By E. R. Perry and R. Torres, Polytech Services

A generator bushing can be the weak link in the electricity supply chain. It is generally anticipated to be maintenance free but several clues prior to failure must be watched.

A number of generator bushing types exist alongside multiple design variations. Once the basics of a generator bushing and the function of each part are understood, it is easier to evaluate the bushing to determine its suitability for continued duty. In most generator bushings, there is a commonality of design and function. Learning what these are makes it easier to determine if a repair or replacement is required.

All generator porcelain bushings are spring loaded to compensate for the variation in thermal expansion of the conductor and porcelain. The air 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. Normally, generator bushing’s outboard end is vented to the atmosphere except when filled with asphalt or a small amount of oil.

The bushing’s generator end has a fixed metal plate anchored to the end of the conductor. Gaskets are placed on both ends of the bushing porcelain to protect the porcelain. On the generator end, the gasket acts to form a gas seal between the generator and the internal bushing. The inboard gasket is most important in preventing leakage through the bushing, as the air end is not normally sealed.

There are several common types of bushings. Here are a few.

Simple Hollow Porcelain Bushings: These bushings are insulated by an air space between the conductor and the porcelain and are used primarily on small generating units. Many date back 40 to 60 years. The most common problem with these bushings is deteriorated gasket materials. Almost all require replacement of the neat cement between the metal 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 by filling the porcelain with asphalt, reducing the temperature. The asphalt also became a liquid seal when hot, to prevent hydrogen leakage. The asphalt is not to provide dielectric integrity to the bushing, but is primarily for heat transfer. Asphalt leaking from the bushing is easily observed. Although it is not always a danger sign, it should be checked carefully. Asphalt leakage is usually caused by old gaskets shrinking, or overheating of the bushing that causes excessive pressure inside the bushing, overcoming the spring pressure and forcing the asphalt out. The primary concern here is to determine why the bushing is overheating. The most common cause is a loose connector or a connector that is not making sufficient area contact.

Figure 1. Diagram of a simple generator bushing showing gaskets, porcelain, conductor, threaded rings and floating rings. Courtesy Polytech Services.Click here to enlarge image

Forced Hydrogen-Cooled Bushings: Hydrogen-cooled bushings are constructed similar to the “simple” and “asphalt filled” bushings already mentioned. They are slightly more complex internally, however, 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 the hollow conductor. Hydrogen is directed under pressure inside the generator end of the hollow pipe. The pipe has exit holes near the air end of the bushing, but still internal to the bushing, where the hydrogen flow is reversed. The hydrogen flows back along the outside of the pipe and in contact with the internal diameter of the porcelain housing (or in some instances along the inside of the conductor if asphalt is being used) where it exits at the generator end of the bushing.

There is a possibility of trouble in this design. Since the bushing is mounted near vertical and since the air end is pointed down, oil can possibly accumulate in the internal air end of the bushing. If sufficient oil accumulates, 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 of the end of the bushing.

Solid Cast Epoxy Bushings: These bushings are simple in design. The solid cast epoxy bushings use 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 eliminates the need for gaskets to seal around the conductor.

While many of the problems associated with porcelain insulated bushings have been eliminated, other problems can occur. Plasticisers in epoxy give it flexibility while new, but they tend to dissipate with time and temperature. The epoxy becomes brittle and no longer follows the thermal expansion and contraction of the copper conductor. This results in either leakage between the conductor and epoxy or cracks appearing in the epoxy. Care must be taken when tightening down the epoxy flanges onto uneven surfaces cracking of the epoxy flange can occur. Another difficulty is that the epoxy dielectrics may be adequate, but the heat transfer in epoxy is marginal, resulting in rapid aging of the epoxy.

Gaskets and Neat Cement

Some designs replace the gasketing on the generator end of the bushing with a thin shell of copper soldered to the conductor and then 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. The copper is corrugated and kept thin to follow the thermal expansion and contraction between the copper and the porcelain.

Older generator bushings used cork gasketing that age rapidly and have 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. This is less prone to shrinkage and can be used without containment of the gasket, such as on a flat surface. The material is still popular today.

