On-line monitoring systems to assess the condition of generator stator windings, bearings and the air gap are now widely employed by hydro-generator plant operators. However, there is no on-line monitor that explicitly determines the condition of the rotor field windings. Although salient pole field windings tend to be very reliable, in each major outage plant personnel spend a considerable amount of time doing the ‘pole drop’ test, to assure themselves that there are no shorted turns on the field poles. In addition, the pole drop test may not be effective in detecting shorted turns in the standstill condition.
For moderate and large size hydro-generators and pump-storage generators, the most common type of salient pole rotor has a ‘strip-on-edge’ type of winding on each pole. Such winding is composed of strips of copper that are fabricated around the pole piece much like a picture frame. Electrical insulation, most commonly fiberglass reinforced epoxy, is used to insulate each copper turn from adjacent turns, as well as provide ground insulation between the copper and the rotor pole.
Failure of this insulation can result in shorted turns. Shorted turns on rotor may lead to unbalanced magnetic pull, which in turn may cause an increase in bearing vibration. However, shorts can exist with no increase in vibration and thus bearing vibration is not an infallible way to detect rotor winding aging.
On high speed turbine generators, rotor shorted turns can be detected at lower cost during normal generator operation by measuring the magnetic flux from each pole as it passes a probe that is placed in the air gap. On line monitoring of rotor has been used for years on high-speed turbine generators.
This paper will describe application of the new flux probe and new test and data analysis method applicable to hydro-generators rotor windings.