Smart Voltage-Source Inverters with a Novel Approach to Enhance Neutral-Current Compensation

© 1982-2012 IEEE. The presence of a neutral current is quite common in three-phase (3P) four-wire (4W) distribution systems due to an unequal distribution of linear and nonlinear single-phase (1P) loads and small distributed generators. However, a high neutral current can overload the neutral conductor and distribution transformer, which can cause electrical safety concerns and even fire. Among several existing neutral current compensators, the 3P four-leg (4L) voltage-source inverter (VSI) provides better control flexibility and more efficient performance than the passive compensators but requires a higher VSI capacity for the fourth-leg operation. To provide a solution to the aforementioned problem, this paper presents a novel control method to utilize the available capacity of a 3P-4L VSI after active and reactive power regulation to enhance the neutral-current compensation. A smart VSI (SVSI) is designed to operate with a solar photovoltaic unit, regulate the ac side voltage, and minimize the neutral current. Case studies are conducted with actual load data from a commercial building in the PSCAD/EMTDC software environment. The designed system with the proposed control method can provide a significant improvement in the neutral-current compensation, phase balancing, and unbalance factor compared to a fixed-capacity 3P-4L SVSI. Experimental results using a TMS320F28335 digital signal processor microcontroller and modified Semiteach 3P-4L inverter are presented to verify the robustness of the designed controller and the enhancement to the neutral-current compensation using the proposed dynamic capacity-control method

Design of External Inductor for Improving Performance of Voltage Controlled DSTATCOM

A distribution static compensator (DSTATCOM) is used for load voltage regulation, and its performance mainly depends upon the feeder impedance and its nature (resistive, inductive, stiff, nonstiff). However, a study for analyzing voltage regulation performance of DSTATCOM depending upon network parameters is not well defined. This paper aims to provide a comprehensive study of design, operation, and flexible control of a DSTATCOM operating in voltage control mode. A detailed analysis of the voltage regulation capability of DSTATCOM under various feeder impedances is presented. Then, a benchmark design procedure to compute the value of external inductor is presented. A dynamic reference load voltage generation scheme is also developed, which allows DSTATCOM to compensate load reactive power during normal operation, in addition to providing voltage support during disturbances. Simulation and experimental results validate the effectiveness of the proposed scheme

Internet of things platform for energy management in multi-microgrid system to improve neutral current compensation

© 2018 by the authors. 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

Management of Distributed Energy Storage Systems for Provisioning of Power Network Services

Because of environmentally friendly reasons and advanced technological development, a significant number of renewable energy sources (RESs) have been integrated into existing power networks. The increase in penetration and the uneven allocation of the RESs and load demands can lead to power quality issues and system instability in the power networks. Moreover, high penetration of the RESs can also cause low inertia due to a lack of rotational machines, leading to frequency instability. Consequently, the resilience, stability, and power quality of the power networks become exacerbated. This thesis proposes and develops new strategies for energy storage (ES) systems distributed in power networks for compensating for unbalanced active powers and supply-demand mismatches and improving power quality while taking the constraints of the ES into consideration. The thesis is mainly divided into two parts. In the first part, unbalanced active powers and supply-demand mismatch, caused by uneven allocation and distribution of rooftop PV units and load demands, are compensated by employing the distributed ES systems using novel frameworks based on distributed control systems and deep reinforcement learning approaches. There have been limited studies using distributed battery ES systems to mitigate the unbalanced active powers in three-phase four-wire and grounded power networks. Distributed control strategies are proposed to compensate for the unbalanced conditions. To group households in the same phase into the same cluster, algorithms based on feature states and labelled phase data are applied. Within each cluster, distributed dynamic active power balancing strategies are developed to control phase active powers to be close to the reference average phase power. Thus, phase active powers become balanced. To alleviate the supply-demand mismatch caused by high PV generation, a distributed active power control system is developed. The strategy consists of supply-demand mismatch and battery SoC balancing. Control parameters are designed by considering Hurwitz matrices and Lyapunov theory. The distributed ES systems can minimise the total mismatch of power generation and consumption so that reverse power flowing back to the main is decreased. Thus, voltage rise and voltage fluctuation are reduced. Furthermore, as a model-free approach, new frameworks based on Markov decision processes and Markov games are developed to compensate for unbalanced active powers. The frameworks require only proper design of states, action and reward functions, training, and testing with real data of PV generations and load demands. Dynamic models and control parameter designs are no longer required. The developed frameworks are then solved using the DDPG and MADDPG algorithms. In the second part, the distributed ES systems are employed to improve frequency, inertia, voltage, and active power allocation in both islanded AC and DC microgrids by novel decentralized control strategies. In an islanded DC datacentre microgrid, a novel decentralized control of heterogeneous ES systems is proposed. High- and low frequency components of datacentre loads are shared by ultracapacitors and batteries using virtual capacitive and virtual resistance droop controllers, respectively. A decentralized SoC balancing control is proposed to balance battery SoCs to a common value. The stability model ensures the ES devices operate within predefined limits. In an isolated AC microgrid, decentralized frequency control of distributed battery ES systems is proposed. The strategy includes adaptive frequency droop control based on current battery SoCs, virtual inertia control to improve frequency nadir and frequency restoration control to restore system frequency to its nominal value without being dependent on communication infrastructure. A small-signal model of the proposed strategy is developed for calculating control parameters. The proposed strategies in this thesis are verified using MATLAB/Simulink with Reinforcement Learning and Deep Learning Toolboxes and RTDS Technologies' real-time digital simulator with accurate power networks, switching levels of power electronic converters, and a nonlinear battery model

