Modular 200 MVA programmable power supply for MST´s poloidal field

The new Poloidal Field Programmable Power Supply will expand the capabilities of the MST reversed-field pinch experiment. The supply, based on IGBT H-bridges, will generate an arbitrary waveform up to +/- 2700 V at currents up to +/- 85 kA to drive the transformer which produces MST´s poloidal field. Additional modules can be added, without other modification, to expand the output current to 115 kA, yielding over 300 MVA. A key feature of the new supply is its modular construction, allowing easy expandability and minimizing interaction between modules. Each module contains its own 900 V capacitor bank with dumping system, IGBT H-bridge, and charging system. High-frequency transformers provide voltage isolation with low parasitic capacitance, allowing modules to be connected in series to achieve the desired output voltage. Isolated capacitor-dumping circuits using depletion-mode MOSFETs do away with expensive shorting relays, eliminating the need to share shorting circuits between modules. Radiated noise is reduced by eliminating all high-frequency interconnections between modules other than the output cables. The isolated construction allows parallel balanced coasting, reducing undesired capacitive current pickup generated during IGBT switching. The supply uses seven-level Pulse Width Modulation; this reduces the magnitude of output voltage steps, further reducing electrical noise generation. This modulation technique also reduces IGBT switching losses, allowing more current per IGBT.