THREE-PHASE VOLTAGE INVERTERS INSENSITIVE TO UNBALANCED LOAD

Abstract

The article is devoted to the systematization and comparative analysis of solutions to the problem of structural and algorithmic synthesis of three-phase bridge voltage inverters (TVI) implemented on 6 transistors in the form of three main switch legs that are able to ensure the required quality of output voltage at unbalanced current load of its phases. To increase the inverter's output voltage, its main TVI switches are controlled in all versions using programmed (hard) PWM in the overmodulation mode with the modulation depth μ = 2.5. It is shown that the simplest solution to the stated given problem is using a capacitive divider in the TVI power supply circuit with connecting its middle (zero) point to the zero point of unbalanced, as well as introducing an individual filter for single-phase load. The disadvantage of this solution is deterioration of the spectral composition of voltages on the load due to the flow of zero sequence harmonic (ZSH) currents in its phases, which results in poorer weight and dimension indicators of the output filters at the preset output voltage quality. To address this shortcoming, three alternative options to solve this problem are considered (including a new one). The common essential feature of all three solutions is insertion of an extra 4th leg of switches into the TVI. In the first of these TVI versions, a so-called ZSH compensating transformer is additionally used, and in the second version a supplementary compensating auto-transformer is incorporated. The third version is free from these electromagnetic components. For the first two TVI versions, a comparative assessment of two types of electromagnetic components is carried out in terms of their power to size ratio, from which preference is given to the second version. For the third TVI version, a new algorithm for control of the 4th switch leg is developed, which eliminates ZSH components in the load (as in the first two TVI versions). The considered solutions for TVI are synthesized with reference to the requirements of aviation applications and are intended for use in synthesizing power generating systems with rated voltage of 115 / 200V and constant 400 Hz frequency at a variable rotational speed of the generator drive shaft. Computer simulation in the OrCAD PSpice Schematics environment is used as the tool for solving the stated problems. The study results and their adequacy to the design concept are explained and confirmed by the waveforms obtained from simulations.