New Leak-Rate Field Test Predicts Future Vacuum Interrupter Failures

Circuit protection protects electrical service personnel, physical assets and production schedules against shorts, faults and dangerous arcing conditions. In addition to protecting equipment from power surges and sags that result in immediate equipment failure, circuit breakers, interrupters and other protective devices also protect equipment from partial failures and faults that shorten the lifetime of electrical equipment. 

For circuit-protection applications with voltages in excess of 1kV, electricians and maintenance personnel traditionally have used circuit breakers with either air-magnetic or oil-based interrupters. More recently, vacuum interrupters (VIs) have supplanted many of these interrupters because of their ability to interrupt power faster—improving equipment and personnel safety—for more cycles than older interrupters. This translates to longer lifetimes for circuit protection equipment and less cost to the user for replacement interrupters. 

Vacuum interrupter manufacturers use three electrical tests to validate the operation of their products before sending them into the marketplace: 1) contact-resistance; 2) high-potential; and 3) leak-rate. Of these three, only leak-rate testing provides data for computerized maintenance management systems (CMMS) and enterprise asset management (EAM) systems. 

Until recently, leak-rate tests could not be conducted in the field because they required large, expensive magnetrons and bulky magnetic coils. Other hurdles for such field testing include calibration-sensitive magnetrons, a lack of compact magnetic coils, standard graphs for measuring ionization current against vacuum pressure and trending and prediction tools and algorithms. 

Solving the problem
New portable magnetron-based test equipment (shown above) and condition-based maintenance (CBM) algorithms developed by Vacuum Interrupters, Inc., recently proved the capability of leak-rate system while field testing 60 GE Power Vac Circuit Breakers at the Lower Colorado River Authority (LCRA) Fayette Power Project facility, in Fayette County, TX.

During field-testing, the VI is inserted into a flexible magnetic field coil (FMFC) developed by Vacuum Interrupters. The FMFCs significantly simplify the process compared to traditional shop-testing procedures. To determine the condition of the VI, the  system measures the relationship between the ionization current and pressure inside the device using the magnetron, coils and a vacuum pump. To create a vacuum-vs-current curve, a VI is opened and a vacuum pump connected to it so the pressure can be gradually decreased, while the magnetron measures the current effects on the magnetic field for each different pressure point. The test automatically enters the data into a CBM diagnostic and predictive algorithm. The algorithm evaluates the results and categorizes the device’s remaining lifetime based on changes between current and historic device measurements, similar to lab tests used to analyze dielectric oils, insulation and other electrical components.  

Of the 60 breakers field tested at LCRA, 10 interrupters were found to need of immediate attention or replacement. Out of those 10, only two would have been identified by traditional contact-wear field tests. MT 

Vacuum Interrupters, Inc.
A Group CBS Company
Carrollton, TX