Study on health assessment of generator winding insulation

Reliable operation of a generator is critically dependent on the integrity of its stator winding insulation. As this insulation is exposed to a wide variety of stresses like thermal, electrical and mechanical, it deteriorates on use over the time leading to failure. These stresses may be continuous, cyclic or intermittent. In addition, factors such as the presence of moisture, corrosive materials and radiation can lead directly to insulation breakdown, or to accelerate the aging process from voltage, temperature and mechanical vibration. Unexpected failure in the insulation may cause serious expenses for material, repair work and unplanned non-availability. To steer clear of unplanned outages periodic diagnostic tests on the insulation have become an integral part of the condition monitoring for generators. Such diagnostic tests generally include non-destructive measurements of various parameters associated with the winding insulation. In the paper efforts have been directed towards the residual life assessment of winding insulation of the existing units in India. This would help in not only the determination of the potential of the existing plants, but would also facilitate in planning for the capacity addition. Some researchers have adopted these methods for life prediction of the samples of the used winding insulations assuming different endurance models and others have tested to the breakdown voltage of winding, which is believed to be related to the residual life of insulation. This paper reviews the various life models under individual stresses and their combinations along with the statistical methods for estimation of residual life of stator winding insulation. The paper also critically discusses the various non-destructive tests and diagnostic parameters, which are helpful in assessing the health of the insulation and HV breakdown and withstand tests to correlate them to its residual life.