MULTILEVEL INVERTER TO GENERATE VOLTAGE WITH COMPLEX HARMONIC COMPOSITION
Authors: Mirgorodskaya Ekaterina E. , Mityashin Nikita P. , Mamonychev Mikhail E. , Tomashevsky Yury B. , Artyukhov Ivan I. 
Published: December 2024

The paper deals with the problem of forming the desired and controllable harmonic composition of the output curve of a multilevel voltage inverter. The problem is relevant when creating installations for multi-frequency induction heating and metal melting.

The purpose of the study is to develop an effective method to form the output voltage of power sources for multi-frequency induction heating installations, the curve of which has a given harmonic spectrum.

Materials and methods. The theoretical part of the work was carried out on the basis of the circuit base of multi-level voltage inverters and switching DC converters using numerical analytical methods of studying nonlinear electrical circuits, methods of solving nonlinear differential equations and approximate harmonic analysis, as well as simulation modeling methods using the MATLAB/Simulink mathematical package. Experimental verification of the theoretical results was carried out in laboratory conditions on the example of a multi-level inverter generating a voltage curve with three operating harmonics. ACS758 series sensors and a four-channel RIGOL DS1104Z digital oscilloscope were used to record the results.

Research results. Achieving of the purpose is based on the use of a universal multilevel voltage inverter, the circuit scheme of which does not depend on the levels number of the generated curve, which ensures a minimum number of power elements for any complexity of the curve spectrum. The method of forming of the output curve of this inverter consists in alternately connecting of the output capacitors of two direct voltage impulse converters to the input of a single-phase bridge voltage inverter using a transistor switch. In this case, voltages of different parity levels are formed by capacitors in the order of their succession, as a result of which a constant-sign multilevel voltage is formed on the input of the bridge inverter, corresponding to the required alternating-sign multilevel voltage. The law of the transistor switching of the bridge inverter ensures the conversion of its input voltage into a load voltage with a given harmonic composition. The peculiarity of the developed converter control method is that the transistor switching of the bridge inverter is carried out in accordance with the change in the sign of the generated curve. The method contains an algorithm to determine levels values formed by capacitors of impulse converters that realizes the required spectrum of the inverter output curve. The efficiency of the proposed method is illustrated by computer modeling of the converter when voltage with four working harmonics of the spectrum (i.e. the required spectrum harmonics) is generated.

The quality indicators of the realized spectrum of the voltage curve are proposed and the analysis results of the influence of the levels number on values of these indicators are presented. The efficiency of the proposed methods is demonstrated experimentally with a laboratory model of the converter.

Conclusions. The paper proposes a method to form the output voltage of a power source, the curve of which has a given harmonic, the efficiency of which is confirmed by the results of computer modeling and laboratory experiments. The proposed technical solutions made it possible to generate an output voltage, the spectrum of the curve of which contains four or more harmonics with specified amplitudes with a minimum number of power elements of the circuit. The paper also proposes models of quality indicators of the realized spectrum of the voltage curve and presents the results of the analysis of the influence of the number of levels on the values of these indicators. The developed methodology allows the development in the direction of controlling the spectrum of the generated voltage in real time, as well as non-periodic alternating voltage that changes according to a given program in accordance with the requirements of the technological process.