Shorted turns due to insulation failure in the windings of round and salient pole rotors in synchronous motors and generators often leads to high rotor vibration, and may limit the output of generators. In fact, high bearing vibration is one of the early indicators that a turn short has occurred. However, since there are many possible causes of high bearing vibration, operators would like independent verification whether such vibration is indeed caused by a rotor insulation problem or not. There are many ways to directly detect shorted turns. Off-line tests such as the voltage to earth, pole drop or the RSO test are often not reliable since the rotor is not spinning and thus faults may disappear (or in fact occur only) in the standstill condition. In addition, these tests require a unit outage and some disassembly.
Users instead prefer an on-line test where the rotational and thermal stresses are normal. The only such direct test for shorted turns involves measuring the magnetic flux in the air gap. The original “air gap flux” test developed by GE over 30 years ago and now widely used around the world on large 2 pole and 4 pole turbine generators, employs a small coil that is normally permanently mounted on one of the stator winding wedges. The sensor detects the “leakage” flux from each rotor slot as it passes the sensor each revolution. If there is a shorted turn in a slot, the leakage flux is reduced from that slot, which can be detected with suitable instrumentation. Since the leakage flux is very small compared to the main airgap flux, in the conventional test one must vary the generator load in several steps between no load and full load, to ensure that there is a “zero-crossing” of the main flux at each rotor slot. This enables the small leakage flux to be measured with some sensitivity for most of the coils.
There are two main limitations with the conventional air gap flux test. The first is that on turbine generators, the sensor can only be installed when the rotor is removed from the stator. Secondly, the load must be maneuvered over a wide range in steps, which causes certain logistical problems for plants under the control of an Independent System Operator, or where reduced load operation reduces revenues. Research in the past 5 years has lead to two innovations that overcome these limitations. The first is a new probe that can sometimes be installed with the rotor in place. The probe is a very thin printed circuit board sensor that is glued to a stator tooth (not the wedge) to measure the main (or total) magnetic flux. The second innovation is a new diagnostic algorithm and instrumentation that is sensitive to shorted turns in any slot while the generator operates at full load, by measuring the main flux. The new instrumentation and algorithm also seems to be effective when used with conventional wedge-mounted sensors. In addition, with some modifications, the new approach can be applied to salient pole rotors in hydrogenerators.