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Online ISSN 1751-8830 Print ISSN 1751-8822

IET Science, Measurement & Technology

Volume 2, Issue 4, July 2008

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Volume 2, Issue 4

July 2008

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    • Streamer propagation along room-temperature-vulcanised silicon-rubber-coated cylindrical insulators
      • Author(s): P.N. Mikropoulos
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 187 –195
      • DOI:  10.1049/iet-smt:20070062
      • Type: Article
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      • p. 187 –195 (9)
        Results concerning streamer propagation along cylindrical insulators coated with room-temperature-vulcanised silicon rubber coatings are presented. Experiments were carried out in a three-electrode arrangement consisting of a 12 cm long parallel-plane gap with an auxiliary needle in the earthed anode. Insulators could be inserted between the plane electrodes adjacent to them and almost in contact with the needle. Positive streamers were initiated by applying at the needle electrode a pulse voltage variable in amplitude and propagated over the insulators towards the upper plane electrode which was stressed by a negative DC voltage. The streamer propagation field at conditions from threshold up to the stable streamer propagation and the associated velocity were measured. The dependence of the streamer velocity on the electric field was investigated with the type of coating as parameter. Both the streamer propagation field and the associated velocity of propagation are higher, when a streamer propagates along the coated insulators, than the reference bare nylon insulator depending on the coating employed. Streamers might be used as an effective probe for studying the surface dielectric behaviour of coated insulators hence also for evaluating the various coatings used to improve the contamination performance of outdoor high-voltage insulators.
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      Positive corona progression over profiled insulator surfaces near the breakdown condition
      • Author(s): B.H. Tan ; H. Rodrigo ; N.L. Allen
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 196 –207
      • DOI:  10.1049/iet-smt:20070042
      • Type: Article
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      • p. 196 –207 (12)
        Experimental observations are presented of the development and propagation of positive impulse corona near the breakdown condition in a point plane gap in air at atmospheric pressure in the presence or absence of insulators having simple cylindrical and profiled surfaces. Optical techniques have been used and the effects of the insulator materials have been demonstrated. All the results with corona were obtained using an applied peak impulse voltage of 0.9U50 in each case, where U50 is the 50% breakdown voltage of the gap. Comparisons were made between air, polytetrafluoroethylene (PTFE) and porcelain surfaces. It has been demonstrated that corona development in the gap is dependent on the relative permittivity of the insulator material, the profiles and the proximity of the insulator to the high voltage point electrode. Simultaneous photomultiplier records and photographs taken on fast film with Quartz optics show details of corona development using the UV region of the spectrum.
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      Evolutionary optimised nonlinear function for linearisation of constant temperature anemometer
      • Author(s): A. Abudhahir  and  S. Baskar
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 208 –218
      • DOI:  10.1049/iet-smt:20070048
      • Type: Article
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      • p. 208 –218 (11)
        A simple software method to derive linearised output for constant temperature anemometer (CTA) is presented. The method uses a nonlinear ratiometric–logarithmic function, which consists of two parameters whose optimal values are determined by minimising the objective function (mean square error) to improve the linearity of CTA signal. Covariance matrix adopted evolutionary strategy algorithm, which generates optimal values consistently, is employed to determine the optimal values of linearisation parameters. The proposed linearisation algorithm was implemented using LabVIEW 7.1 Professional Development System in a personal computer that provides the facility to interface with the National Instruments data acquisition module PCMCIA-NI DAQCard-6024E. Experimental studies have been carried out using practical air-flow velocity measurement data obtained form Dantec Dynamics practical guide. The performance measures such as full-scale error and root mean square error are considered to compare the performance of the proposed method with the methods reported for linearisation of transducers. Experimental results reveal that the proposed evolutionary optimised nonlinear function-based software lineariser works well for CTA, and it can be suitable for computer-based flow measurement/control systems.
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      New vector signal measuring system, featuring wide bandwidth, large dynamic range and high accuracy
      • Author(s): B.K. Sternberg  and  S.L. Dvorak
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 219 –232
      • DOI:  10.1049/iet-smt:20070088
      • Type: Article
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      • p. 219 –232 (14)
        A new measurement system, with two receiver channels per measurement port, has been developed that provides absolute magnitude and absolute phase relationship measurements over wide bandwidths. Gain ranging is used at radio frequency to provide optimum noise performance and a swept yttrium iron garnet (YIG) preselector filter is used to avoid spurious signals. A new absolute vector error correction method is used to calibrate the measurement system in order to allow for absolute vector measurements, and it also removes the time-varying responses caused by the swept YIG preselector filters. A quasi-reciprocal mixer with a characterised non-reciprocal ratio is used to provide the absolute calibration standard. The two receiver channels can be adapted to a wide variety of applications, including wide bandwidth vector signal analyser measurements, mixer measurements and harmonic measurements. The two channels can also be used as an absolute calibrated transmitter/reflectometer.
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      Mechanism behind high-frequency circulating bearing currents in view of motor structures
      • Author(s): T. Kovanen ; L. Söderlund ; L. Kettunen
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 233 –243
      • DOI:  10.1049/iet-smt:20070099
      • Type: Article
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      • p. 233 –243 (11)
        Frequency converter feeds can cause high-frequency current pulses through motor bearings. Consequently, the lifetime of bearings may decrease remarkably. A study of the effect of motor structures on these currents may offer a way out of the problem. The conditions under which circuit analysis can be applied in a model taking into account the electric and magnetic fields inside the motor are reviewed. This investigation is applied to the mechanism behind the circulating type bearing current. This leads to a comparison of a modified model with a previously utilised one. Results indicate that the present models are somewhat oversimplified.
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      Modifying the feature-selective validation method to validate noisy data sets
      • Author(s): J. Knockaert ; J. Peuteman ; J. Catrysse ; R. Belmans
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 244 –257
      • DOI:  10.1049/iet-smt:20070063
      • Type: Article
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      • p. 244 –257 (14)
        Objective validation and ranking of measurements and simulations may be done by methods such as feature selective validation (FSV). FSV is used to compare two EMC-measurement results. Owing to the noisy nature of these type of data, the FSV results are corrupted. The reasons are discussed and solutions are proposed to make FSV feasible in a broader area of applications. The final solution is a combination of denoising the data and changing the weight of the data to be in accordance with our visual interpretation.
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      Quasi-reciprocal mixer as an absolute vector signal standard at microwave frequencies
      • Author(s): B.K. Sternberg  and  S.L. Dvorak
      • Source: IET Science, Measurement & Technology, Volume 2, Issue 4, p. 258 –267
      • DOI:  10.1049/iet-smt:20070089
      • Type: Article
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      • p. 258 –267 (10)
        A reciprocal mixer has the potential to extend an absolute calibration of magnitude and phase relationships at baseband to an absolute calibration at microwave frequencies. Unfortunately, recent publications have shown that mixers are not in general reciprocal devices. The authors' tests have shown that the up- and down-conversion transmission responses for a mixer change with time and temperature. The ratio of the up- and down-conversion transmission responses, however, can be remarkably constant with time and temperature for some mixers. The authors refer to this ratio as the characterised non-reciprocal ratio (CNR) for the mixer. By using a new calibration circuit, they are able to utilise this CNR for a quasi-reciprocal mixer and provide an effective absolute vector calibration standard at microwave frequencies.
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