B. Nageshwar Rao

Study on diagnosis of HV rotating electrical machine insulation based on structural changes due to ageing

The model bars of generator stator winding manufactured under resin poor process and laminates made using both resin rich and resin poor systems were subjected to accelerated electrical, thermal and combined stress ageing. The insulation curing status and behaviour on ageing was studied by thermo analytical techniques like Differential Scanning Calorimeter (DSC), Thermo gravimetric analysis (TGA) and Thermo Mechanical Analysis (TMA). Various dielectric properties like Dissipation factor, Capacitance, Partial Discharge, Polarization currents etc. were measured at regular intervals during ageing. The structural and chemical changes that could have occured during ageing were examined using techniques like Fourier Transform Infrared Spectrometer (FTIR), Scanning Electron Microscope (SEM), and X-ray energy spectral analysis. The glass transition temperatures of fresh (sound) and thermally aged samples determined by DSC and TMA indicated post-curing effect to some extent. From the TGA study it was found that there was no change in total weight loss of sound and aged specimen. The decomposition temperatures of the specimens though are not that distinct from the normal curves, the derivatives precisely show the peaks at which decomposition is occurring. FTIR spectra of aged specimen showed a new peak at 1720 cm−1 resulting from carbonyl (C=O) group in resin rich laminate. SEM Pictures showed a significant difference in surface morphology on account of heterogeneity.

Optimum threshold estimator for de-noising partial discharge signal using wavelet transform technique

Condition monitoring of electrical power equipment is a vital step in extending the lifetime of inservice equipment which are quite aged. Among the various condition assessment techniques partial discharge (PD) monitoring has emerged as one of the useful technique for condition assessment of high voltage equipment. However the major challenge during PD measurement at site is that PD signals are severely affected by external noises and disturbances like white noise, random noise, Discrete Spectral Interferences (DSI), which are generated due to broadcasting stations, stochastic noise and pulses from power electronics at site conditions. Extracting PD signals from these noises is a challenging task. Several methods have been proposed by researchers for de-noising PD signals. This paper proposes a new method of threshold estimation for de-noising PD signals using wavelet transform (WT) technique. The threshold estimation is done by considering the entire range of interferences thereby eliminating most of the noise without affecting the original signal. This method was used to de-noise simulated PD signals and the results show that the new threshold estimator produces better results when compared to the existing methods. The performance of the proposed method is evaluated using four different parameters namely, Signal to Noise Ratio, Cross Correlation Coefficient, Pulse Amplitude Distortion and Mean Square Error where it consistently outperformed the existing methods. The results are presented and discussed.

Application of RVM technique to study the polarisation and depolarisation characteristics associated with thermal ageing of epoxy mica composite

The Recovery voltage measurement technique was used to investigate the polarization processes in the dielectric to assess the insulation ageing. Two different types of insulation systems are evaluated, resin poor and resin rich mica composites. This paper presents and discusses the data on polarization and depolarization characteristics of thermally degraded epoxy mica composites. Beyond polarization spectrum, additional spectra slope spectrum and peak time spectrum of Recovery voltage measurement technique were used. The combined spectrum: a plot of initial slope (V/s) against Max. Recovered voltage, Vr(V) showed a clear peak in the form of “bun” and “nose” distinctly which probably could give more information on the bigger polar species and the polarization phenomena. Multiple peaks are seen in case of thermally aged samples which clearly exhibit the degradation phenomena.

Failure analysis of Medium Voltage Cable accessories during qualification tests

Medium Voltage Cable Network plays an important part in reliability of Power Distribution system. The Joints and terminations which form an integral part of the cable network are the weakest link and prone to failures during service due to various operational stresses and ageing. The integrity of accessories with respect to various service conditions are evaluated through qualification tests. Failure of cable accessories during qualification tests occur due to various reasons such as wrong selection accessory components, poor design, Poor quality of materials / Improper Conductor connections etc. The authors have evaluated a large number of cable accessories as per national and international specifications in the laboratories of CPRI. In this paper, analysis of cable accessories failed during Qualification tests are presented and discussed.

Assessment of water logged cables by very low frequency tan delta testing technique

Distribution cables rated 22 kV which were submerged under water for more than 45 days due to flash floods in one of the processing industry were assessed using VLF technique in addition to other techniques like insulation resistance and dc voltage withstand test. Special technique used for extraction of water from the cable termination is discussed. The consequences of bad crimping of cable lugs and improper cable terminations on the penetration of water into the cable length are highlighted. The study showed abnormal dielectric losses in the cable insulation as a result of highly polar contaminants in the cable. The precaution to prevent water entry into the cable are suggested in this paper.

Evaluation of fire gases from polymeric materials

This paper presents and discusses the toxicity of the products of combustion in terms of small molecular species arising when a sample of a material is completely burnt in excess air under specified conditions. The evaluation of the toxicity is made through the determination of the following gases: Carbon oxides, halogen acids, Prussic acid, nitric oxides, acrylonitrile, phenol, phosgene etc. Cable sheathing materials, combustible materials like fibre reinforced plastic used in rail coaches for interiors: the floors, side wall, window frames etc and floor covering materials: Nylon carpets, styrene butadiene rubber mats; Curtains, Drapes, seat cushioning materials/covering materials have been evaluated. The results are presented and discussed.

Assessing smoke and fire hazard of burning electric cables

This paper presents and discusses the techniques for assessing the fire smoke hazard of burning electric cables. Power cables, communication cables, data cables and wires used for various applications in Power plant, Refineries, automobiles and other applications have been evaluated for heat release measurement. The behavior of cables have been studied at various thermal irradiances. Power cable Individual components have been evaluated at different heat fluxes. Parameters like time to ignition, mass loss rate, total heat release, heat of combustion, specific extinction area of smoke, rate of production of yields CO/CO2 ratios are also measured and discussed.

Low frequency dielectric response of thermally degraded epoxy-mica insulation

Dielectric spectroscopy measurement is essentially a dissipation factor measurement performed at multiple frequencies ranging from few mHz to kHz. Low frequency dielectric spectroscopy measurement has been performed on number of sound and thermally aged epoxy-mica representative laminates and on actual motor coils. Dielectric loss angle, Power Factor, capacitance was measured at frequencies ranging from 1 mHz to 1 kHz. The dielectric response exhibited by of thermally aged epoxy-mica insulation at lower frequencies shows the predominant influence of interfacial polarization on the total dielectric response of epoxy-mica layer.