Condition Assessment Of Insulation Systems Research Articles

Dielectric Response Model for Transformer Insulation Using Frequency Domain Spectroscopy and Vector Fitting

This paper proposes a rational approximation-based approach to find positive real parameters for the extended Debye model (EDM), aimed at condition assessment of insulation systems of power transformers. The EDM can model the slow and fast polarization phenomenon, including relaxation mechanisms with different relaxation times within a composite dielectric material. In the proposed approach, the complex permittivity of the transformer’s composite insulation is approximated via rational functions, as given by the vector fitting (VF) software tool, and the EDM parameters are identified from the obtained poles/residues. To guarantee positive real parameters, i.e., a physically realizable circuit, VF is internally modified to calculate the final residues of the rational approximation via a constrained linear least-squares problem without resorting to further post-processing algorithms, as in existing methods, hence without affecting fitting accuracy. The effectiveness of the parametrized EDM is demonstrated in two ways: (a) by reconstructing frequency domain spectroscopy (FDS) curves provided via measurements in new oil-immersed power transformers and (b) by the comparison of the calculated polarization current given by EDM versus real measurements in time domain. The achieved fitting accuracy in most of the cases is above 99 percent for the reconstructed FDS curves, while the polarization current waveform is reproduced with good agreement.

Dielectric response measurement by impulse stimulus on AC: Measurement considerations, and laboratory testing on a bushing

Dielectric response (DR) measurement is commonly used for condition assessment of insulation systems of power components. A method for measurement of dielectric response using the stimulus of natural transients such as lightning and switching impulses was proposed in earlier work. Its desirable features include the ability to make measurements online over a range of frequencies, without requiring a voltage source. This article presents a laboratory demonstration of the method on a 150 kV service-aged transformer bushing, where the stimulus was a standard lightning impulse voltage superimposed on an AC voltage. Several aspects of the measurement and data processing that affect the results are studied experimentally and numerically. The results are compared with low-voltage frequency domain spectroscopy (FDS). Reasonable accuracy for monitoring changes in DR can be achieved by suitable choice of measurement circuit and data acquisition. The study suggests an approach for online monitoring of dielectric properties of power transformer bushings, and diagnostics of defects that affect the high frequency region of DR, such as moisture content.

A method for discriminating original pulses in online partial discharge measurement

Partial discharge (PD) measurement is a promising method for condition assessment of insulation systems in high voltage equipments. Conventional PD measuring systems have some difficulties in the measurements especially in online conditions and noisy environments. There is a growing tendency to use new measuring systems for separation of PD pulses from different sources and rejection of external noises. In this investigation a new ultra wideband PD measuring system and relevant methodology are discussed. The proposed system uses an efficient detection method for extracting PD pulses embedded in noisy signal, meanwhile, a wavelet multiresolution analysis is applied to reduce background and communication noise level. In this paper some new and modified features for pulse classification procedure have been proposed. The proposed system, methodology and features have been implemented on a hydrogenerator and also a power transformer. Finally performance of the system is discussed.