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

Empirical thesis.Bibliography: pages 184-196.Chapter 1. Introduction -- Chapter 2. Literature review and system modelling -- Chapter 3. Penetration of renewable energy sources into LV network -- Chapter 4. Multifunctional operations with three-phase four-leg PV-SVSI -- Chapter 5. Capacity improvement of four-leg smart inverter -- Chapter 6. Experimental setup and results analysis with SVSI -- Chapter 7. Conclusion and future work -- Appendix -- References.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.Mode of access: World wide web1 online resource (xxv, 196 pages) colour illustration

Impact of controlling zero sequence current in a three-phase four-wire LV network with PV units

The dynamic impacts of controlling zero sequence current in a three-phase four-wire low voltage (LV) distribution network with a four-leg voltage source inverter (VSI) and PV installations are investigated in this paper. The PV VSI is designed to control active power at unity power factor (p.f) with additional neutral and zero sequence current controls. Inherently, most of the three-phase four-wire LV distribution networks exhibit voltage unbalance characteristics due to the connection of single- and three-phase nonlinear loads as well as occurrence of asymmetrical faults in the networks. The control over zero sequence current can improve the inherent voltage unbalance in the LV networks. The transient characteristics of the unbalanced network are investigated utilizing a widely used software environment, PSCAD/EMTDC, for the Energex 11 kV/420 V distribution network in Brisbane, Australia. The performance of the designed four-leg VSI is compared with the traditional three-leg VSI experiencing different asymmetrical faults. The results show that, with the additional degree of freedom from the four-leg VSI, faster fault recovery and lower MVA requirement can be achieved for unbalanced LV networks.5 page(s

A multi-purpose interlinking converter control for multiple hybrid AC/DC microgrid operations

This paper presents a multi-purpose interlinking converter control design along with a simplified centralized multi-microgrid operation controller (MMOC) to achieve smooth and coordinated operations among multiple hybrid AC/DC microgrids with electric vehicle-energy storage systems (EVESSs). The primary purpose of the multi-microgrid operation is to share required active/reactive power among multiple microgrids. In order to achieve this objective interlinking converters of each microgrid is designed with embedded droop controllers. The conventional droop controllers are replaced with proportional-derivative (PD) compensated droop controllers. The PD compensated droop controller ensures proper damping and avoids the limitations of power sharing accuracy associated with the conventional droop controllers during islanded to multimicrogrid mode transition. A simplified centralized MMOC has been designed to control the initiation and the termination period of the multi-microgrid operation. The MMOC also generates necessary reference signals for the interlinking converters to achieve successful multi-microgrid operation among multiple hybrid AC/DC microgrids. Two islanded hybrid AC/DC microgrids are designed in MATLAB/ SIMULINK environment based on N44 and N05 buildings in Griffith University, Australia. The PD compensated droop method has been embedded into the interlinking voltage source inverter (VSI) controllers of each microgrids. Simulation has been carried out for islanded mode, multi-microgrid operational mode and the transition between these two modes under variable loading and three-phase fault condition. All the simulation results show that the designed controller provides superior and robust performance for variable scenarios.6 page(s

Implementation of independent improved neutral current controller using four leg PV-VSI

An independent improved neutral current controller is implemented in a three-phase (3p) four-wire (4w) low voltage (LV) distribution network with a four-leg voltage source inverter (VSI) and photovoltaic (PV) installations in this paper. The PV VSI is designed to control active power at unity power factor (p.f) with additional improvement in the neutral current controller using the load generated neutral current as a direct reference. As, inherently, most of the three-phase four-wire LV distribution networks exhibit voltage unbalance characteristics due to divergent load connection types such as single-and three-phase loads, the independent control over neutral current at the point of common coupling (PCC) becomes imperative especially for decentralized controllers. The improved performance of the neutral current controller both at customer's installation and at the distribution transformer (DT) terminal is investigated by implementing the designed PV-VSI with actual Australian (Energex) LV network model in PSCAD/EMTDC software environment. The performance of the designed four-leg VSI is compared with the traditional active neutral compensator with actual single phase customer loads. The results show that, with the proposed independent neutral current controller, improved neutral compensation can be achieved for unbalanced LV networks for whole day operations.6 page(s

Internet of Things Platform for Energy Management in Multi-Microgrid System to Improve Neutral Current Compensation

In this paper, an Internet of Things (IoT) platform is proposed for Multi-Microgrid (MMG) system to improve unbalance compensation functionality employing three-phase four-leg (3P-4L) voltage source inverters (VSIs). The two level communication system connects the MMG system, implemented in Power System Computer Aided Design (PSCAD), to the cloud server. The local communication level utilizes Modbus Transmission Control Protocol/Internet Protocol (TCP/IP) and Message Queuing Telemetry Transport (MQTT) is used as the protocol for global communication level. A communication operation algorithm is developed to manage the communication operation under various communication failure scenarios. To test the communication system, it is implemented on an experimental testbed to investigate its functionality for MMG neutral current compensation (NCC). To compensate the neutral current in MMG, a dynamic NCC algorithm is proposed, which enables the MGs to further improve the NCC by sharing their data using the IoT platform. The performance of the control and communication system using dynamic NCC is compared with the fixed capacity NCC for unbalance compensation under different communication failure conditions. The impact of the communication system performance on the NCC sharing is the focus of this research. The results show that the proposed system provides better neutral current compensation and phase balancing in case of MMG operation by sharing the data effectively even if the communication system is failing partially