The control and operation of the five level diode clamped inverter
This thesis describes an investigation of three and five level diode clamped inverters for motor drive applications. The work was completed as a PhD project at the University of Nottingham with funding from EPSRC and Heenan Drives Ltd. The investigation of the three level converter describes the design, development, control and operation of an 11kW prototype. Included in the design is a review of typical switching strategies employed for control of the output voltage. New improvements to the sub-harmonic pulse width modulation method are presented which allow an improved output waveform to be obtained. The problem of DC link capacitor voltage balancing (Neutral Point Control) is addressed and a novel balancing control method is presented based on the addition of a DC offset to the modulation pattern. This method is verified through mathematical analysis and experimental operation. The operational limits of the control are analysed. Improvements to the technique are presented to expand its operating limits. The development of a prototype five level converter is then described. The design again features improvements to the sub-harmonic modulation strategy to provide enhanced output waveform generation, particularly for transient operation. The current demands on the DC link capacitors for the five level arrangement are analysed and it is concluded that the capacitors cannot be regulated by simple modifications to the output switching pattern. A novel circuit is presented to achieve capacitor balancing within the DC link. The circuit behaviour is described and analysed. Operation is confirmed through simulation and experimental implementation. High dynamic performance is demonstrated via the use of a vector controlled induction motor. Neutral point control is successfully achieved through a similar method to that used for the three level inverter. Having demonstrated the principle of operation of the three and five level inverters on low voltage prototypes, the thesis concludes with a review of the main considerations required to implement the configurations as medium voltage drives.