PD activity in void type dielectric samples for varied DC polarity

This paper discusses the DC testing of a dielectric sample (with voids) under DC conditions. The ramp test method was employed to assess whether the tests were repeatable. Three DC ramp tests were performed in quick succession. The polarity of the first/third tests was positive and the polarity of the second test was negative. The resultant partial discharge activity was analyzed for each series of tests and compared for positive and negative conditions. The results show that the inclusion of a negative ramp between two positive ramps enabled similar PD activity to be recorded in the first and second positive ramp tests. The negative cycle was used to re-initialise the test sample and allows trapped charge to migrate enabling similar PD activity in the subsequent positive ramp test. Changes in PD behavior were observed in the distribution of PD data between the tests

Long term testing and analysis of dielectric samples under DC excitation

This paper details testing conducted under DC conditions on a dielectric sample containing internal voids. The DC testing was conducted using a ramp method to vary the voltage applied to the dielectric sample. The dielectric sample was de-energised for a week prior to two separate identical ramp tests and the results are presented showing the variability of PD activity. After the second ramp test an additional ramp test was performed in quick succession and PD activity was reduced, emphasizing the importance of de-energising the sample between tests. A major challenge associated with void type dielectric samples is ensuring that repeatable results are generated and possible approaches are discussed

External lightning protection system for wind turbine blades - power performance

Previous studies conducted by the authors on the aerodynamic performance of turbine blades have shown unfavourable effects of externally mounted lightning down conductors. Owing to this, one could ask whether or not externally mounted lightning down conductors could linearly affect the power performance of the whole wind turbine as well. Therefore, this paper presents a study of the power performance of one wind turbine with an externally mounted lightning down conductor. An untwisted blade profile of NACA 4418 with and without external conductor was used on the full wind turbine model. Numerical simulations were carried out on turbine power output derived from Blade Element Momentum (BEM) theory. The results are compared and discussed. The preliminary results indicate that degradation in power performance may not be significant

Antibacterial activity of Sargassum polycystum C. Agardh and Padina australis Hauck (Phaeophyceae)

Seaweeds are used in pharmaceutical and biochemical applications as they possess interesting biological activities that contribute to the discovery of natural therapeutic agents. In this study, the antibacterial activity of n-hexane, dichloromethane and methanolic extracts of brown seaweeds (Phaeophyceae), Sargassum polycystum C. Agardh and Padina australis Hauck, was examined using the disc diffusion and broth microdilution methods. The bioactivity of the seaweed extracts was expressed as minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The antibacterial activity against Gram-negative bacteria (beta-lactamase positive and negative Escherichia coli, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus, Bacillus cereus) was discussed. Gram-positive bacteria especially B. cereus was more susceptible to the seaweed extracts (MIC = 0.130 to 0.065 mg/ml). Generally, S. polycystum extracts exhibited higher bacteriostatic activity (lower MICs) against all the tested bacterial strains when compared with P. australis. However, P. australis extracts showed a narrow spectrum of bactericidal activity against B. cereus. n-Hexane extracts of S. polycystum exhibited promising bacteriostatic agents against B. cereus (MIC = 0.065 mg/ml) with MIC value lower than the standard MIC of potential antimicrobial drug (0.100 mg/ml). Since only crude seaweed extracts were tested in this study, further purification and isolation of bioactive compounds from the extracts are essential in future studies in order to optimize their antibacterial activity.Key words: Phaeophyceae, disc diffusion test, minimum bactericidal concentration (MBC), minimum inhibition concentration (MIC)

Transfer function characterization for HFCTs used in partial discharge detection

High frequency current transformers (HFCTs) are widely employed to detect partial discharge (PD) induced currents in high voltage equipment. This paper describes measurements of the wideband transfer functions of HFCTs so that their influence on the detected pulse shape in advanced PD measurement applications can be characterized. The time-domain method based on the pulse response is a useful way to represent HFCT transfer functions as it allows numerical determination of the forward and reverse transfer functions of the sensor. However, while the method is accurate at high frequencies it can have limited resolution at low frequencies. In this paper, a composite time-domain method is presented to allow accurate characterization of the HFCT transfer functions at both low and high frequencies. The composite method was tested on two different HFCTs and the results indicate that the method can characterize their transfer functions ranging from several kHz to tens of MHz. Results are found to be in good agreement with frequency-domain measurements up to 50 MHz. Measurement procedures for using the method are summarized to facilitate further applications

Effect of voltage reduction in minimising partial discharge activity in cables - experimental study

A common cause of insulation degradation in high voltage (HV) cables is due to partial discharges (PD). PD may be initiated through imperfections and contaminants present in the insulation. It can be hypothesized that a reduction in system voltage can potentially reduce PD, which will correspondingly extend the service life of the cable. Currently, industry voltage statutory requirements permit ±6% tolerance setting on nominal voltage on distribution networks. Such ±6% voltage reduction on may have little or adverse effect on PD magnitude which depends on the nature of defect. Hence the accuracy and type of measuring PD while reducing system voltage is more critical for this application. Power cable as a low pass filter which attenuates and disperse the PD pulses. In this paper effect of voltage reduction on PD is investigated using standard electrode geometries such as point-plane, point-point and point- rod using IEC60270 measurement system together with frequency domain measurements. It has been observed that PD frequency characteristics at different voltages varies in the wideband frequency spectrum

Rough set theory applied to pattern recognition of partial discharge in noise affected cable data

This paper presents an effective, Rough Set (RS) based, pattern recognition method for rejecting interference signals and recognising Partial Discharge (PD) signals from different sources. Firstly, RS theory is presented in terms of Information System, Lower and Upper Approximation, Signal Discretisation, Attribute Reduction and a flowchart of the RS based pattern recognition method. Secondly, PD testing of five types of artificial defect in ethylene-propylene rubber (EPR) cable is carried out and data pre-processing and feature extraction are employed to separate PD and interference signals. Thirdly, the RS based PD signal recognition method is applied to 4000 samples and is proven to have 99% accuracy. Fourthly, the RS based PD recognition method is applied to signals from five different sources and an accuracy of more than 93% is attained when a combination of signal discretisation and attribute reduction methods are applied. Finally, Back-propagation Neural Network (BPNN) and Support Vector Machine (SVM) methods are studied and compared with the developed method. The proposed RS method is proven to have higher accuracy than SVM and BPNN and can be applied for on-line PD monitoring of cable systems after training with valid sample data

Laboratory and field partial discharge measurement in HVDC power cables

A range of experimental and field measurements of partial discharge (PD) activity under high voltage direct current (HVDC) conditions have been conducted with the goal of developing effective monitoring techniques for PD in HVDC cables and ancillary equipment, particularly in offshore renewable energy HVDC grid installations. Laboratory measurements on insulation test objects and cross linked polyethylene (XLPE) cable samples have been conducted to better understand the characteristics of PD activity under direct current (DC) stress in comparison with AC. In addition, long-term PD measurements carried out at both an HVDC cable aging laboratory and an in-service HVDC interconnector circuit are presented together with a description of the monitoring system architecture