Wyle has been the largest supplier of medium voltage circuit breaker retrofits to the North American nuclear industry. Wyle has designs for many types of breakers including most GE Magneblast breakers from 4 KV to 13 KV both vertical and horizontal, Westinghouse DHP 350 4 KV, Allis Chalmers MA 250 4 KV, and ABB 7.5HK circuit breakers. Wyle has accomplished this through our partnership with Siemens. Siemens has installed over 250,000 of these same types of breakers throughout the world. Siemens role is the designer and manufacturer of the breaker. Wyle is the nuclear supplier who performs all of the qualification engineering/testing and provides a nuclear quality program that meets CSA Z299 and 10 CFR 50, Appendix B.
Siemens has incorporated frequently asked questions on Medium Voltage switchgear into Technical Topics papers. TechTopics is a series of papers which discuss issues of interest to users or specifiers of medium voltage electrical equipment.Siemens has built new manufacturing plants in the U.S. to better serve its customers over the long run.
Wyle offers these breakers to the nuclear power industry for both safety related and non-safety related applications.
There are a number of very compelling reasons why nuclear utilities should upgrade their medium voltage circuit breakers with the Siemens/Wyle option now.
Pay-back for the vacuum upgrade has been calculated by one nuclear utility to be less than 5 years. New circuit breakers featuring the very latest technology can be provided for about the cost of refurbishment.
All Siemens replacement 3AF/3AH breakers have virtually the same operating mechanism regardless of interrupting rating; therefore, the number of required spare parts is further reduced. Simpler more reliable mechanisms require less maintenance and are therefore more cost effective than alternate breakers. Siemens recommends that the 3AF/3AH breaker be lubricated every 10,000 operations or every 10 years. As this number of operations is approached, contact shortening should be checked especially if there have been a number of high fault current interruptions. This is easily accomplished by checking the position of an erosion mark on the exposed movable contact stem of the interrupter. Replacement of vacuum interrupters, closing solenoid, opening solenoid and trip free drive bar mechanism is recommended every 10,000 operations. An operations counter with a 5 digit register is installed on every breaker.
Lubrication, which is recommended every ten years under normal conditions, may be expected to require less than one man-hour per breaker. A major overhaul, if ever required, may be expected to require less than eight man-hours.
Vacuum circuit breaker replacements can be accomplished quickly. They provide maximum interchangeability because the Siemens vacuum breakers are designed to replace the entire Magne-Blast™ breaker with a new but dimensionally interchangeable truck thus avoiding the need to modify the original switchgear. This allows the upgrade to be made in minimum time. Breakers can be changed out during a routine outage.
The modern technology featured in the Siemens vacuum circuit breakers produce the most reliable medium voltage circuit breaker available today. The simplified operating mechanism as well as modern manufacturing techniques assures upgraded reliability and greatly reduced maintenance. 
Long term technical support is insured by both Siemens and Wyle. Both Siemens and Wyle are reputable companies and leaders in their respective fields. Long range plans of both companies carries them well into the 21st century. This guarantees availability of first quality safety related renewal parts for years to come.
The Siemens vacuum circuit breakers features the Siemens 3AF operating mechanism and a vacuum interrupter. Because a vacuum is the ideal dielectric, contact travel is very small, on the order of 12.5 mm (~ 1/2 inch). The movement is accommodated by a stainless steel bellows that is welded in place at the factory and eliminates all leak paths. Small contact travel also requires a very small breaking time. All this translates into less required stored energy to break the circuit. Less stored energy and short breaking time simplifies the operating mechanism. For instance the 3AFS operating mechanism has approximately one third as many parts as a comparable SF6 breaker. All Siemens 3AFS breakers have virtually the same operating mechanism regardless of the interrupting rating, therefore the number of required spare parts for an extremely reliable device is further reduced. The Mean Time To Failure (MTTF) of the Siemens 3AFS mechanism has been calculated at nearly 1,000 years.
Improvements in vacuum technology over the past several years has resulted in most of the world's medium voltage switchgear manufacturers adopting the vacuum option. This is the case of virtually every U.S. manufacturer, 80 percent of the German manufacturers, and more than 60 percent of the British manufacturers. More than half of the world's current annual production of medium voltage circuit breakers are vacuum circuit breakers. In order to properly evaluate the vacuum versus SF6 breaker designs, it is necessary to determine the basic criteria which affect the reliability of circuit breakers. The three main considerations that contribute to reliability are mechanical reliability, breaking reliability and contact design and construction.
At initial separation, the vacuum circuit breaker is already in a state of breaking at the zero current point. Vacuum is the ideal dielectric and the breaking time is extremely low. Therefore, there is no need for generating artificial conditions for arc quenching and contact separation is minimal. All this translates to less required energy storage and a less complicated breaking mechanism than any other type of circuit breaker.
The SF6 circuit breaker requires regeneration of the dielectric at natural current zero by re-injecting fresh dielectric between the contacts. This is performed directly by the operating mechanism (gases compressed by a pump) or by the arc itself which generates the needed pressure. Each of these processes requires a longer period of time and more importantly a contact stroke and speed of much higher energy.
This is important when considering that most of the circuit breaker failures are due to the operating mechanism. Some of the operating mechanism parts are severely stressed during operation and suffer from degradation due to the operating environment.
Vacuum breakers have less than 30 percent as many parts as comparable SF6 breakers and an even lower percentage when compared to the older air breakers. More significantly, vacuum breakers have only 10 percent as many inaccessible moving parts. Fewer parts with lower energy mechanisms and totally enclosed breaking contacts means less maintenance, fewer failures and higher reliability.
The relative small motion of a vacuum interrupter is accommodated by metal bellows which are sealed during manufacturing. These bellows are joined at either end by electron beam welding or other advanced welding techniques, thus there are no organic materials and no leak paths.A vacuum can be guaranteed because vacuum losses can only occur due to air migration through the constitutive material of the interrupter or through out-gassing from the copper and other materials.
Therefore, vacuum loss is constant in time and can already be tested and measured accurately at the time of manufacture. It is, therefore, absolutely useless to have a constant indication of the vacuum in service as there are no degradable leak paths such as gaskets and o-rings or leak detectors.
Vacuum interrupters can be qualified as sealed by IEC standards when the pressure guarantee has a validity of 10 years but for most applications vacuum integrity exceeds 20 years.
The SF6 circuit breaker manufacturers have indicated on many occasions that a vacuum breaker needs a loss of vacuum detector to be equivalent to an SF6 circuit breaker with a leak detector. No one has ever developed a practical loss of vacuum detector on a vacuum breaker. Installation of any type of leak detector on a vacuum breaker requires changes to the interrupter construction which introduces a leak path and radically increases the likelihood that a leak will occur.
The design of the contact assembly is extremely critical to the successful operation of vacuum circuit breakers. The materials of construction must provide a high degree of electrical conductivity so that normal load currents will not cause excessive heating. Thermal conductivity is an important characteristic as the contacts must rapidly dissipate the thermal energy generated during arc interruption.
The rapid build-up of the dielectric strength in the contact gap enables the arc to be safely extinguished even if the contact separation occurs immediately prior to current zero.
The arc drawn into the vacuum breaker is not cooled. The metal vapor plasma is highly conductive and the resulting arc voltage only attains values between 20 and 200 V. For this reason and because of the short arcing times, the arc energy developed in the break is very small. This also accounts for the long electrical life expectancy of the vacuum interrupter.
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Robert Francis
7800 Highway 20 West
Huntsville, AL 35806
(256) 837-4411 ext 315
(256) 837-3363 fax
robert.francis@wyle.com