Polytech Shares Generator Bushing Inspection & Renovation Experience

Polytech Services Corp.rator bushings are subjected to a variety of adverse conditions compared to transformer and circuit breaker bushings. They must stand up to vibrations, heat, and thermal overloads, much more so than other types of bushings. These conditions must be recognized and generator bushings closely inspected for signs of deterioration.

<%=company%> relies on 18 years of experience inspecting and renovating generator bushings, and often detects a variety of flaws that escape casual observation by power plant personnel. Hairline cracks nearly invisible to the unaided eye and deteriorated flange cement rank among the most common of these problems. Either of which could result in catastrophic failures.

Bushings before Polytech Services used its proprietary Polymer Ceramic material and processes to repair.

After repairs.

Because a generator bushing is porcelain, it is often thought that not much can happen to it over a period of time. On the contrary, bushing destruction remains subtle and often difficult to detect until a catastrophic failure takes place. Once this happens, the cost is magnified a thousand times over the cost of properly maintaining the bushing. There are many indications of bushing deterioration other than the obvious.

Cement Deterioration a Subtle, Undetectable Problem
Deterioration of the cement that attaches the flange to the porcelain remains one of the subtler, normally undetectable problems. Long-term vibration tends to pulverize the cement. The cement looks smooth and solid from the exterior, but the cement has been weakened by vibration, and shrinkage occurs from thermal cycling, reducing the adhesion of the cement to the porcelain. Hydrogen cooled generators often leak hydrogen through the cement on several bushing designs.

Black-Tar Substance More Noticeable
Leaks of a black-tar mixture coming from the outboard end of the bushing seem a more noticeable problem. These leaks occur when the springs in the end cap weaken, the bushing overheats, or the gasket deteriorates with time.

The tar mixture hardens and prevents the end cap from sealing, which allows moisture or air to progressively enter the insides of the bushing. When a gasket in the end internal to the generator fails after long-term usage, the hydrogen leaks past the internal tar mixture and out the outboard end cap of the bushing.

Hairline Cracks Occur for a Number of Reasons; Talcum Powder Helps Detection
Hairline cracks in the porcelain occur for a number of reasons, including overheating, or thermal shock. It is more usual to find these hairline cracks on the inboard end of the bushing. Hydrogen leaks through the bushing are the first indication of these types of cracks. These cracks are very difficult to detect.

Close visual inspection with good lighting is one method used to detect these hairline cracks. This can be supplemented by wiping talcum powder roughly over the porcelain surface. Hairline cracks show up better with the white talcum powder against the brown porcelain glaze.

Thermal Shock from Explosion, Short Circuits or Other Impacts
A bushing suffering from a hydrogen explosion, excessive short circuit, or mechanical impacts caused by workmen presents the worse scenario.

Older porcelains contain built in thermal stresses and are filled with micro cracks that allow the porcelain to shatter rather easily. Often the pieces are held into place only by the pressure exerted on the porcelain by the end caps or springs.

Renovation, Repair, Replacement
Anytime a generator is down for inspection or maintenance; the bushings should be carefully inspected for any of the above symptoms. Any generator 20 years old or older should be considered ready for renovation at the earliest possible time.

With the technical capability and expertise to completely renovate generator bushings, Polytech Services estimates the turn-around time for renovating a bushing at about a week or less. In emergencies, bushings can be repaired or renovated and returned the same day it is received in Polytech's facilities.

Polytech Services asks its customers to notify the company as early as possible that they plan to send a bushing to allow fast scheduling and preparation for its arrival. Companies should provide as close a description as possible of the bushing type and suspected or known problems with the bushing.

For broken porcelains, Polytech uses a process developed during the past 18 years of repairing bushings. It uses a proprietary Polymer Ceramic material matching the coefficient of thermal expansion and contraction of porcelain to either repair, or reconstitute a broken porcelain bushing.

Sometimes, the end of a porcelain bushing can be badly shattered from an explosion or mishandling. When this occurs, Polytech removes it and recasts its original form with the Polymer Ceramic material. This is a fast procedure, and the repaired bushing meets all applicable standards for a similar new bushing.

For hairline cracks, Polytech opens them with high speed diamond grinding wheels, and after additional preparation, it fills the opening with the Polymer Ceramic material to obtain a strong and leak proof repair of the porcelain.

The company uses the same Polymer Ceramic material to replace the weakened flange cement. Polytech removes the old cement and replaces it in a manner that does not disturb the orientation between the flange and the conductor, eliminating the need for time-consuming jigs and fixture construction and application.

Some of these repairs can be made in the field, but if possible, Polytech recommends that customers send them to Polytech's service facility for complete disassembly and thorough inspection. Polytech makes quotes from photos that can be sent via e-mail or descriptions over the phone.

Service Examples
Polytech can manufacture new generator bushings, renovate old bushings, and repair broken bushing porcelains.

• Polytech designed and manufactured three new generator bushings for a large utility in the Mid-Atlantic area with only the very basic dimensions made available. It accomplished the job in three weeks to get them back into service.
• Another utility in the Northeast flew a bushing to Polytech's facility, arriving at 9 a.m. The end of the bushing was completely shattered. Polytech removed and recast the shattered end, shipping the bushing back to the utility the same day.
• A Midwest utility sent a special truck delivery in with a cracked bushing leaking hydrogen. The truck arrived in the morning and while the truck driver slept, Polytech repaired the bushing. It arrived back at the utility by 10 p.m. and was reinstalled by the utility's crew waiting on standby.
• Most recently a large company started up a new generator when it discovered an epoxy high voltage bushing cracked at the flange and leaking hydrogen. The bushing could not be removed from the installation without delaying the startup. A Polytech engineer arrived on the job Sunday afternoon and worked until 3 a.m. to repair the bushing and stop the hydrogen leaks, without removal of the bushing.

For more information, contact Polytech Services, E.R. Perry, 703-257-1500, fax 703-257-1915, matdevco@aol.com, www.polytechservicescorp.com.

Edited by April C. Murelio
editor@poweronline.com