This paper presents a new soft-switching system for three-phase two-level voltage source inverters. In this system, capacitors are not connected in parallel to the main transistors, and inductors are not connected in series with the auxiliary transistors. Avoiding these types of connections protects inverter transistors against damage in cases of disturbances in the inverter control system. The control algorithm of the main transistors of the inverter is the same as that of the transistors of an inverter operating with the hard-switching technique because the auxiliary transistors are switched depending on the operation state of the main transistors. The structure and operation principles of the new soft-switching solution are comprehensively described herein. Additionally, the control algorithm and the principles for selecting the elements of the new soft-switching circuits are discussed. The validity of the system operation was verified by performing laboratory tests with a three-phase voltage source inverter (3 kW, 250 V, 12 A, 3 kHz) supplying an inductive-resistive load or an induction motor, whereas numerical calculations were performed for inverters with a higher rated power. The switching losses of the inverter operating with the proposed soft-switching solution were compared with those of an inverter operating with the hard-switching technique.

This paper describes a simple method of approximating hysteresis changes in electrical steel sheets. This method is based on assumptions that flux density or field strength changes are a sum or a difference of functions that describe one curve of the limiting hysteresis loop and a certain ‘transient’ component. Appropriate formulas that present the flux density as functions of the field strength and those that present inverse dependencies are proposed. An application of this approximation requires knowledge of the measured limiting hysteresis loop and a few minor loops. Algorithms for determining changes in the flux density or field strength are proposed and discussed. The correctness of the proposed approximation of hysteresis changes was verified through a comparison of measured hysteresis loops with the loops calculated for several different excitations of the magnetic field occurring in dynamo and transformer steel sheets. Additionally, an example of the application of the proposed approximation of hysteresis changes is discussed in the paper. The proposed approximation of hysteresis changes is recommended for numerical calculations of the magnetic field distribution in dynamo and transformer steel sheets.

This paper presents a new soft-switching solution recommended for three-level neutral-point-clamped inverters. The operation principles of the proposed solution, working stages, selection of elements, and the control algorithm are comprehensively discussed herein. The control method of the inverter main switches is the same as that of the switches of an inverter operating according to the hard-switching technique. The correctness of the proposed solution was confirmed by the results of different tests using a laboratory neutral-point-clamped inverter with rated parameters of 3 kW, 2 × 150 V, 12 A, and 3 kHz. Numerical analyses were performed for the inverter of rated power 1.2 MW. The switching losses of the inverter operating with the proposed solution were compared with those of an inverter with hard-switching method. The proposed soft-switching solution increased the inverter efficiency and its competitiveness in relation to other proposals because there were no connections between switches and capacitors or inductors, which pose a risk of damaging the inverter when disturbances in the control system appear.

This article presents a modified soft-switching system for three-phase voltage source inverters. Similar to the basic soft-switching system, capacitors are not connected in parallel to the main transistors, and inductors are not connected in series with the auxiliary transistors. These connections protect the inverter from damage when disturbances occur in the control system. Unlike the basic version, the modified soft-switching system does not allow resonant processes to affect the voltage waveform on the load, and the modified system features a simpler control algorithm for the main and auxiliary transistors. The structure and operating principles of the modified soft-switching solution are comprehensively described herein. Additionally, the principles for selecting the elements of the modified soft-switching system are discussed. The system's operation is validated through laboratory tests using a three-phase voltage source inverter (10 kW, 400 V) to supply a squirrel-cage induction motor, whereas numerical calculations are performed for inverters with higher rated power. The switching losses of the inverter operating under the modified soft-switching solution are compared with those of an inverter operating under the hard-switching technique. Results show that the proposed modified soft-switching system increases inverter efficiency .

Purpose: The purpose of this paper is to present the method which relatively easily allows to approximate the hysteresis loop of the dynamo or transformer steel sheets. The paper also looks into the formulation of an equation allowing determination of distribution of the flux density and eddy currents in cross-section of these sheets. Design/methodology/approach: An exponential function was applied in the presented method relating to the approximation of the hysteresis loop. When the field strength changes its value, then, the flux density are the sum or difference of a function, describing the lower or upper hysteresis curve and some “ransient” component. On the basis of Maxwell’s equations and Amper’s law, one non-linear differential equation was formulated which allows to calculate the flux density and eddy currents in a cross-section of a transformer sheet. Findings: The method which relatively easily allows approximation of the hysteresis loop of ferromagnetic material is presented in the paper. The paper presents the derivation of one non-linear differential equation, allowing calculation of the flux density and eddy currents in the cross-section of the transformer sheets, taking into account the hysteresis phenomenon. Practical implications: The paper presents the method that can be used in modeling of the hysteresis loops of dynamo or transformer sheets, and the final non-linear differential equation can be applied in calculations of the magnetic field and eddy currents in cross-section of the transformer sheets. Originality/value: The paper refers to important issues of modeling and calculations of the magnetic and eddy current field distribution in transformer steel sheets.

This article presents proposals to increase the efficiencies of energy conversion systems from renewable sources using a soft-switching technique in three-phase voltage source inverters. The first part of this article briefly presents basic systems for generating energy from renewable sources. Special attention is paid to both photovoltaic and wind power plants. The next section describes the voltage source inverter with the soft-switching system of transistors, which is resistant to disturbances in the control systems of inverters. Laboratory tests on cooperation between the voltage source inverter and the AC grid are carried out for two cases, when energy is transmitted from the DC circuit to the AC grid and vice versa. In the final part, the efficiencies of energy conversion systems operating under the voltage source inverter with the soft-switching technique are compared with those of an inverter using hard switching of transistors. A comparison is made for energy conversion systems with a rated power of 100 kW and 1 MW.