More recently the tendency has been to use a Buna-N material with hydrogen cooling as it has less porosity to hydrogen than most other gasket materials. The Buna-N material has good resilience for proper conformity under pressure. The result is an excellent seal. A Buna-N gasket must be contained in a groove as it will otherwise cold flow under pressure.

The neat cement between the bushing flange and the porcelain is itself a gasket of sorts. The purpose of the cement is to form a leak-proof seal and provide a mechanical bond to support the porcelain to the mounting flange. Neat cement has the unique property of forming a strong mechanical bond while providing sufficient resilience to compensate for the difference in thermal expansion and contraction between the metal flange and porcelain body.

Older bushings need to have the neat cement inspected for leakage and mechanical strength deterioration. Generator heat and vibration tend to pulverize the neat cement, resulting in a loss of its sealing and mechanical properties. Older bushings may not have had the quality material and control available today. This often results in large voids internal to the neat cement. These voids are not apparent from the surface, but lie internally. Older bushings should have the neat cement removed and replaced with newer neat cement or a polymer ceramic material presently available. The polymer ceramic material has greater resilience and mechanical strength than the older neat cements.

Porcelains

Porcelain materials in a generator bushing vary in strength and configuration. Older bushings did not have the clay formulations to provide the mechanical strength of modern porcelains. Better clays, combined with alumina, and more precise processing have resulted in added strength and greater uniformity.

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 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 can match the porcelain’s thermal expansion and contraction and is stronger mechanically and electrically. 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.

Broken porcelain skirts or chips can be replaced in the field or factory in a simple process using high-speed diamonds to mechanically prepare the broken area. The area is chemically treated for maximum adherence of the polymer ceramic material. A metal mold is constructed and applied to the broken area to shape the polymer ceramic to conform to the original porcelain surface. The polymer ceramic material is mixed and poured into the mold to cure in minutes. The mold is stripped from the repaired area and the surface is coated with a fluorourethane material to match the porcelain glaze in color and gloss. The repair of a typical broken skirt takes two to four hours.

Damage to porcelains can be quite severe and still repairable. Repair of severely damaged porcelains can only be done in the factory where proper equipment is available. Fortunately, these repairs can be done rapidly, usually within one to two days. The repaired sections are of equal or greater mechanical and electrical strength than the original porcelain.

Generator bushings are generally larger in diameter than their equivalent voltage in substation equipment. This is a result of higher currents and conductor diameters required by the generator bushings. Materials such as the asphalts used in generator bushings are more to dissipate heat away from the conductor and toward 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.

New Design and Materials

A new material in now available to replace porcelain and epoxy materials for generator bushings. 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 and requires no heat for curing. It can be poured into a silicon rubbera mold and cures chemically in less than 20 minutes.

A polymer ceramic generator bushing simplifies the bushing design. The material matches the coefficient of thermal expansion and contraction of metals. It appears to be a rigid material, but sufficient flexibility in the material exists to match the differences in thermal expansion and contraction of copper, steel and aluminum. This allows these metals to be directly cast onto the polymer ceramic.

The resulting design requires no gaskets, neat cements or spring loading as is required when using porcelains and 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 metal flange to act as labyrinth seals. No conventional flange seals are required.

Heat is dissipated from the conductor along its entire length. The polymer ceramic has a high heat conductance due to its high silica content and is cast into intimate contact with the conductor for heat transfer. The solid cast generator bushing is more efficient in removing the heat from the conductor than a simple porcelain, asphalt-filled design or hydrogen-cooled application. This results in cooler terminal connections as the heat generated in the middle section of the bushing is dissipated uniformly outward to the outside air 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 taken samples of the polymer ceramics from liquid nitrogen temperatures and immediately placed them in a propane flame with no cracking. 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. If standard current ratings are used, the polymer ceramic bushing runs considerably cooler than a porcelain bushing. Hydrogen cooling is no longer required.

In emergencies, the polymer ceramic generator bushing 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 polymer ceramic bushings will be standard in the future as a result of their simplicity of construction, less maintenance requirements and cooler operating temperatures.

Authors: E.R. Perry and R. Torres are with Polytech Services based in Gainesville, VA.