Wu, Dan

Project title: Coordinated Control for Power Quality in Grid Connected and Islanded Three-Phase MicroGrids

PhD period: 2012.10.01 – 2015.09.30.
Section: Power Electronic Systems
Research Programme: MicroGrids
Supervisor: Josep Maria Guerrero Zapata
Co-Supervisor: Juan Carlos Vasquez
Contact information

Collaborator: China Scholarship Council.
Funding: China Scholarship Council and scholarship from the Dept. of Energy Technology. 

Abstract

The control of a MicroGrid can be done by means of hierarchical control, which consists of three levels: primary control, secondary control and tertiary control. The primary control is a local control that allows the power electronics converters to operate independently but at the same time cooperating among them.

This Thesis will study power quality issues related to the primary control of MicroGrids. Traditionally the primary control of MicroGrids is based on the droop control method, which consist of adjusting frequency and amplitude of the inverter output, according to the active and reactive power demanded (also named P-f and Q-V droops). This control mimics the behaviour of synchronous generators in large power systems. In a MicroGrid, the frequency is the same in every place, so that active power is well distributed. However, reactive power will not be properly shared, as it depends mainly on the voltage, which can be different in each point of the MicroGrid. On the other hand, in small MicroGrids the lines are mainly resistive, so that the active power flow depends mainly on the voltage, thus P-V and Q-f droops are used. This means that in MicroGrids the active power is difficult to properly share. This phenomenon will be studied in detail.

In order to obtain additional features, a virtual impedance loop is often used together with the droop control. This way, better power sharing can be obtained in spite of the line impedance. Also, selective harmonics and negative and positive sequences can be processed individually to deal with harmonics and voltage unbalances. However, a trade-off between power quality and power sharing appears. Furthermore, the design of the droop control and the virtual impedance is totally disconnected in the literature. In the Thesis, the co-design of droop controllers with virtual impedances will be developed.

Another possibility is to use reverse droop functions, which consist on adjust active and reactive power references according to the measured frequency and voltages (f-P and V-Q). This approach is called virtual inertia control loop or virtual synchronous generator. The Thesis will study and compare both approaches (droop control and virtual inertias). Since the droop control and the virtual inertias can make inverters work like non-ideal voltage and current sources, next step will be to study the interaction of these two kind of controlled inverters coexisting in a MicroGrid. That can be interesting to give the role of voltage sources to the energy storage systems and current sources to the generators of the MicroGrid.

Papers

Publications in journals and conference papers may be found at VBN