Load characteristics of plastic film high voltage

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Analysis of load characteristics of high voltage corona treatment of plastic film in series resonance mode

Abstract: corona treatment technology of plastic film is widely used in packaging and printing industry. In order to enhance the surface adhesion of plastic film, surface corona treatment must be carried out. The characteristics of gas discharge in corona treatment are introduced. The equivalent circuit and volt ampere characteristics of corona load working in series resonance mode are analyzed in detail, and the equivalent circuit model is given. Through the simulation of the model, it is compared with the experimental test

key words: corona treatment; Dielectric barrier discharge; Series resonance

1 introduction

corona treatment technology is to use high-voltage discharge technology to ionize the air between the discharge electrodes into corona discharge. When the treated object such as polymer receives discharge through the discharge space, its surface will generate polar groups, and at the same time, strong ion impact will coarsen the treated surface, so as to enhance the penetration and adhesion of ink and glue on the surface of the treated material. Corona load is similar to ozone generator and belongs to dielectric barrier discharge. Generally, the corona treatment device uses the series resonance technology. The inverter outputs 5 ~ 8Kv high-voltage square wave with a frequency of about 20kHz through the step-up transformer, and the load resonates with the leakage inductance of the step-up transformer to the voltage required by the process (10 ~ 20kV). Although thin film corona treatment is widely used, there are few domestic manufacturers, and there is little research on the equivalent circuit and volt ampere characteristics of the load. This paper makes experiments and Simulation Analysis on it

2 corona load characteristic analysis

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2.1 corona discharge characteristic

corona discharge is a dielectric barrier discharge, and the medium is generally rubber or ceramic with good heat resistance. Figure 1 is the schematic diagram of dielectric barrier discharge of corona treatment device

Figure 1 Schematic diagram of dielectric barrier discharge

the medium is only covered on a single electrode, with a thickness of LD and a discharge air gap of LG. When the voltage acting on the electrode is vs, it is assumed that the medium density is uniform, while the electric field strengths ed and eg between the medium and the discharge gap are different


then vs=lded + lgeg

therefore, the electric field intensity on the medium and air gap is

ed=, eg=

where: G and D are the dielectric constants of the discharge air gap and the medium respectively

due to the existence of dielectric layer between electrodes, the working voltage of dielectric barrier discharge must be alternating. According to the frequency difference of alternating voltage, the discharge characteristics are different. Usually it can be divided into low frequency dielectric barrier discharge and high frequency dielectric barrier discharge after 10 years of exploration. The frequency range of the former is 50Hz to 10kHz, and the frequency of the latter is above 100kHz. The equivalent circuit of these two mediums 5. The detection and blocking discharge of finished and semi-finished products is shown in Figure 2

(a) low frequency (b) high frequency

Figure 2 equivalent circuit of dielectric barrier discharge at different frequencies

in Figure 2, CD is the dielectric capacitance, CG is the air gap capacitance, and R is the equivalent resistance of discharge, Usually CG CD, VOP is the peak value of the applied voltage, and V * and VOP * are the voltage values when the integral current in the circuit is zero under low-frequency and high-frequency conditions respectively.

Figure 3 shows the relationship between the air gap voltage and the external applied voltage. In the electrical characteristics of dielectric barrier discharge, the discharge voltage VD is an important electrical parameter, and its size is directly related to the discharge power. An average discharge voltage VD can be defined, which is a Constant. When the voltage VG on the discharge gap is less than VD, no discharge occurs. At this time, the circuit is composed of dielectric capacitance CD and gap capacitance CG in series. When the discharge gap voltage VG reaches VD, micro discharge occurs and remains until the external voltage reaches the maximum value VOP. During the whole time of micro discharge, the value of VG VD and discharge voltage VD is between the voltage value when the micro discharge occurs and after the cut-off. This value is an average value, which mainly depends on the composition of the discharge gas, the particle concentration in the gap and the gap width

Figure 3 Relationship between external applied voltage vs and discharge gap voltage VG

3.2 corona load equivalent circuit

corona discharge property is equivalent to a nonlinear lossy capacitor, which is simulated with CG and RG, and the drum with dielectric on the surface is simulated with CB. The equivalent circuit is shown in Figure 4. For the circuit in Figure 4 (a), CG and CB are connected in series before the air gap is broken down; After breakdown, the nonlinear gas discharge is connected in series with CB. This circuit is only roughly equivalent, and the characteristics in the process of gas discharge cannot be equivalent

(a) (b)

