A Novel Single-Phase Grid-connected PV Inverter System

Renewable energy sources, especially solar energy, continue to gain popularity and are ready to become a significant part of global energy portfolio. Grid-connected inverter based distributed generator is becoming increasingly popular due to its advanced control flexibility. Power quality and reliability are attracting much attention in such systems. In order to meet requirements of future applications and maximize the value of inverter system, advanced inverter functions are expected to provide more functionalities. This dissertation proposes a novel single phase inverter system combining proposed advanced control schemes and active power decoupling technique. This dissertation firstly investigates the existing power control schemes for single phase grid-connected inverter and then proposed an independent power control scheme, which is implemented in stationary reference frame. The synchronization function is combined in power loop directly to eliminate the use of conventional phase locked loop. The proposed controller with double-loop current controller based on proportional resonant compensator is proved to achieve good power tracking performance even under distorted grid conditions. Active damping function for resonant peak problem is also implemented in controller. Inverter based distributed generators may operate in different conditions and transition between different operating conditions may result into voltage spikes across the local loads and inrush currents into the grid due to the failure of synchronization on point of common coupling voltage. In this dissertation, a novel control scheme based on model predictive control is proposed for grid connected inverter to enable the capability to operate in both grid-connected and island conditions and the capability to seamless transfer between different conditions through proposed synchronization and phase adjustment algorithm. The auto-tuning strategy of weight factor is presented as well as the stability analysis on the system. Compared with the conventional methods, the proposed seamless transfer control strategy has simpler structure and exhibits good transient performance. Double line frequency ripple power is inherent in single phase rectifiers and inverters and can be adverse to system performance. Therefore, numerous active power decoupling techniques have been introduced to decouple that. All existing active topologies are investigated. Comprehensive comparison is conducted on the minimum required capacitance for power decoupling, the dc voltage utilizations, the current stresses, the modulation complexity and even application evaluations except for power rating and component counts. Based on the investigations and generalized comparison results, a new active power decoupling circuit composed of dual buck converters is proposed together with its control and modulation strategy. The ripple power is stored in split dc link capacitors with high energy utilization. The proposed power decoupling circuit could reduce the storage capacitance needed. The proposed power decoupling circuit does not have shoot-through concern, thus it could enhance the overall system reliability and decoupling performance

Coordinated active power reduction strategy for voltage rise mitigation in LV distribution network

Integration of renewable energy systems by the utility, customers, and the third party into the electric power system, most especially in the MV and LV distribution networks grew over the last decade due to the liberalization of the electricity market, rising energy demand, and increasing environmental concern. The distributed rooftop PV system contributes to relieve the overall load, reduce losses, avoid conventional generation upgrade, and better matching of demand on the LV distribution network. Originally, the LV distribution network is designed for unidirectional current flow, that is from the substation to customers. However, a high penetration of rooftop solar PVs (with power levels typically ranging from 1 – 10 kW) may lead to the current flowing in the reverse direction and this could result in a sudden voltage rise. These negative impacts on the network have discouraged the distribution network operators (DNOs) to allow increased PV penetration in the LV distribution network because some customers load, and equipment are sensitive to voltage perturbation. Presently, the most applied voltage rise mitigation strategy for high rooftop solar PV penetration is the total disconnect from the LV distribution network when the voltage at the point of common coupling (PCC) goes above statutory voltage limits. However, the sudden disconnection of the PV system from the grid can cause network perturbation and affect the security of the network. This action may also cause voltage instability in the network and can reduce the lifetime of grid equipment such as voltage regulators, air conditioner etc. Due to this negative impact, different voltage rise mitigation strategies such as the active transformer with on load tap changers (OLTC), distributed battery energy storage system and reactive power support (D-STATCOM, etc.) have been used to curtail voltage rise in the distribution network. However, the implementation of D-STATCOM device on a radial LV distribution network results in high line current and losses. This may be detrimental to the distribution network. Therefore, in this thesis, a coordinated active power reduction (CAPR) strategy is proposed using a modified PWM PI current control strategy to ramp down the output power and voltage of a grid-tied voltage source inverter (VSI). In the proposed strategy, a reactive reference is generated based on the measured voltage level at the PCC using a threshold voltage algorithm to regulate the amplitude of the modulating signal to increase the off time of the high frequency signal which shut down the PV array momentary in an extremely short time and allow the VSI to absorb some reactive power through the freewheeling diode and reduce voltage. The proposed CAPR strategy was designed and simulated on a scaled down simple radial LV distribution network in MATLAB®/Simulink® software environment. The results show that the CAPR can ramp down the PV output power, reduce reverse power flow and reduce the sudden voltage rise at the point of common coupling (PCC) within ±5% of the standard voltage limit. The study also compares the performance of the proposed CAPR strategy to that of the distributed static compensator (D-STATCOM) and battery energy storage system (BESS) with respect to response time to curtail sudden voltage rise, losses and reverse power flow. The investigation shows that the D-STATCOM has the faster response time to curtail voltage rise. However, the voltage rise reduction is accompanied by high current, losses and reverse active power flow. The introduction of the BESS demonstrates better performance than the D- STATCOM device in terms of reverse power flow and losses. The CAPR strategy performs better than both D-STATCOM and BESS in terms of line losses and reverse power flow reduction