POWER QUALITY CONTROL AND COMMON-MODE NOISE MITIGATION FOR INVERTERS IN ELECTRIC VEHICLES

Inverters are widely utilized in electric vehicle (EV) applications as a major voltage/current source for onboard battery chargers (OBC) and motor drive systems. The inverter performance is critical to the efficiency of EV system energy conversion and electronics system electro-magnetic interference (EMI) design. However, for AC systems, the bandwidth requirement is usually low compared with DC systems, and the control impact on the inverter differential-mode (DM) and common-mode (CM) performance are not well investigated. With the wide-band gap (WBG) device era, the switching capability of power electronics devices drastically improved. The DM/CM impact that was brought by the WBG device-based inverter becomes more serious and has not been completely understood. This thesis provides an in-depth analysis of on-board inverter control strategies and the corresponding DM/CM impact on the EV system. The OBC inverter control under vehicle-to-load (V2L) mode will be documented first. A virtual resistance damping method minimizes the nonlinear load harmonics, and a neutral balancing method regulates the unbalanced load impact through the fourth leg. In the motor drive system, a generalized CM voltage analytical model and a current ripple prediction model are built for understanding the system CM and DM stress with respect to different modulation methods, covering both 2-level and 3-level topologies. A novel CM EMI damping modulation scheme is proposed for 6-phase inverter applications. The performance comparison between the proposed methods and the conventional solution is carried out. Each topic is supported by the corresponding hardware platform and experimental validation

Can nanotechnology potentiate photodynamic therapy?

Photodynamic therapy (PDT) uses the combination of nontoxic dyes and harmless visible light to produce reactive oxygen species that can kill cancer cells and infectious microorganisms. Due to the tendency of most photosensitizers (PS) to be poorly soluble and to form nonphotoactive aggregates, drug-delivery vehicles have become of high importance. The nanotechnology revolution has provided many examples of nanoscale drug-delivery platforms that have been applied to PDT. These include liposomes, lipoplexes, nanoemulsions, micelles, polymer nanoparticles (degradable and nondegradable), and silica nanoparticles. In some cases (fullerenes and quantum dots), the actual nanoparticle itself is the PS. Targeting ligands such as antibodies and peptides can be used to increase specificity. Gold and silver nanoparticles can provide plasmonic enhancement of PDT. Two-photon excitation or optical upconversion can be used instead of one-photon excitation to increase tissue penetration at longer wavelengths. Finally, after sections on in vivo studies and nanotoxicology, we attempt to answer the title question, “can nanotechnology potentiate PDT?”National Institutes of Health (U.S.) (RO1 AI050875)United States. Air Force (Medical Free Electron Laser Program (FA9550-04-1-0079)

Multifunctional Three-Phase Four-Leg PV-SVSI With Dynamic Capacity Distribution Method

The unequal single-phase load distribution in three-phase (3P) four-wire (4W) low-voltage (LV) networks can cause significant neutral current and neutral to ground voltage rise problems at both customer and distribution transformer terminals. High neutral current can overload the neutral conductors and can cause electrical safety concerns to the users. To mitigate the high neutral current problem in an unbalanced residential LV network, a multifunctional 3P four-leg (4L) rooftop photovoltaic (PV) smart voltage source inverter (SVSI) is designed with improved active neutral current compensation along with active power export and point of common coupling (PCC) voltage regulation. A novel dynamic capacity distribution (DCD) method is proposed using the available SVSI capacity after active and reactive power operations to achieve higher capacity neutral compensation at the PCC. The performance of the designed 3P-4L PV-SVSI with the DCD method is compared with a traditional 4L SVSI with fixed unbalanced compensation capacity and a passive unbalance compensator, such as a zig-zag transformer, in PSCAD/EMTDC software. Several case studies, such as balanced and unbalanced load changing effects, are presented with actual residential loads connected to an Australian 3P-4W LV network. A Semikron Semiteach modified inverter and a real-time TMSF28335 DSP microcontroller are also used to provide experimental verification on the improvement of the proposed neutral current compensation with the DCD method. Detailed simulations and experimental studies are presented to verify the robustness and efficacy of the proposed control strategy with the designed 3P-4L PV-SVSI

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