Figure 4 several corona discharge equivalent circuits

Figure 4 (b) is another equivalent circuit, considering the loss of the air gap, using RG to approximate the energy loss in the equivalent air gap, and RS represents the resistance of the load conductor and the leakage resistance of the capacitor. Although the energy loss in the process of gas discharge is considered in this circuit, the constant voltage characteristic in the process of discharge is not shown

corona treatment under series resonance uses the parasitic leakage inductance of step-up transformer and the series resonance of corona load. Figure 5 shows the load equivalent circuit based on the experimental waveform analysis. LP in the figure is the leakage inductance of the step-up transformer, which is generally in tens of MH. The equivalent capacitance value of the load is mainly determined by CG, and RG is the loss in the air gap, which is a variable resistance with large resistance value. In order to achieve good corona treatment effect, the general load resonance frequency is about 20kHz. Considering that 20kHz is in a medium and high frequency band, the equivalent model is established by combining the two cases in Figure 2. When the leakage inductance of the transformer cannot meet the requirements of resonant frequency, the primary side or secondary side can be connected in series to match the load. At this time, LP is the total inductance. The value of RG is a quantity related to temperature. With the increase of the temperature of the whole treatment load, the equivalent resistance decreases and the output current increases

Figure 5 corona load equivalent circuit

when the air gap is not broken down, the load is equivalent to two capacitors in series. When the output voltage of the inverter increases from zero to the rated voltage, the electrode discharges from partial sparse discharge to countless random irregular micro dense discharge channels, and the equivalent capacitance changes nonlinearly from small to large. After air gap breakdown and discharge, it is similar to a quasi constant voltage source and loses energy at the same time. At this time, the voltage change on the air gap is not obvious when the output voltage of the inverter is increased

3.3 volt ampere characteristics of load

a 12KW corona treated high frequency and high voltage generator is designed. In the experiment, the volt ampere characteristics of load are analyzed by recording the voltage and current (average value) on the DC bus of inverter. Each of the high-voltage discharge electrodes used is 1.6m long and consists of four knife edges. Figure 6 shows the volt ampere characteristic curve at different voltages from before the air gap breakdown to the full power output DC bus. The circuit works in a slightly inductive resonant state, and the current lags behind the voltage by a small angle. Since the current of the load is related to the magnitude of the phase-locked hysteresis angle set in the inverter control circuit, the data measured in the experiment is the volt ampere value in this state (the current hysteresis angle is about 10)

Figure 6 corresponds to the load volt ampere characteristics of the DC bus

in Figure 6, 7 electrodes and 9 electrodes. It can be seen from Figure 6 that when corona discharge begins (about 100V), the current increases rapidly. When it reaches almost complete discharge (about 300V), increase the output voltage and slow down the current increase. At this time, most of the energy consumed is in the air gap discharge. The resonant frequency of the load decreases with the increase of the output power because the aforementioned equivalent capacitance increases. The output power of corona discharge is related to the resonant frequency f. under the same conditions, the higher the frequency, the greater the output power. Therefore, in order to obtain sufficient processing power, we must try to increase the resonant frequency, one of which is to reduce the leakage inductance of step-up transformer

4 simulation and experimental results

the simulation model is established by the equivalent circuit in Figure 5, and pspice9.2 is used to simulate for a specific condition. The resonant frequency of the whole circuit is mainly determined by LP and CG, and its parameters are lp=38mh (the same as the transformer leakage inductance measured in the experiment), cg= 1.67nf, cd=500nf, rg=10k. At this time, the resonant frequency of the circuit is about 20kHz. The step-up transformer model is omitted and directly replaced by 20kHz, 200V square wave power supply, The breakdown voltage of the two regulators is set to 500V

Figure 7 (a) shows the simulated voltage and current waveform. The current waveform is a distorted sine wave. In the process of voltage jump from positive to negative or negative to positive, the current waveform is distorted, which is caused by the constant voltage source characteristics after air gap breakdown. When the applied electric field jumps, the air gap discharge extinguishes and the reverse electric field is re established. The distortion of this waveform can be seen from the simulation waveform and the experimental waveform. Strictly speaking, this is no longer in the series resonance state. After the resonance voltage reaches a certain value, it is clamped on the air gap average discharge voltage VD to prevent it from rising further

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the output voltage and current waveform measured in the experiment is shown in Figure 7 (b), in which the voltage is 50v/div and the current is 25a/div. It can be seen from the waveform diagram that the circuit works in a slightly inductive resonant state, and the voltage and current waveform is close to ideal. The burr near the current peak is caused by corona discharge

(a) simulation waveform

(b) experimental waveform

Figure 7 Simulation and experimental voltage and current waveform

5 conclusion

based on Simulation and experiment, this paper analyzes the load characteristics and equivalent circuit of high voltage corona treatment on the surface of plastic film. The gas shows constant voltage characteristics during corona discharge. The nonlinearity and variability of corona load also bring some difficulties to the design of inverter. The realization of corona treatment by using series resonance to meet the requirements of output high voltage and power has been widely used in industry. The analysis of load characteristics is the basis of reasonable design of corona treatment generator

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