Rectifilter for electric arc plasma plant

This article presents the results of the development and research of a new type of AC-DC converter a filterrectifier (or rectifilter). The object of application of the proposed device is an industrial plant of electric arc plasma processing, the operation of which requires maintaining a constant value of the arc current and is accompanied by a high level of harmonic distortion and consumed reactive energy. From the point of view of the reactive power compensation and harmonic filtering strategy, the rectifilter can be attributed to active electric power filters and FACTS, however, from the position of the main function performed, it is a direct current source, an AC to DC converter, an active rectifier. This work describes in detail the principles of operation of the control system and the hardware architecture of the device. Mathematical modeling of the rectifilter and analysis of the graphs of the consumed alternating current, rectified current, analysis of the harmonic composition and power factor in comparison with the thyristor rectifier of an electric arc plasmatron were carried out.

The proposed solution for constructing a harmonic filtering system and reactive power compensation is as follows. Instead of a controlled rectifier on single-operation thyristors of the plasmatron converter, a rectifier should be installed on fully controlled semiconductor switches with a maximum switching frequency exceeding the order of distorting harmonics. Such a rectifier, on the one hand, will play the role of a current source for the formation and maintenance of the electric arc, on the other hand, it will form the effective consumption of electric current. Based on the value of the required electric arc current, as well as the instantaneous values of currents and voltages in phases at the connection point, the rectifier control system will set the waveform of the consumed current. Ensuring operation speed can also be achieved by optimizing the algorithms for generating control pulses of the control system.

Experimental plant 2.1. Hardware
The rectifier of the experimental plant is a single-level three-phase bridge rectifier on IGBTs with blocking diodes. The schematic diagram of the power unit is shown in Fig. 1.
On IGBT gates provide switching signals from the control system. The control system receives: -measured instantaneous values of phase currents consumed from the main and line voltages at the connection point of the plant (from the measuring unit MB), -measured current values in a DC circuit (from CS current sensor), -the measured values of the potential difference between the anode and cathode (from the voltage sensor VS). The plasmatron rectifier is connected to the main through the reactor blocks L1 -L3 and linearly connected capacitors C1 -C3. They are installed to reduce, respectively, ripple current and voltage. VD1-VD6 diodes are required to block reverse diodes in the IGBTs VT1-VT6. In addition to the CS current sensor, a choke L4 is included in the DC link, which serves to accumulate energy and maintain a constant direct current value. The circuit, consisting of the reverse diode VD7 and resistor R, serves to discharge the inductance, and protects the converter from overvoltage at the time of breaking the DC circuit. The anode of the electric arc plasmatron is connected to the positive contact of the converter, and the cathodeto the negative.

Control System
The essence of the proposed control method is to determine the waveforms of the consumed phase currents, which, on the one hand, will provide the required value of the arc current in the DC link of the converter, and on the other hand, will correspond in harmonic composition and phase shifts to the waveforms of the corresponding phase voltages. The functional diagram of the rectifilter control system is shown in Fig. 2.
The operation of the control system is as follows.
Based on considerations of simplifying the logic of the formation of control pulses, we will limit the number of combinations N = 6 so that three of them and their corresponding inverse combinations commute exclusively two phases at the input of the converter. Thus, the state Q of the converter switches will be determined by the following system of equations: h -the width of the hysteresis zone of the hysteresis controller HC, [A]. It is usually set at 5 ÷ 10% of the nominal consumed phase current.
This system of equations is presented in the control system in the form of a switching combinations table SCT and hysteresis controllers HC.
The difference is determined, as explained above, by the following equation   One of the important elements of the proposed control system is also a prioritizer, the operation of which is determined by the priority calculation algorithm (PCA) and consists in controlling the signal transmission on each of the IGBT control lines depending on the level of harmonic distortion and the magnitude of the phase shift of phase currents. This algorithm allows to solve the problem of the appearance of additional distortions in the waveform of the consumed current during regulation.

Modelling Results
The operation of the proposed system was simulated in MATLAB Simulink. Fig. 3a shows graphs of the phase voltage a u and the consumed phase current a i during the operation of the rectifilter without a prioritizer.
As can be seen from the phase current graph, such a rectifilter operation leads to additional distortions in the current waveform. These distortions appear in cases when the control system simultaneously tries to fulfill several conditions for compensation of linear currents. The result of the prioritizer operation reflects the current graph in Fig. 3b. As can be seen, the waveform of the phase current a i repeats the waveform of the phase voltage a u without distortion. given level of arc current is confirmed by the graphs in Fig. 4. From the graphs of the current it can be seen that the actual value of the arc current corresponds to the reference value. Fig. 4. Graphs of giving and actual arc currents

Conclusions
Making a conclusions about the originality of the proposed solution, one should pay attention to a similar class of devices -active rectifiers AFE (9), which have a voltage inverter topology and are not suitable for systems powered by a current source. There are solutions that allow the use of this type of rectifiers as current sources (6,25), however, the use of direct current at the output of the rectifier as a target parameter of the control algorithm leads to a high level (relative to active power filtering systems) of harmonic distortions of the consumed current and does not allow control reactive energy flows. In view of the use of instantaneous values of phase currents at the input of the converter as a target parameter of the control algorithm in the proposed solution, such a converter should be compared with flexible ac-transmission systems. When comparing the control systems of the filter-rectifier and active power filters, it should be noted that in the latest solutions using the topology of the current source (4), the PQ method is used as an algorithm for determining the reference values of the recovered phase current, based on calculating the power averaged over the period and filtration, and characterized by lower performance in comparison with the proposed method. Also, in such a solution (4), PWM modulation is taken as the main method for generating control pulses, which in this topology differs in significant voltage ripples on the AC side, rather than when using hysteresis regulators used to control rectifier filters.