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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