Advanced control of converters with multitask functionalities in distribution grid systems based on conservative power theory

Abstract

Distributed generation (DG) play a very important role in the modernization of electric power systems, it is estimated their increasing share of operation in the near future. In addition, there is growing concern on the environmental issues, lack of transmission capacity and limitation in constructing new lines, and increasing demand of energy, that would support a more flexible inverter control, capable of interacting users with the utility grid. The main objective of such a system is providing active power, which is the primary use to balance loads. However, power electronic systems can provide power quality improvement and DG systems would then be used as multi-functional compensators for improving power quality, instead of only balancing or selling active power to the grids. In this context, this dissertation first studies the well-known instantaneous current decomposition theories highlighting the important contributions based on the Instantaneous Power (PQ) theory and the Conservative Power Theory (CPT) and presents a comprehensive comparison from performance and computational complexity perspectives. Although these theories are quite distinct in their formulations, the central idea is to make a comparative study between the current portions and their respective portions of power, in order to show the similarities and divergences between them in terms of characterization of the physical phenomena and in terms of disturbing current compensation. The studied instantaneous current decomposition techniques are then used to provide selective functionalities in distribution systems. Therefore, it is possible to inject active power plus compensate selectively unwanted current terms (reactive, unbalance, and distortion), enabling full exploitation of the inverter capability and increasing its overall cost-benefit and efficiency. Afterwards, control structures with multitask functionality to the grid side converter of the renewables to carry out the power quality ancillary services in the distribution system are developed. The key diversity of the methodologies we proposed in this project with respect to others in the literature is that the developed control structures on the grid side converters are based on the CPT theory. This choice provides decoupled power and current references for the grid side inverter control, which offers very flexible, selective and powerful functionalities. These qualities make the system to be the benchmark for achieving 100% renewable and sustainable grid with multifunctional capabilities. This thesis then proposes the coordinated control of the aforementioned multifunctional interfacing DG systems to enhance the operation of microgrid systems. Based on our proposed method, a hybrid cooperative strategy is developed that overcomes limitations in communication-based and non-communication-based approaches for the coordinated operation of multifunctional distributed generators in islanded microgrid systems. Two important issues that are addressed are the power quality and undesirable current sharing, particularly in the low-voltage distribution network, where electronic devices are drawing distorted and unbalanced currents. The interactions of such current disturbances with high feeder/line impedances, in a low voltage system cause considerable voltage deterioration and possibly affect sensitive loads showing the requirement for power quality enhancement. Finally, this thesis explores the study and implementation of cascaded multilevel converters, in which the primary concepts relating to modulation, structure, and control schemes are detailed. These topologies are composed of series-connected H-bridge converters with isolated DC links. Therefore, it is possible to integrate renewable energy and storage resources to power grids. The experimental findings validate the applicability and performance of the proposed control strategies in distribution grid systems.