Despite the Myth Engineers Can Put Things Back Together After Taking Them Apart: Repair of the Rotor Arm at Ohau A Power Station

摘要

Reliable, efficient, and available generating assets are key attributes that contribute towards the success of a generating company within the New Zealand energy market. Expectations from the market are high with respect to these key performance indicators and the cost of failure can be significant, especially in terms of lost revenue potential, Electricity Governance Rules compliance and company reputation. Asset owners are thus driven to ensure that asset management practices are optimised through the application of engineering process to ensure a timely and cost effective repair is implemented prior to any plant failure and that maintenance investment appropriately mitigates or removes the exposure of plant failure. Ohau A power station, commissioned in 1979/80, comprises of four hydro units with an installed capacity of 264 MW. During routine inspection of the generator in 1996 a crack was discovered in one of the rotor spider arms of Unit 5. In 2002 the defect was reported to have grown thus sparking concern of the unit's operational durability. Over the past three years the defects have grown in number and size. Routine monitoring has been used to track the cracks growth rate and projected time to failure. Engineering assessment, including finite element analysis, concluded that the defect could compromise the integrity of the generator, with the worst scenario resulting in extensive damage to the rotor and stator. This paper describes the complex task of replacing a rotor spider arm, an undertaking that far exceeds the normal routine maintenance carried out on a hydro generator. A synopsis of the rotor defects, condition monitoring and engineering assessment that led to the decision to repair the generator rotor are outlined. Moreover, the physical repair works are presented including removal of the rotor rim using induction heating, arm manufacture and replacement, to reassembly and commission. The work conducted highlights the benefits of routine engineering assessments and a proactive approach to asset management.