical bond for
support of the porcelain to the mounting flange. The neat cement
has the unique property of forming a strong mechanical bond while
at the same time providing sufficient resilience to compensate for
the different thermal expansion and contraction between the metal
and porcelain.
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Older bushings need to have the neat cement inspected for
leakage and deterioration of mechanical strength. Years of heat and
vibration while installed in a generator tend to pulverize the neat
cement, causing it to loose its sealing and mechanical properties.
It also appears there was less quality control in the old days and
this often resulted in large voids internal to the neat cement. It
is possible the voids were caused by a chemical reaction when the
neat cement was hydrolyzed. These void are not apparent from the
surface, but lie internally. On older bushings, it is desirable to
have the neat cement removed and either replaced with more neat
cement or a new material such as a polymer ceramic material
presently available. The polymer ceramic material has greater
resilience and mechanical strength than the older neat cements, and
no voids due to chemical reactions.
PORCELAINS
The porcelain materials, used as the principal insulation of a
generator bushing, varies widely in strength and configuration. The
older bushings did not have the clay formulations available today
to provide the mechanical strength of modern porcelains. Better
clays combined with alumina in modern porcelains and more precise
processing resulted in added strength and better uniformity to
porcelains.
Until recently, broken porcelains could not be repaired. If a
generator bushing suffered a thermal crack or a broken skirt, the
bushings had to be replaced. New materials and new processing
procedures have made porcelain repairs more commonplace. It is now
possible to repair or replace broken sections of porcelain with a
polymer ceramic material. The unique characteristics of a polymer
ceramic are that it can match the thermal expansion and contraction
of the porcelain, and is stronger mechanically and electrically
than porcelain. The polymer ceramics are formed chemically and do
not require firing or heat. This eliminates the hazards of
subjecting the original porcelain to possible thermal cracking.
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Broken porcelain skirts or chips are replaced in the field or
factory. It is a simple process of cleaning the broken area with
diamond abrasives. The porcelain surface is then prepared
mechanically, and treated chemically for maximum adherence of the
polymer ceramic material. A metal mold is field or factory
constructed and applied to the broken area to shape the polymer
ceramic to conform to the porcelain surface. The polymer ceramic
material is mixed and poured into the mold. After a few minutes to
allow the polymer ceramics to cure, the mold is striped from the
repaired area. The surface of the new area is coated with a
fluorourethane material to match the porcelain glaze in color and
gloss. The repair of a typical broken skirt takes approximately 2-4
hours in the field.
Often porcelains are damaged in the field due to excessive
thermal stress or hydrogen explosions. These damages can be quite
severe. Repair of severely damaged porcelains can only be made in
the factory with proper equipment available. Fortunately, these
repairs can be accomplished rapidly, usually within one to two
days. The repaired sections are of equal or greater mechanical and
electrical strength than the original porcelain.
DIELECTRICS & TESTING
Generator bushings are generally large diameter compared to
their equivalent bushings in substation equipment. This is a result
of the lower voltage and higher currents required by the generator
bushings. It is seldom a problem to meet the dielectric
requirements of a generator bushing, as there is more than ample
room between the conductor and the mounting flange to provide
adequate dielectric strength. Most dielectric tests (such as the
Doble power factor test) are used to detect cracks in the
porcelain, rather than to determine the adequacy of the bushings
dielectric strength.
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Materials such as the asphalts used in generator bushings are
more to dissipate the heat away from the conductor and to the
porcelain, than to provide dielectric strength. Simple Megger tests
in the field and or a 60Hz Hi-Pot test in the factory or field is
generally adequate. Checking partial discharge during the 60Hz
Hi-Pot test is desirable to ascertain the adequacy of the design
and dielectrics of the bushing.
NEW BUSHING DESIGN & MATERIAL
Within the last few years, a new material and design for
generator bushings has been introduced to the industry. The
material is a polymer ceramic with a history of over 20 years in
the field as an outdoor insulator on utility systems. It is
chemically constructed as slurry, consisting of 87% silica and 13%
resin composition. It can be poured into a mold to cure without
heat. The curing reaction is completely chemically. Cure time is
less than 20 minutes.
The polymer ceramic material has been thoroughly tested both in
the laboratory and in the field. The polymer ceramic test data is
attached (see Fig 1) and compared with fired porcelain test data
used for bushing manufacturing. In all electrical and mechanical
tests, it is the equivalent of, or exceeds the requirements of
porcelain.
A polymer ceramic generator bushing simplifies the design of the
bushing. The material matches the coefficient of thermal expansion
and contraction of metals. It appears to be a rigid material, but
there is sufficient flexibility in the material to match the
differences in thermal expansion and contraction of copper, steel
and aluminum. This allows these metals to be directly cast onto the
material.
The resultant design of a generator bushing using the polymer
ceramics is simple. There are no requirements for gaskets, neat
cements or spring loading as is required when using porcelains. It
is a one-piece construction. The polymer ceramics are cast directly
onto the conductor, forming a vacuum tight seal. The mounting
flange is embedded from the outside into the polymer ceramics with
grooves machined into the inside diameter of the metal flange to
act as labyrinth seals. No conventional flange seals are
required.
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Heat is dissipated from the conductor along the entire length of
the conductor. The polymer ceramic has a high heat conductance due
to its high silica content, and is cast into intimate contact with
the conductor. The continuous polymer ceramics in intimate contact
with the conductor and with its high heat conductance is more
efficient in removing the heat from the conductor than a simple
porcelain or asphalt filled design. In some instances, it is more
efficient than hydrogen forced cooled bushings. This results in
cooler terminal connections as the heat generated in the middle
section of the bushing is dissipated outward to the outside air or
hydrogen, through the bushing body and is not transmitted to the
terminals at the end of the bushing.
The polymer ceramics are not susceptible to cracking, from
either heat/cold, or impact. Tests have been conducted, taking
samples of the polymer ceramics from liquid nitrogen temperatures
to be immediately placed into a propane flame and no cracking has
occurred. The material is non homogeneous and is practically immune
to impact. While conducting tests for the manufacture of high
voltage outdoor insulators, 69kv Station Post insulators were shot
with a 30.06 caliber rifle and only small chips were knocked loose,
but the insulators were not cracked. In general, identical designs
of porcelain and polymer ceramic bushings will provide a 10% to 25%
greater current carrying capacity for the same current rating. If
standard current ratings are used, the polymer ceramic bushing runs
considerably cooler than a porcelain bushing.
In emergencies, the polymer generator bushing is especially
valuable. Starting from a blank design sheet, the polymer ceramic
bushings can be designed, manufactured and delivered in less than
four weeks.
A number of polymer ceramic generator bushings have been
installed in large generator units for several years now and are
operating without incidents. It is anticipated the trend will be
more towards the polymer ceramic bushings in the future as a result
of their simplicity of construction, less maintenance requirements,
and cooler operating temperatures.
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Figure 1
POLYMER CERAMIC MATERIAL TECHNICAL DATA
Polymer Porcelain Ceramics _________
1. Dielectric Strength, volts/mil 450-650 55-300 2. Dielectric
Constant 4.5 5.4 - 7 3. Dissipation Factor and 0.019 0.9-1.12
30 days @96-98% Rel. Humidity 0.065 4. Material and Processing
Cost, cents/pound 7-20 40-60 5. Energy Required, BTU/pound