Digital Control of Converters for Distributed Power Generations

Abstract

Pulse width modulated converters are becoming increasingly popular as their cost decreases and power rating increases. The new trend of smallscale power producers, often using renewable energy sources, has created new demands for delivery of energy to the grid.

A major advantage of the pulse width modulated converter is the ability to control the output voltage at any point in the voltage period. This enables rapid response to load changes and non-linear loads. In addition it can shape the voltage in response to the output current to create an outward appearance of a source impedance. This is called a virtual impedance.

This thesis presents a controller for a voltage controlled three phase pulse width modulated converter. This controller enables operation in standalone mode, in parallel with other converters in a microgrid, and in parallel with a strong main grid.

A time varying virtual impedance is presented which mainly attenuates reactive currents. A method of investigating the overall impedance including the virtual impedance is presented.

New net standards have been introduced, requiring the converter to operate even during severe dips in the grid voltage. Experiments are presented verifying the operation of the controller during voltage dips.