Fig. 6 Evaluation of PDM minimum serial pulse number
Taking into consideration that applied voltage is dividing by Ca and Cg capacitances, applied voltage across the ozone generation tube must be higher than certain discharge starting value (in tested tube 9kV-10kV). When number of serial pulses is more than three, peak voltage achieves required value of 10kV. Thus, at least four serial pulses are necessary for inverter operating cycles to start a stable discharge. In the developed system, 4 pulses are introduced for producing 10% power; respectively full power level is equal to 40 pulses in one period.
During zero power injection periods, it is effective to apply signal with increased frequency to decrease the rising time of the current when next power injection period begins. Therefore, during zero power periods, the inverter frequency is set to 11kHz. As it is can be obtained from the relations in Fig. 7, on this frequency, output power is almost equal to zero.
Thus, in this ozonizer PDM and PFM (Pulse Frequency Modulation) hybrid control method is applied to provide more effective operation.
Zero-voltage-switching (ZVS) in a wide range of output power for this inverter is mainly achieved by using lossless capacitive snubbing circuit as it is mentioned before, therefore power losses in the inverter are extremely low, and just low efficiency of electric energy-to-ozone conversion and cooling capabilities restrict ozone production output.
Furthermore, as shown on Fig. 8, the output power can be linearly regulated over wide power range. The range of ozone output is from 10% to 100% of full power (3kW) and accuracy of the regulating characteristic is 2.5 % that is equal to one working pulse.
Fig. 7 Frequency characteristics
Fig. 8 Regulating characteristics
5. CONCLUSIONS
In this paper power supply and control schemes for ozone generation silent discharge load have been presented. Ozone generator developed in accordance with pulse density modulation strategy, has been shown high performances of power regulating, high efficiency of the ozone generation and reasonable cost that believed to aid wider applying of ultraviolet and ozone utilizing system in marine shipbuilding.
Although this ozonizer has been demonstrated for industrial purposes; the same design procedures can be applied for smaller-scale ozone generation systems that could be applied on ship.
A novel voltage source type soft-switching series inductor compensated resonant load high-frequency inverter using IGBT power modules has been developed for efficient ozonizer, its experimental operating characteristics were compared with simulation results and have good agreement. New control strategy based on PDM regulation associated with PFM was implemented and its efficiency for such applications as silent discharge type ozonizer has been proved in experiment.
In the future such possibilities as using higher frequencies to increase energetic characteristic of the ozone generation, eliminating the auxiliary control pulses during zero power periods, and comparing with current-fed scheme should be investigated further. Nevertheless, developed system has shown superior performances, which appear to urge wider employment of ozone.
Taking into consideration, that ultraviolet generation is based on the principle of the silent discharge, application of the developed inverter will be performed for UV generation source, which can be used in perspective onboard water treatment systems. On the other hand, ozone and ultraviolet systems seem to be beneficial on passenger vessel and so on, where treatment of a big amount of drinking water and utilizing of wastewater are essential needs.
REFERENCES
[1] Rip G. Rice: 'Ozone in the United States of America-state-of-the-art', 13th ozone World Congress, Vol.2, pp.1029-1040 (1997).
[2] N. Matsumoto and K. Watanabe: 'Foot prints and future steps of ozone application in Japan', 13th Ozone World Congress, Vol.2, pp.1041-1050 (1997).
[3] Jae-Duk Moon and Sang-Taek Geum: 'Discharge and ozone generation characteristics of a ferroelectric-ball/mica-sheet barrier', IEEE Transactions on Industry Applications, Vol.34, No.6, pp.1206-1211 (1998).
[4] Y. Nomoto, T. Ohkubo, S.Kanazawa and T. Adachi: 'Improvement of ozone yield by a silent-surface hybrid discharge ozonizer', IEEE Transactions on Industry Applications, Vol.31, No.6, pp.1458-1462 (1995).
[5] S.M. Cargill, D.G. Edwards: 'Corona screen effectiveness in large machines under high voltage, high frequency transient conditions', IEE Proceedings Electric Power Applications, Vol.145, No.5, 469-474 (1998).
[6] R. Feng. G.S.P. Castle. S. Jayaram: 'Automated system for power Measurement in the silent discharge', IEEE Transactions on Industry Applications, Vol.34, No.3, pp.563-570 (1998).
