The cause of the August 3rd outage was related to a failed speed signal to the Whitehorse Hydro Unit #4 governor. The governor controls the electrical output from a hydro unit. The failure of this speed signal causes the hydro unit to shut down as a safeguard to prevent the unit from becoming uncontrollable.
The loss of the speed signal was caused by a control board failure. The equipment was repaired, tested and the unit was placed back on line.
Whitehorse Hydro Unit #4 is the largest hydro unit on the grid. When the generation was lost, the feeders tripped off in an attempt to balance the customer load to remaining generation.
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