Mechanical considerations for uprate and rehabilitation of hydro-generators

Electrical design procedures for the purpose of generator uprating are relatively straightforward calculations, based on the space available in the slot, desired insulation system, desired operating parameters and the magnitude of the uprating. More involved, and sometimes overlooked, are the mechanical design calculations to ensure that these machines are safe to operate at the higher levels. In a recent generator refurbishment and uprate by the author's company, large cracks were found in the spider arm to hub area. This machine would have failed mechanically at the higher rating. Its rotor cracks were extensively evaluated with root cause determination, and subsequently repaired. Before looking at this particular case history, a general overview of the mechanical calculations required for uprating are given.