Lifetime extension experience on rotating machine insulation using on-line PD testing

Rotating machine failures due to insulation breakdown can cause catastrophic damages to the equipment, loss or de-rating of power output, lengthy forced outages and heavy costs to the utility. Therefore it is desirable to detect the onset of significant insulation deterioration and identify potential failures as early as possible. As in the medical field, regular testing and early detection of problems mean better chances of survival for the 'patient'. Faced with increasing pressure to increase unit availability yet reduce maintenance costs, utilities are moving away from time-based maintenance practices to condition-based maintenance efforts. Owners of generators and large motors are using on-line partial discharge analysis (PDA) systems for condition monitoring of the stator insulation system. Regular condition monitoring of the stator winding insulation can give users the confidence to safely operate machines, even ones that have suffered some degree of insulation degradation. This paper presents two case studies of how utilities applied an online PD testing system on a generator and a motor with insulation degradation to avoid substantial costs and forced outages. Long-term online PD monitoring has given the utilities confidence to continue running machines that have the potential for insulation failures.