There are many on-line and off-line methods of testing and inspecting Turbine Generator Stator (TGS) windings. All involve some degree of investment in equipment and hardware – at times quite expensive. However, a visual inspection of a generator can speak volumes to you, with regard to its operating condition. Dusting is one of the characteristics that a generator may exhibit at some point in its lifespan. Caught early, corrections to this issue can add literally years to the life of you generator. It is important that we all learn to read the tale-tell signs.
Dusting is created by one of two different phenomena. Under the right circumstances, both can be working in consort with one and another within the same area. Mechanical dusting is generated when two components fret against one another. This activity leaves a deposit of fine particles. Left unchecked, this phenomenon could potentially abrade through critical insulated components leading to dielectric failure. Corona dusting can be generated within the stator slots, slot exits, end windings, and connections. Slot discharges occur when the coils become loose, causing the winding capacitive charging current to be dissipated to ground through concentrated contact points. This in turn generates localized areas that appear to glow, spark, and eventually burn. Left to continue, this phenomena will eventually compromise the ground wall insulation system and result in a winding failure to ground. Stress gradient materials, in particular those comprised of paints or varnishes, are susceptible to deterioration due to high thermal factors. Eventually, the interface between the stress gradient and the semi-conductive components is breached. This gap allows for the inception of discharging. Although this discharge activity can appear
pronounced and wide spread, it is considered to be a rather slow acting mode of deterioration. Corona discharges might also occur in stator slot and based on other visual symptoms, part of the slot should be unwedged to see extent of corona activity going on in the slot.
Our experience over the years has demonstrated that the UV camera is an important tool for simulation and visualization of corona activity while generator stator winding is energized with AC Voltage. This gives significant inputs in overall reliability assessment of stator winding and in determining the solution required to avoid/minimize the corona activity.
Devinder Singh / WoodGroup.com