A NEW SENSOR FOR DETECTING PARTIAL DISCHARGES IN OPERATING TURBINE GENERATORS

Partial discharges (pd) are a symptom and/or a direct cause of the deterioration of the groundwall insulation used in high voltage stator windings. By measuring the pd activity in a machine, a utility can determine if conditions such as polluted endwindings, loose wedges, insulation delamination or surface stress control coating degradation exist. On-line pd testing is important to utilities since expensive testing outages can be avoided. In addition, certain critical conditions such as loose wedges and slot discharge only create steady discharges when the machine is operating. A decade ago, an on-line pd test for hydrogenerators was developed which permitted utilities to assess the condition of the stator winding, and plan maintenance well before catastrophic insulation failure occurred. Unfortunately, a reliable on-line pd test for turbine generators was not possible due to the very high level of electrical interference which commonly occurs in such large machines. Under the sponsorship of the Canadian Electrical Association (CEA), a new pd sensor has now been developed which is able to unequivocally differentiate between pd in the winding and all types of electrical interference. Thus the new sensor will permit an on-line test for turbine generators, similar to the test now used for hydrogenerators. The new sensor requires no high voltage connection to the winding, and is easily installed in the stator slot, underneath the wedges. Noise is differentiated from pd by the shapes of the electrical pulses from the sensor. The new sensor, called the Stator Slot Coupler (SSC), has been installed in six large turbine generators, and poses no mechanical or electromagnetic risk to the generator.