Enhancements in Dielectric Response Characterization of Insulation Materials

Licentiatavhandling, 2013

The increasing demand of integrating various renewable energy recourses in power system requires extensive use of power electronic solutions, such as HVDC and FACTS, as these techniques allow energy conversion between different frequencies and serves to stabilize the network. Consequently, electric stresses other than traditional 50/60 Hz sinusoidal voltage stress are acting on high voltage insulation materials. Therefore, a need for fast and accurate characterization methods, which can be used to study the influence on insulation materials of different types of voltage waveforms, has arisen. Dielectric response measurement is one commonly used non-destructive insulation test technique and has more than 100 years of history. Limitations like precision of the testing voltage waveform, high demand on a stable testing environmental condition, intricate specimen-electrode preparation, severe electrode geometric influences, etc., are however restraining the use of such measurements. In this project, several methods that are based on the Arbitrary Waveform Impedance Spectroscopy (AWIS) technique have been developed to enable fast and more accurate solid dielectric characterization. A test setup that is based on the AWIS technique can be optimized by implementing several techniques such as voltage divider, voltage follower, shielding box, etc., to reduce the measurement noise and to avoid crosstalk between adjacent signals. By utilizing optimized harmonic limited waveforms in the AWIS test setup, dielectric responses over a wide frequency spectrum can be determined simultaneously without degradation by signal aliasing. This also enables monitoring of the dielectric properties under fast changing test conditions as a source of systematic error is eliminated when all frequencies are obtained at the same time and at the same conditions. An air reference method and a contact-free electrode arrangement are described in this work to enhance the dielectric characterization accuracy by avoiding contact problems at the electrodes. It is shown that by performing a calibration with electrode gap filled with air under the same conditions as the material is tested, the air reference method can improve the measurement accuracy substantially. This type of approach also eliminates the need of a detailed model of the analog measurement circuit. In conjunction with the contact-free measurements, the approach allows for avoiding complicated and time-consuming specimen preparation procedures. The measurement methodology as well as the electrode arrangement and error estimation are presented and evaluated using different dielectric response instruments and materials. To improve the accuracy of dielectric permittivity, two methods are suggested in this work by either compensating or shielding the electrode setup from undesired geometric influences. One approach is to estimate a correction factor from a finite element method (FEM) model to improve the accuracy. The other approach uses a shielding guard electrode to avoid the geometric influence on measurement. Measurements of several PET films are used as an example to illustrate the use of correction factors in the application of contact free measurement. The latter method is only verified in FEM calculation.