Decentralised control of a modular cascaded multlevel converter

As the demand for higher power, higher voltage electrical conversion systems increases, it becomes less and less feasible to use two-level power electronic converter topologies in these applications because of rating limits with semi-conductor devices and demands on their harmonic performance. Multilevel converters are an elegant solution to these constraints; they allow the series connection of semi-conductor devices, thereby increasing the overall voltage rating of the converter, and at the same time they can limit the voltage stress across individual devices while simultaneously achieving substantially improved harmonic performance. One approach to the design of these converters is the concept of modularisation, which packages hardware and software into reusable building blocks. This approach allows more flexible and extendable designs. However, while hardware modularisation of multilevel converters has been quite successful, the control system for these converters typically remains highly centralised and reliant on a high bandwidth communication network. This thesis proposes a strategy to decentralise the open loop control and closed loop current regulation of a modular multilevel converter. The main concept is to give the responsibility for determining the converter’s switched outputs to local controllers in each converter module, using local sensors, a separate current regulator and a local modulator. These local controllers then coordinate to achieve the overall control objective of the multilevel converter. This thesis focuses on two key aspects of this approach. The first is an in-principle theoretical development, which begins by evaluating existing open and closed loop control strategies to select the most appropriate strategies and then adapting and extending them to suit the decentralised application. The intra-converter communication system is then enhanced to determine the minimum communication requirements for this system. The second aspect of this work is the practical implementation of this control system. This required a detailed analysis of the non-ideal circuit conditions which can adversely affect the performance of the system, and the development of techniques to manage them. The control strategies developed in this thesis have been tested extensively in both simulation and in practice using an experimental 12-kVA three-phase 5-level modular cascaded converter. The resultant performance of the system matched the best state of the art centralised control system in terms of transient and steady state output, while also achieving the same levels of harmonic distortion in the switched output voltage waveforms.