Pulse-amplitude-synthesis-and-control (PASC) inversion system for single-source diagonally connected MHD generators

In this paper, the problem of power waveform synthesis using the PASC technique to produce a desired output from a single dc source is introduced. Although the specific example discussed here is for a diagonally connected magnetohydrodynamic (MHD) generator, the technique can be applied to any single terminal pair source, with appropriate modification in power extraction interface and computer control program to match source and load impedance characteristics. The method as discussed here, employs banks of pulse transformers, one bank per phase, in which the primaries are connected in parallel through a switch matrix to the dc source and two opposite polarity primaries per transformer are pulsed alternatively in time to produce an oscillatory output waveform. The timing sequence in which the primaries are injected with power from the source under switch control determines the resulting ac waveform. If a repetitive mono-polarity pulse waveform is desired, a capacitive interface network is used since it can support an average dc output component. The PASC process utilizes a switching array to connect/disconnect one or more dc sources in an overlapped fashion under computer actuation, along with error output signal feedback, to generate the control necessary to match the desired waveform as specified in the computer. The principal benefit of this system for the single output source diagonal MHD generator is that quantized sinusoids can be generated having low harmonic content and power factor control without the need for large and expensive filtering and VAR compensation such as is required for conventional inverters. After an introduction to the converter system topology for a single dc source along with the transformer representation employed, the simulation results for two and eight level PASC inverter systems are presented. Results for triangular synthesis and sinusoidal synthesis simulations are given for different load impedance to source impedance ratios. As expected the results indicate strong dependence on source impedance if the load to source impedance is not large. Uncontrolled output waveform results show very small total harmonic distortion.