Development of an antimicrobial blended white LED system containing pulsed 405-nm LEDs for decontamination applications

This study details the design, build and testing of a prototype antimicrobial blended white light unit containing pulsed red, yellow, green and 405nm LEDs. With a push for alternative methods of disinfection, optical methods have become a topic of interest. Ultra-violet (UV) light is widely known for its antimicrobial properties however; 405nm light has demonstrated significant antimicrobial properties against many common hospital acquired pathogens. In this study, a pulsed, blended, white-light prototype with a high content of 405 nm antimicrobial light, was designed, built and tested. Antimicrobial efficacy testing of the prototype was conducted using Staphylococcus aureus and Pseudomonas. aeruginosa, two bacteria which are common causes of hospital acquired infections. These were exposure to 3 different light outputs from the prototype and the surviving bacteria enumerated. Results showed that the mixed light output provided a much better CRI and light output under which to work. Also, the light output containing 405 nm light provided an antimicrobial effect, with decontamination of 103 CFUml-1 populations of both bacterial species. The other light content (red, yellow, green) had no beneficial or adverse effects on the antimicrobial properties of the 405nm light. The results suggest that with further development, it could be possible to produce an antimicrobial blended white light containing pulsed 405nm light that could supplement or even replace standard white lighting in certain environments

Efficacy of Pulsed 405-nm LEDs for antimicrobial photodynamic inactivation : effects of intensity, frequency, and duty cycle

Objective: This study investigates possible advantages in pulsed over continuous 405-nm LED-light for bacterial inactivation and energy efficiency. Background: Alternative non-antibiotic methods of disinfection and infection control have become of significant interest. Recent studies have demonstrated the application of systems using 405-nm light-emitting diodes for continuous disinfection of the clinical environment, and also for potential treatment of contaminated wounds. Methods: Liquid suspensions of 103 CFU/ml populations of Staphylococcus aureus were subject to pulsed 405-nm light of different frequencies, duty cycles and intensities, and for different lengths of time. Results: Pulsed exposures with the same average irradiance of 16 mWcm2 and varying duty cycle (25%, 50%, 75%), showed very similar performance compared with continuous exposures, with 95-98% reduction of S. aureus achieved for all duty cycles. The pulsing frequency was varied in intervals from 100 Hz - 10 kHz and appeared to have little effect on antimicrobial efficacy. However, when comparing pulsed with continuous exposure, an improvement in inactivation per unit optical energy was achieved, with results showing an increase of approximately 83% in optical efficiency. Conclusions: These results suggest that under pulsed conditions a lower energy consumption and lower perceived brightness could be achieved, thus potentially providing improved operating conditions for medical/infection-control applications without compromising antimicrobial efficacy

Thermoplastic materials aging under various stresses

The most popular cable insulation material used is XLPE due to its excellent electrical and thermal properties. However, it does not lend itself to ease of recycling. As a result of an increase in concern worldwide regarding environmental protection, it is the objective of this work to investigate whether a thermoplastic material could be used to replace XLPE for cable insulation. Among thermoplastic materials, HDPE is regarded as one with the most similar properties as XLPE. Although it is clear that the performance of polymeric material changes with different stresses, especially polymer nanocomposites aging process under AC electric field stresses, there are also not many publications on how a superimposed AC voltage would affect the insulation’s performance in HVDC power systems. This paper reports the dielectric properties of HDPE under thermo-electrical stresses. DC stress with and without a superimposed AC stress were applied in the experiments undertaken. The degradation of materials with change in frequencies are summarized and discussed

Aging behaviour of polypropylene under various voltage stresses

Practical HVDC systems have superimposed AC or transients and the synergistic effect of these factors on polymer aging would be of interest. Although it is clear that partial discharges under AC stress will gradually degrade the insulation behavior of a polymeric material, there are not many publications detailing the effect of superimposed AC voltages on polymer performance in a HVDC power system. Assuming polypropylene (PP) is suitable for use as electrical insulation, this paper considers the behavior of PP under various voltage ratios and temperatures. Factors which cause the degradation of PP will be summarized and explained. To simulate the working condition, electro-thermal aging equipment will be used. Fourier transform infrared spectroscopy - attenuated total reflection (FTIR-ATR) measurement and dielectric spectroscopy measurement will be carried out before and after aging

Assessment of HDPE aged under DC voltage combined with AC harmonic stresses of various frequencies

One of the challenges on the increasing reliance on isolated renewable generation sources is the transmission of power from these sources to centers of power demand. One possible approach is the use of high voltage direct current (HVDC) transmission. The power electronic converters are key components in HVDC transmission system. The converters produce the intended DC voltage for transmission but there may also be AC harmonics superimposed. The superimposed harmonics on the HVDC may have synergistic effects and may lead to further degradation in the cable insulation. Previous research has shown that partial discharge was the main cause of degradation in polymeric insulation under AC stress. However, few publications have demonstrated the effect of combined stress on cable insulation degradation. Additionally, the most popular cable insulation material, cross-linked polyethylene (XLPE), cannot be recycled. Alternative materials which can be recycled have been proposed and one such solution could be thermoplastic materials. In this study, HDPE was investigated as a reference material for thermoplastics and their potential use as insulation in HVDC cables. In this paper the effect of frequency on HDPE degradation under superimposed stresses was studied using the following approaches; equivalent phase resolved partial discharge (PRPD) plots, fourier transform infrared spectroscopy - attenuated total reflection (FTIR-ATR) and dielectric spectroscopy (DS) measurements were carried out. The results show that during aging and with a frequency increase, the voltage of PD events increased which in turn created more polar molecule groups on the surface. The amount of polar molecule groups was found to affect ε' and tanδ, with both increasing when more polar molecules were created. The results show that applying a higher AC frequency enhances polymer degradatio

