Fault Tolerant Cascaded Multilevel Converter
Multilevel converters have been receiving increasing attention due to their potential to be used for high-power applications. Compared with two-level converters multilevel topologies have several advantages, such as high efficiency due to lower switching frequencies and low electromagnetic emission due to low voltage changes. Multilevel converters also have a high number of semiconductor switches, making them inherently more tolerant to internal faults than two level converters. Cascaded h-bridge (CHB) converters are a multilevel topology which have found widespread use in active filters, renewable energy conversion systems, and traction motor applications. The CHB uses several series connected single-phase inverters, called cells (or modules), each of which is powered by an isolated dc source. In KSU's Power Electronics Research Laboratory a 3 kW, 3-phase CHB with 4 cells per phase is being built. As shown in the figure below, one leg of the inverter has been completed. This custom built equipment is being used to verify algorithms designed to implement Adaptive Sinusoidal Pulse Width Modulation (ASPWM), a technique developed to allow time-variant dc inputs cell inputs to be used to generate a regulated ac output voltage.
Once the multilevel converter is completed, further experimental verification can be completed. Future work includes:
- Modifying ASPWM to allow for fault identification and correction if an h-bridge switch has an open circuit failure.
- Implementing fundamental phase shift compensation for the case where voltages can take on any value within a continuous range (current methods utilizing lookup tables require a discrete number of cases).
- Developing other fault tolerant algorithms to control a three-phase CHB.