Performance Evaluation of Medium Voltage Cascaded H-Bridge converter for Low frequency High power variable power factor Load

Abstract

Cascaded H-Bridge converters are promising alternative to cyclo-converters in MV drives in many applications like grinding mills in mines, petrochemical industry, conveyer belts etc. It uses low voltage devices, has input power factor close to unity and low dv/dt in output voltage. Conventional H-Bridge converters has isolated power supplies and filter capacitors for each module and such several modules are connected in series per phase. Good amount of work is reported on improved topologies like Multi Modular Converter (MMC), having common high voltage DC bus and no isolated supply for H-Bridges. Though popular but it has serious individual capacitor charging and voltage balancing issues demanding additional complex circuits and control methodology. Also at low output frequency and variable load PF for large MVA, makes problem more complex. The conventional H-Bridge converter has modular construction, very simple control and avoids complex circuitry and algorithm for capacitor voltage balance etc. The voltage balance problem is tackled with passive filters yet not compromising with good drive availability. Also simple and modular construction allows to build redundancy in hardware so that downtime is minimized. Such drive systems are very critical for demanding loads like grinding mills in mines where availability is prime concern to avoid business interruption loss.This paper simulates the conventional H-Bridge converter for MV drive with 4950 volt, 5.574 Hz voltage source converter for 20 MVA load and power factor varying from 0.3 to unity. All parameters like input and output harmonics for each power factor load and effect of filter capacitor on these parameters are brought out. Also performance parameters like efficiency and input power factor for each load power factor is plotted by variation of DC link filter capacitor value.