Characterization of Electrical Insulation Exposed to Arbitrary Voltage Waveforms

Doctoral thesis, 2010

The extensive use of power electronics in power systems today gives rise to other voltage waveforms than the conventional 50/60 Hz sinusoidal and consequently other types of electric stress act on high voltage insulating materials. Previous studies has shown that Pulse Width Modulated (PWM) voltages and similar waveforms increase stress in Adjustable Speed Drives (ASD) as compared with sinusoidal waveforms and it has also been shown that partial discharge (PD) activity is intensified during PWM voltages and that rise time of square like pulses affect PD inception voltage. In the light of these findings a need for new characterization and diagnostic methods which can be used to study the influence of different types of voltage waveforms on insulating materials and systems has arisen. In this project a method called Arbitrary Waveform Impedance Spectroscopy (AWIS) has been developed which is based on conventional dielectric response measurement techniques but capable of using any voltage waveform to perform measurements. By utilizing the harmonic content of the waveform the dielectric properties of an insulating system can be determined at several frequencies simultaneously. This provides an advantage compared to other dielectric response techniques where the applied voltage needs to be carefully controlled. With AWIS a test object can be stressed with any waveform and the influence on the dielectric properties studied. The accuracy level has been found to be in the same range as for commercially available instruments, but additional complications related to aliasing and spurious/harmonic distortion have to be taken into account. AWIS has been used to study PD behaviour in systems where different analysis methods have to be applied depending on the relationship between the magnitude of the discharge and dielectric current. A small discharge current compared to the dielectric current requires an approach which relies on the increase of noise levels during PD activity and the amount of information possible to extract is limited. For larger discharge currents PD behaviour over time can be monitored both in frequency and time domain and in certain objects it is also possible to detect a conductive current most likely related to charge relaxation. The measurement system has also been used to monitor the behaviour of dielectric properties during electrical tree growth in low density polyethylene samples having capacitance less than one picofarad. The tree length was found to be correlated with capacitance increase which can be modeled and used in order to estimate tree growth rate. This possibility appears especially useful if the samples are opaque as conventional CCD cameras used for such measurements are no longer applicable. In addition to the development of the measurement system and the application on PD and electrical treeing measurements, the influence of the voltage waveform on dielectric heating was studied. A method for estimating the increased amount of power generated in different insulating systems due to the harmonic content of the voltage waveform was developed and evaluated. AWIS was used to measure the dielectric properties of different insulating systems and based on these two different dielectric heating parameters was calculated using the proposed method.