Voltage support control for distributed generation systems
Creators: name: Kabiridehkordi, Roozbeh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400803
subject: Electrical energy generation (incl. renewables, excl. photovoltaics)
classificationCode: 400805
subject: Electrical energy transmission, networks and systems

description: Distributed Generation (DG) - defined as small-scale electricity generation near the end user - is becoming more prevalent in recent years with an increasing penetration of renewable generation emerging in utility distribution grids. Flexible operation of these DG units is a major objective for such smart grids. In the operational control framework, DG units should be considered as a part of the system, so that system reliability can be enhanced by ancillary service provided by DG systems.<br><br>As conventional electrical networks were not built to accommodate large DG penetration levels, several new challenges have emerged. An important issue that appears in low voltage feeders is voltage regulation, which can limit the level of DG penetration that can be tolerated and this also affects protection systems. On the other hand, DG control enables a wide range of new support functions which can help to mitigate distribution problems.<br><br>The first objective of this thesis is to investigate the application of DG systems to provide system level support functions within a grid. Initially, voltage regulation of a feeder is investigated using PV inverter reactive power injection. The inadequate effect of this form of control on voltage regulation was identified, in part due to the higher resistance characteristics of a typical feeder. To resolve this limitation a combined voltage regulation strategy, is proposed where an electronic tap-changer is incorporated into the feeder distribution transformer with the PV DG systems at each feeder bus then providing local reactive power support. The investigation accounted for a variety of issues such as feeder impedance, dynamic transformer tap changing, different load types and levels of PV penetration.<br><br>The second objective of this thesis is to propose a new DG current regulation method under unbalanced voltage conditions which can achieve desired positive and negative sequence current control objectives. The complexity and practical limitations of existing control methods inevitably compromise the current regulator’s performance, particularly during transient events. A novel current regulation method under unbalanced voltage conditions is presented in this thesis which eliminates the need for current sequence extraction and at the same time significantly improves the dynamic response of the current regulation strategy.<br><br>Finally, the third research objective of this thesis is to investigate inverter power control methods which can contribute to voltage quality, in particular during unbalanced voltage conditions. A detailed theoretical analysis is developed for the output power control of DG inverters under such conditions, incorporating factors which can contribute to mitigation of abnormal voltage conditions such as real and reactive power oscillatory terms, and differing grid impedance characteristics ranging from resistive to inductive. The results are used to identify the most effective control approach which achieves better voltage quality at the point of common coupling. Furthermore, fault ride through capabilities can be added to the DG inverter functionality to support the grid voltage during such symmetrical/asymmetrical faults, which consequently allow increased penetration of Distributed Energy Resources (DER).