Design and implementation of smart voltage source inverter (SVSI) with renewable energy source (RES)

Renewable energy sources (RES), such as photovoltaic (PV) and battery energy storage(BES) systems, are becoming popular due to their ease of installation, reduction in greenhouse gas emissions and economic benefits from electricity bill reduction. However, the increasing amount of RES penetration into the low-voltage (LV) network is making the existing passive distribution network face many challenging issues such as voltage rise at the point of common coupling (PCC), voltage unbalances, power quality degradation etc. Additionally, the unbalanced distribution of linear and nonlinear single-phase loads and RES installations are causing high neutral current generation along with neutral to ground voltage rise at the PCC in three-phase four-wire LV networks. Therefore, in this dissertation, a multifunctional smart voltage source inverter (SVSI) is designed with a PV system to provide optimised and coordinated voltage regulation and improved neutral current compensation performance at customer installation points. The first contribution of this research is the development of a hierarchical control selection method to mitigate the voltage-rise associated with increasing PV penetration in a balanced three-phase three-wire Australian LV network. The proposed method utilises five control modes based on the PV penetration level in the LV network. The voltage regulation method provides a step-by-step requirement of different voltage regulation devices such as a distributed static synchronous compensator (D-STATCOM), D-STATCOM/BES, residential SVSI, BES and power sharing among neighbouring RES units for critically voltage-sensitive areas in the LV network. The developed control selection method provides an optimised and economical way to achieve 100% penetration of RESs into the LV network without any voltage constraints. The second contribution of this research is the design and application of a multi-functional three-phase (3P) four-leg (4L) PV-SVSI with a novel neutral current control which can significantly compensate for the load-generated current at different network locations. The relationship between the load-generated neutral current and the zero sequence R/X ratio(R0/X0) of the transmission-line neutral conductor is developed. The 3P-4L PV-SVSI is designed to operate robustly with variable R0/X0 ratios and system fault conditions. Comparisons of neutral current compensation operation with existing passive and active neutral compensation methods are presented to verify the efficacy and novelty of the proposed system. The third contribution of this dissertation is the development of a novel dynamic capacity distribution (DCD) method to improve the neutral current compensation from the 3P-4L PVSVSI. The DCD method distributes the available capacity after active and reactive power regulation operations from the SVSI to the neutral current controller for higher capacity neutral current compensation. Traditional current limiters with dynamic value assigning function are used to utilise the DCD method in the four-leg inverter to achieve better unbalanced compensation than that provided by existing methods. The final contribution of this research is the construction and application of a 3P-4L SVSI hardware prototype for experimental results verification. The four-leg VSI system is constructed by modifying the Semiteach three-leg teaching module, and the fourth leg is controlled independently in the system. The same inverter system is operated in three- and four-leg inverter configurations with a real-time digital signal processor (DSP) controller module provided by Denkinetic Pty Ltd and using Code Composer Studio (CCS) software. Different case studies are conducted with both inverter configurations in the power system computer aided design/ electromagnetic transient DC (PSCAD/EMTDC) software platform and in an experimental setup to verify the efficacy of the proposed methodologies. Proper electrical connection standards are also ensured in the designed PV-SVSI system, such as total harmonic distortion less than 5%, voltage unbalance factor less than 2%, and neutral to ground voltage less than 1 V. The case studies’ results show that the designed multifunctional PVSVSI can provide stabilised performance with the proposed methods in voltage regulation and neutral current compensation, despite the variations in sun irradiance, customer load profiles,network parameters, and different fault conditions.