Current regulator based control strategy for islanded and grid-connected microgrids

摘要

Distributed generation has been primarily developed to harvest renewable energy resources such as wind and sunlight. Due to low unit power capacity and dispersed unit placement, integration of distributed generators is facilitated if a cluster of generation sources and their dependent loads are considered as one self-controlled network, commonly referred to as a microgrid. This thesis presents a new control strategy that allows microgrids to continuously operate with current controlled inverter based distributed generators. This approach offers significant benefits over the conventional voltage control strategies used for these inverters. A microgrid must be controlled carefully to operate according to a set of desired requirements such as voltage and frequency regulation criteria. It is desirable that the distributed generators share active and reactive power generation equally between themselves in order to increase the reliability of the microgrid, and a microgrid also has to be able to connect to or disconnect from the main utility grid with a smooth and seamless process. This thesis starts by investigating the present common method for microgrids known as droop control. It is shown that existing strategies that use voltage controlled converters and droop control have a number of significant shortcomings. These include slow dynamics due to the structure of dual-loop voltage controllers; poor performance of voltage controllers compared to current regulators; sluggish power sharing response as a result of feeding back the measurement of the generated powers in the droop function; and significant sensitivity to the microgrid structure and impedance of the distribution lines. Next, a new method for controlling the distributed generators in a microgrid using current controlled converters is proposed, challenging the commonly accepted belief that current regulators cannot be used with an islanded microgrid and actually achieving superior performance compared to the common voltage control based approach. The new strategy calculates the reference of the current controlled converters without making any changes to the current controller structure, using a simple predictive voltage controller that does not compromise the current controller’s dynamic performance. Furthermore, by using a hitherto undiscovered characteristic of PR resonant linear regulators in the stationary reference frame, automatic frequency and load power factor matching is readily achieved. Finally, a new algorithm is proposed to implement a droop function for current controlled distributed generators, which achieves equally shared rated power. Finally, the applicability of the proposed control strategy is analysed in various situations that can be expected in a real world microgrid. Scenarios investigated include operation in islanded and grid-connected modes and the transition between the two; supplying power to a various range of loads; implementation of the microgrid in single-phase as well as three-phase; application for low voltage or high voltage networks; and so on. It is shown that a microgrid controlled with the new proposed control structure has very satisfactory operation for all these identified situations. The findings of this thesis have been tested and verified using computer simulations and also by experimental confirmation using laboratory-scale power electronic converters.