A BIL rating on a transformer is an assurance that it will be able to routinely withstand power surges up to the given rating emanating from any number of common sources such as lightning, heavy motor start-ups, industrial load switching, grid overloads, etc.
Partial discharge or Corona is defined by ASTM as a “type of localized discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value” The partial discharge (Corona) level has a pronounced effect on the life of the transformer.
As the voltage stress is increased, anomalies in the insulation system will result in Partial Discharge or Corona inception. As the voltage stress is reduced, Corona extinction will occur.
The Partial Discharge cycle repeats rapidly during the operation of the coil. The excess energy dissipated during these repeating cycles causes deterioration of the insulation system resulting in premature failure of the coil.
Corona testing requires specialized equipment and cannot be characterized with standard dielectric or induced voltage testers.
Bicron is able to minimize Partial Discharge or Corona through its very extensive design experience, repeatable manufacturing processes, and specialized test equipment and procedures to measure Corona inception and extinction levels.
BIL rated equipment traditionally had been the exclusinve domain of utility equipment manufacturers and gas or liquid filled transformer manufacturers. No longer. Recently, power grids, especially in metropolitan areas are reaching a level of saturation that result in a greater susceptibility to load variances, including lightning strikes. Consequently, AC switches and high voltage industrial equipment are exposed to voltage surges normally absorbed by the grid. BIL rated components are now being routinely specified for a much broader range of power control devices to greatly reduce the possibility of catastrophic system damage.
Bicron minimizes Partial Discharge and Corona through the elimination of nanovoids and contaminants during the design and manufacturing process. Construction materials and manufacturing process controls are designed for maximum product performance.
Bicron applies three tests to insure the desired level (BIL) of
Corona and Partial Discharge protection is achieved.
• Applied Voltage or Dielectric Test
A high voltage AC or DC test voltage is applied between the windings, and from the windings to ground. The test is designed to verify the adequacy of the system in terms of coupling capacitance (AC) or resistance (DC).
• Induced Voltage Test
The Induced Voltage Test reveals defects that might eventually result in a shorted turn and excessive excitation current. This test is important when winding layers are not properly insulated.
• Partial Discharge (Corona) Test
The partial discharge test is a nondestructive method of identifying the potential for premature insulation breakdown. The test is capable of detecting small defects and nanovoids in the insulation system that could lead to catastrophic transformer failure.