Chapter 10: Electrical Techniques: Discharge Monitoring

10.1 Introduction

Chapter 9 concentrated on the perturbations to current, flux and power at the terminals due to faults in the machine. However, there is another effect at work in high-voltage electrical machines and that is perturbations to the voltage and current wave fed to the machine as a result of electrical disturbances or discharges in the insulation system.

This chapter deals with a more specialised terminal analysis than considered in Chapter 9, which has the potential to detect those discharge activities present in the machine winding insulation, particularly in high voltage machines. One of the reasons this technique has received so much attention is because the insulation system lies at the heart of every electrical machine, as described in Chapter 2, and its deterioration can be relatively slow, as described in Chapter 3. Therefore it should be a good target for condition monitoring.

10.2 Background to Discharge Detection

Discharge behaviour is complex and can be categorised in ascending order of energy and damage as

• corona discharge,

• partial discharge,

• spark discharge,

• arc discharge.

A well-made insulation system will exhibit low-level corona discharge on the surface of the insulation at AC voltages above 4 kV rms to ground. If there are voids inside the body of the insulation system those voids will also exhibit partial discharges at the points in the voltage cycle when the local electric field strength exceeds the Paschen curve level for the gas in the void at that temperature. Neither that surface or body activity is necessarily...