Study of the Impact of Power Converters Equipped with LC-LCL filters interfacing renewable energy sources to the grid

Abstract

The demand and variety of power converters interfacing with renewable energy sources have been increasing steadily in recent years, presenting new challenges for the current centralized power grid. The microgrid is a new alternative that increases the penetration rate of the renewable energies (wind and photovoltaic energy, etc.) while the electrical power quality is improved. However, the increased penetration rate of the distributed generators DGs systems can pose problems with reverse power flow, voltage fluctuations and other issues to distribution networks. Beside this, the loads variations, the power generated by the renewable energy sources, and also the transition between grid-connected and the islanded modes impact the microgrid stability. The instabilities are linked to the fact that the power flow in this type of network is bidirectional. In addition, the power fluctuations generated by the renewable energy sources in MGs require the coordinated control of DGs, which allows further stable MG operation. Therefore, the reliability and stability are the major concerns in a microgrid system. On the other hand, distributed generators are equipped with an LC-LCL filter to produce higher power quality while relieving it for reactive power compensation and harmonic elimination. As part of this doctoral thesis, the local control of a distributed generator which is connected to the loads and to the grid via an LCL filter has been optimally designed as to have the active damping based on LQR control. In addition, this proposed control takes into consideration the perturbation and the tracking of a desired reference model in order to guarantee the stability and robustness of the distributed generator. The controllers have been experimentally implemented using real time with dSPACE. Two applications have been successfully completed; first application is a distributed generator connected to the grid via an LCL filter (PUC5-LCL) and the second one is a single stage multifunctional SPV-APF-LCL which was designed to achieve optimal multifunctional operation for residential power supply. In order to improve the power sharing between the distributed generators in a microgrid, a new droop control in the primary control has been proposed to keep the voltage and frequency of the microgrid constant. This droop control is based on supplementary control loop (stabilizer) added in the conventional droop control and is applied to several converters interfacing with renewable energy sources connected in parallel on the AC bus. The microgrid application with the proposed droop control has been verified in the real time environment using dSPACE and RT-LAB. The application of its commands involves the modeling of these complete systems as well as the design of the appropriate regulators to deal with the resonances problems, instabilities and the high quality of energy transfer as well as the suitable power sharing performance. All the theoretical studies are completely validated by simulation and experimentation analyses and discussions.

Date	12 Jun 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Kamal Al-Haddad (Supervisor)