Methods of characterisation of DC partial discharge in polymeric cable insulation

Partial discharge monitoring is frequently used in AC cable systems, and there exists a strong desire for the same for DC cables within the electrical power industry, given their recent increased use. However, DC PD is a less well understood phenomenon. This paper provides analysis of three methods of partial discharge characterisation: pulse duration and amplitude analysis, frequency-domain spectra analysis, and partial discharge inception voltage analysis in artificially-created voids in polymeric cable insulation samples (polyethylene and polypropylene) under both AC and positive and negative DC excitations. From these a ‘finger-print’ of the defects can be determined based on the distribution of energy within the discharge frequency spectrum, and the inception voltage

Statistical analysis of pulsed micro-discharges and ozone generation in dielectric barrier discharges

Pulsed micro-discharges produced by dielectric barrier discharges (DBDs) in a sub-millimeter gap were investigated under 200 cycles of sinusoidal ac voltage at 5 kHz in this work. The impulsive current in the external circuit was accurately measured by an oscilloscope with a bandwidth of 2.5 GHz and maximum sampling rate of 40 GS/s to calculate the filamentary current in the discharge gap. The amplitude, pulse duration and transferred charge of a single filamentary current and the micro-discharge energy acquired over the 200 voltage cycles were statistically analyzed for different discharge gaps and gas pressures. The micro-discharge parameters and ozone generation efficiencies for different conditions were compared. The ozone production efficiency was found to increase with increasing pressure from 1 bar absolute to 2 bar absolute, and the gap length from 0.2 mm to 0.5 mm. The maximum ozone production efficiency achieved in the work was 217 g/kWh, with a gap length of 0.5 mm, 2.0 bar absolute pressure, and an applied voltage of 5.5 kV at 5 kHz

Impulsive discharges in water : acoustic and hydrodynamic parameters

Underwater spark discharges are used in multiple practical applications including plasma closing switches, water treatment, plasma channel drilling and mineral processing, waste recycling, treatment of metals and medical lithotripsy. Spark discharges in water have been studied for several decades, however, despite significant research efforts and progress in this area, further investigation into the efficiency of plasma-acoustic sources and their optimisation is required in order to expand their practical applications. This paper is aimed at investigation of the electrical and hydrodynamic parameters of underwater plasma-generated cavities, including plasma resistance, energy delivered into the plasma cavity, period of cavity oscillations and characteristics of pressure impulses. Different energy levels, breakdown voltages and gap distances were used in the present study to allow systematic analysis of these electrical and hydrodynamic parameters. Empirical scaling laws which link the maximum acoustic pressure and the period of cavity oscillations with the energisation parameters and the resistance of plasma have been obtained. These empirical functions can be used for optimisation of the plasma-acoustic sources and for tailoring their parameters for specific practical application

A study of energy partition during arc initiation

This paper presents an analysis of energy partition among various physical processes during the formation of an electrical arc. Experimental work was conducted to accurately determine the energy delivered into the discharge. The transient voltage and current waveforms have been measured. An analytical model was developed which allows estimation of the energy partition in the discharge to be performed in order to evaluate risks associated with different energy components: thermal, kinetic-acoustic and light. Approximately 60% of the electrical energy is converted into mechanical work, subsequently contributing to the pressure rise. The results obtained will help in studies of safety considerations regarding hazards associated with plasma discharges in transient faults/sparks and during the onset of arcing faults (flash and blast hazards)

Field-time breakdown characteristics of air, N2, CO2, and SF6

The dielectric performance of gases in insulation systems used in high voltage power and pulsed power applications is a subject of intensive theoretical and experimental investigations. Transient breakdown processes in gases stressed with short, high-field impulses, have been studied for many decades. However, there are still significant gaps in the understanding of the main breakdown processes and mechanisms associated with fast transient breakdown processes in gases. This knowledge is important for the optimization of gaseous insulating systems and for the coordination of gaseous insulation in power and pulsed power apparatuses. This information is also required for the development of gas-filled components such as circuit breakers and plasma closing switches. This article is aimed at the analysis of the field-time breakdown characteristics of air, N2, CO2, and SF6, using kinetic and drift-diffusion approaches. The kinetic approach is based upon the avalanche-to-streamer transition criterion, while the fluid drift-diffusion model requires self-consistent numerical solution of the continuity equations for charged species, and the Poisson equation for the electric field. The time to breakdown as a function of the applied field was obtained for all investigated gases. The obtained analytical results agree well with the experimental data reported in the literature, which suggests that both approaches can be used for insulation coordination, and for the development of gas-insulated power and pulsed power systems and components