From these relations parameters of this equivalent circuit can be obtained together with some other discharge characteristics. Thus, distance between lines of the Lissajous figure crossing voltage axis is equal to 2Vz.
Electric energy utilized during one working period is related to the area of the Lissajous figure and full discharge energy Po can be estimated as,
Po = f・Co・Sl (3)
where, f; frequency of AC voltage
Co; auxiliary condenser for current integration
Sl; area of lissajous figure
3. HIGH-FREQUENCY INVERTER SYSTEM
Figure 4 displays schematic configuration of the power conversion conditioning and processing system for the inverter type ozonizer developed here.
It consists of three-phase bridge rectifier with a DC smoothing capacitor, voltage source-fed series inductor-compensated load resonant inverter using IGBT modules and a novel prototype of high-frequency (7kHz) ozone generation tube.
Due to using of IGBTs with lossless capacitors, soft-switching commutation can be provided over all operating range in experiments.
Actual efficiency of the conversion of electric energy to ozone does not exceed 10-20 percents in up-to-date generation tubes that restrict achievable efficiency of full system. Therefore, even if the efficiency of the power supply system is high, the energy converted directly into ozone does not exceed one-fifth part of full consumed power. This makes full system efficiency minor important factor and achieving maximum level of the power delivered to the load and therefore providing maximum ozone output becomes the central problem. Thus, since the main objective is the delivering to the load power as high as possible, the resonant type inverter topology appears to be the most effective.
The capacitance of ozone generation tube is compensated by the additional series inductance Ls, which also includes the leakage inductance of high-frequency high-voltage transformer in simulation model. Since operating power of the developed system is quite high, the physical size of the compensating capacitor is relatively large, thus possibilities of applying higher frequencies and using only transformer leakage inductance as compensation should be investigated further.
Optimum value of the compensating inductor Ls can be estimated by simulation, to realize series resonant conditions for such changing capacitive load. To provide soft-switching conditions, lossless snubbing capacitors are connected in parallel with the active switches. Since working frequency of this inverter is constant, soft-switching commutation is fulfilled over all regulation range as was observed in experiment.
4. PERFORMANCES OF THE CONTROL SCHEME
The principle of PDM control scheme is basically illustrated in Fig. 5.
Control procedure of the inverter is based on variation of pulse density by changing number of pulses in operation period, keeping working frequency constant at 7kHz, that represents PDM (Pulse Density Modulation) strategy.
Operation time is divided into the operation cycles; each operation cycle divided, in its turn, into power injection periods and zero power periods. During power injection periods, working pulses of full length and frequency of 7 kHz are applied to the load. Zero power periods not include working pulses, thus, no ozone is generated at this time. The length of the operation cycle is kept constant and delivered power is regulated by changing the duration of the power injection period.
Peak voltage of the ozone generation tube cannot achieve discharge starting value when number of pulses is too low. Figure 6 shows relation of the peak voltage across ozone tube and output power of ozone generation tube depending on number of pulses.
Fig. 4 System configuration of inverter-fed type ozonizer
Fig. 5 Principle of PDM control
