Analysis and Design of Methods for Condition Monitoring of Capacitors in Multilevel Converters

Multi-level converters are an important class of power electronics based systems that enable seamless conversion of electrical power from one form to another. Due to its distinct merits, it finds a vast scope of application in the fields such as renewable energy, electrical power transmission, adjustable speed drives, uninterrupted power supplies and custom power devices. These merits often come at a cost of increased complexity, higher number of power semiconductor devices and higher number of energy storage elements. Multi-level converters generates staircase waveform by use of high density capacitor banks. These capacitor banks are often subject to failure due to vaporization of electrolyte forming weakest link in reliability context. This thesis addresses reliability issue by proposing an online condition monitoring method for a three-level neutral point clamped multi-level converter which can be easily integrated with existing control methods. The proposed method provides an online estimate of existing capacitance in DC-link and helps increase in reliability in terms of preventive maintenance. The validity of proposed technique is obtained by verification of the method on a 3KVA laboratory developed experimental prototype. It also addresses reliability by developing tool in terms of analytical expressions which can be used as a ready reckoner for proper design of capacitor bank employed in five-level active neutral point clamped multi-level converter. Results from this developed tool are quantitatively verified with the results obtained from converter models developed over MATLAB Simulink environment confirming their accuracy