Distributed droop schemes for standalone and interlinked AC microgrids

• Main Author: Inam Ullah
• Other Authors: Loh Poh Chiang, Andrew
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Power electronics; DRNTU::Engineering::Electrical and electronic engineering::Electric power; DRNTU::Engineering::Electrical and electronic engineering::Electric power::Production, transmission and distribution
• Online Access: https://hdl.handle.net/10356/59385
• Institution: Nanyang Technological University

Distributed Generators (DGs), when clustered to form small microgrids, offer many advantages like resource optimization, improved power quality, stability and reliability. The formed microgrids can certainly be controlled by centralized management systems, but for widely dispersed DGs where communication links are not viable, autonomous droop schemes might be more appropriate. So far, the common objective focused by the droop schemes has been proportional power sharing among the DGs based on their respective ratings. This objective is fine if the DGs are of the same type, just like the classical control of parallel synchronous generators. It is however not the case for microgrids, where different types of DGs usually exist. Proportional power sharing based on ratings alone might therefore not be suitable for microgrids. Other factors like costs, efficiencies, pricing schedules and emission penalties should be considered, just like in most centralized cases, where power dispatch commands are usually decided from a combination of factors rather than ratings alone. The same thought has however never been tried with droop control for an autonomous standalone microgrid. To fill the shortfall, a number of cost-based droop schemes has been proposed, whose power sharing in the steady state will give rise to a lower total generation cost (TGC) for the considered microgrid when compared with the traditional droop schemes. Experimental results have verified the reduction or cost saving achieved. Like the grouping of DGs to form a single standalone microgrid, the grouping of multiple microgrids to form a larger interlinked system is possible and likely given its more efficient reserve sharing without demanding for standby generators. The interlinked microgrids are strictly free to have their own voltages and frequencies that better match with their sources and loads. This applies to existing “standard” power grids too, where 50 Hz can be found in some countries, while 60 Hz prevails in others. Frequency of 400 Hz can also be found in aircraft supply networks. It is therefore likely to have different conditioned microgrids, whose interlinking will certainly require the insertion of interlinking power converters. Power flows through these interlinking converters must thoroughly be studied and controlled appropriately, which presently, are lacking for autonomous systems. It is hence the second objective of the thesis to investigate on autonomous droop schemes for the interlinking of converters. Proper tuning of the schemes has been suggested, whose improved performances have been verified in experiments.