Monitoring and detection of partial discharges in stator windings of turbine generators

Overview of techniques for PD monitoring

The most common way to plot results from partial discharge measurements is the so called phase-resolved partial discharge pattern. The magnitude of each partial discharge is plotted with respect to its position on the 50 Hz voltage. In addition the number of partial discharge pulses with respect to the 50 Hz voltage is plotted.

Results can be plotted either in 2-D diagrams or in a 3-D diagram. Examples of on-line measured 2-D and 3-D partial discharge patterns are given in Figure 1 below.

Phase-resolved partial discharge patterns are useful in identifying the most important failure mechanisms of stator windings.

The first systems to monitor partial discharges during regular operation were developed in the early 1970’s by Westinghouse in the USA and Ontario Hydro in Canada. The stimulus for this development were problems, at that time, with broken sub-conductors in the Roebel-bars of large turbine generators and slot-discharges in the windings of hydro generators.

For the Westinghouse method, PD currents are detected in the neutral grounding connection of the generator by means of a HF current transformer.

The currents are recorded via a spectrum analyzer, which is used as a tunable narrow-bandwidth filter. The analyzer is tuned to a well-chosen frequency where partial discharge signals dominate.

Disadvantages of this method are:

1. The low sensitivity,
2. It is difficult to determine the phase where a discharge takes place,
3. Due to the large self-inductance, only a small fraction of the PD current will flow through the grounding connection, and
4. Interference currents can easily be induced into the large loops that are formed by the grounding system. Nevertheless, the method is still widely used.

PD monitoring in this thesis

Practical on-line partial discharge measurements take place in an industrial environment, where many sources of interference are present. As we saw in the previous section, available methods all have problems with this interference.

In this thesis we have chosen to:

1. Concentrate on interference-free measurements by:
   1. Electromagnetic compatibility (EMC) methods to suppress interference,
   2. External sensors with a bandwidth which is much higher than the common 5 MHz,
   3. Measuring methods to suppress remaining interference.
2. Locate abnormal partial discharge activity by means of additional sensors
3. Test the developed methods under real conditions in several power plants.

Especially at frequencies up to 5 MHz, intense interference is caused by e.g. the rotor excitation system. As a consequence, the question – What can be observed under realistic conditions ? – has to be answered.