Air Blast Switchgear - The Explosive Cost of Failure

The cost of poor performing hygrometers is often measured as a financial loss. On occasion, the loss can be far more dramatic

 

In September 1987 two 230 kV CGE type AT air blast circuit breakers at the Cherrywood Transformer Station (ON, Canada) failed explosively within a one week period. The root casue of the failure was attributed to excessive moisture contamination within the 7000 kPa compressed air system. Due to the nature of these two failures, extraordinary measures had to be expended to ensure adequate protection for personnel whilse rehabilitating the remaining circuit breakers and the associated air systems.

Cherrywood was the largest and most important transformer station in the province of Ontario, a crucial link tying all the eastern provincial hydraulic generation to the power system.

When the first failure occured, in late September, this was dismissed as an isolated incident as the operating company had not experienced any similar failures within its organisation. Initial studies suggested that the fault lay within the porcelian air column. The priority was the repair of surrounding equipment (insulators, bushings, support columns and other air columns), which had been damaged by flying debris.

One week later a second, identical failure occured. This time, beacuse of the explosive nature of the failures, all planned work in the switchyard was halted and access was restricted.

The company insigated an emergeny investigation. The conclusions and consequences were shocking:

 

  • Moisture built-up in the compressed air system due to saturated desiccant in the chemical dryers.
  • The air blast breakers were becomming heavily contaminated with moisture.
  • The plant's moisture analsyers had malfunctioned and measurements taken over a period of months contained significant errors.
  • In addition, a high moisture alarm had failed - plant operators were unaware that moisture was spreading through the plant.
  • Excess moisture in the breaker caused distortion, erosion of insulation and the release of free carbon.
  • Eventually, the line to ground voltage within the breaker reached critical levels.
  • The resulting explosion propelled debris across the switchyard at a velocity of 71 metres / second.
  • During the resulting repair operation, technicians had to wear high-grade bomb protection suits (borrowed from the local police force)
  • The outage in the switchyard had serious knock-on effects, as the local nuclear reactor was forced to scale-down its output.

 

With the entire switchyard in an unknown state of detrioration, it was clearly time for the company to reconsider its moisture measurement methodology. After a consulatation and evaluation, it was decided that Manlytical's (MCM) Temperature Controlled Silicon Sensor technology offered the best level of performance. A new suite of analysers were supplied and the need bomb safety equipment became a thing of the past.

The learning process had been catastrophic and painful, but the company very soon realised the value of accurate and relaible hygrometers.

For the full story, download the detailed report below.

 

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