Controlled Switching for energizing large transformers

Residual magnetic fields found in large transformers can make energizing them a tricky business.  The resulting voltage depressions and harmonics can lead to flickering lights, equipment damage and even widespread outages for customers.

Special circuit breakers with pre-insertion resistors can reduce the problem, but don’t eliminate it. They are also expensive, getting hard to replace at 230 kV and below, and require frequent maintenance.

To solve this issue and to reduce costs, Manitoba Hydro investigated the application of controlled switching (also known as Point-on-Wave (POW) switching)  — a relatively new technology that can safely energize large power transformers using standard circuit breakers.

Manitoba Hydro applied state of the art transformer modeling techniques and learned that the standard approach of using capacitor voltage transformers for measuring remnant flux inside the transformer was not accurate enough.

These modeling techniques and study approaches were enhanced for the case of parallel HVDC converter transformers. This allowed two HVDC converter transformers connected in parallel to be safely energized using controlled switching — a world first!

Other utilities can now save potentially millions of dollars by using transformer controlled switching with confidence, knowing that Manitoba Hydro has thoroughly investigated, tested, and successfully implemented this technology.