Symmetry in Renewable Energy and Power Systems

This book includes original research papers related to renewable energy and power systems in which theoretical or practical issues of symmetry are considered. The book includes contributions on voltage stability analysis in DC networks, optimal dispatch of islanded microgrid systems, reactive power compensation, direct power compensation, optimal location and sizing of photovoltaic sources in DC networks, layout of parabolic trough solar collectors, topologic analysis of high-voltage transmission grids, geometric algebra and power systems, filter design for harmonic current compensation. The contributions included in this book describe the state of the art in this field and shed light on the possibilities that the study of symmetry has in power grids and renewable energy systems

Power quality improvement utilizing photovoltaic generation connected to a weak grid

Microgrid research and development in the past decades have been one of the most popular topics. Similarly, the photovoltaic generation has been surging among renewable generation in the past few years, thanks to the availability, affordability, technology maturity of the PV panels and the PV inverter in the general market. Unfortunately, quite often, the PV installations are connected to weak grids and may have been considered as the culprit of poor power quality affecting other loads in particular sensitive loads connected to the same point of common coupling (PCC). This paper is intended to demystify the renewable generation, and turns the negative perception into positive revelation of the superiority of PV generation to the power quality improvement in a microgrid system. The main objective of this work is to develop a control method for the PV inverter so that the power quality at the PCC will be improved under various disturbances. The method is to control the reactive current based on utilizing the grid current to counteract the negative impact of the disturbances. The proposed control method is verified in PSIM platform. Promising results have been obtaine

Design of power converters with embedded energy storage for hybrid DC-AC applications

The high penetration of renewable energies into power systems is leading to a revolution in the structure of modern power grids. In this context, the present thesis investigates the design of power electronics converters with extended capabilities due to the embedding of energy storage within the topologies. Thus, the research objective is to propose power converters with capabilities of integrating energy storage technologies to provide further services required for the operation of hybrid dc-ac systems. The thesis contains two parts, first part shows the work developed for low- and medium-power applications, while the second part describes the investigation performed for high-power systems. The first part of this thesis explains the design and operation of a three-port dc-dc-ac converter developed for integrating energy storage into hybrid dc-ac applications. The topology is based on a conventional two-level dc-ac converter, and it uses a single power conversion stage to control the power flow between three ports, minimising the required components. Simulation and experimental results validate the operation of the proposal, showing that a multi-variable control system allows exploiting the degrees of freedom to manage power interactions of multiple elements without needing extra power converters. Furthermore, a comparative analysis is carried on to showcase the advantages and limitations of the proposal as opposed to state-of-the-art solutions in the same context. The study concludes that the proposed topology is suitable for low- and medium-power systems with bidirectional power flow capabilities among all ports and limited voltage boost needs. Simulation analysis shows that efficiencies up to 95.94% can be reached for a 3 kW design, which compares to efficiencies of similar state-of-the-art topologies. Moreover, the operation is also validated in a reduced-scale prototype allowing to test the multi-variable control scheme in a real-time implementation. The second part of the thesis focuses on the design and operation of a Modular Multilevel Converter (MMC) topology with integrated energy storage using new parallel branches in the phases of the converter. This topology allows the integration of partially-rated Energy Storage Systems(ESS) to decouple the ac and dc sides of a High Voltage Direct Current~(HVDC) substation. Thus, it enables the provision of ancillary services such as fast frequency response, black-start capabilities and load-levelling, which are required by modern hybrid dc-ac power grids. Results show that the proposal allows the addition of up to 37% power from the ESS considering similarly rated power semiconductors in a simulated 1 GW MMC substation. Analysis shows that extra device losses remain under 1% for an additional +-10% of ESS power on top of the nominal substation-rated power. Furthermore, a laboratory-scale experimental rig was built to demonstrate the operation of the proposed design. In conclusion, two different topologies are proposed and analysed for integrating energy storage into hybrid dc-ac applications depending on the power rating required. The study is supported by simulation and experimental results obtained during the project to validate both proposals

Designs for ultra-high efficiency grid-connected power conversion

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 213-215).Grid connected power conversion is an absolutely critical component of many established and developing industries, such as information technology, telecommunications, renewable power generation (e.g. photovoltaic and wind), even down to consumer electronics. There is an ever present demand to reduce the volume and cost, while increasing converter efficiency and performance. Reducing the losses associated with energy conversion to and from the grid can be accomplished through the use of new circuit topologies, enhanced control methods, and optimized energy storage. The thesis outlines the development of foundational methods and architectures for improving the efficiency of these converters, and allowing the improvements to be scaled with future advances in semiconductor and passive component technologies. The work is presented in application to module integrated converters (MICs), often called micro-inverters. These converters have been under rapid development for single-phase gridtied photovoltaic applications. The capacitive energy storage implementation for the double-line-frequency power variation represents a differentiating factor among existing designs, and this thesis introduces a new topology that places the energy storage block in a series-connected path with the line interface. This design provides independent control over the capacitor voltage, soft-switching for all semiconductor devices, and full four-quadrant operation with the grid.by Brandon J. Pierquet.Ph.D

Model based forecasting for demand response strategies

The incremental deployment of decentralized renewable energy sources in the distribution grid is triggering a paradigm change for the power sector. This shift from a centralized structure with big power plants to a decentralized scenario of distributed energy resources, such as solar and wind, calls for a more active management of the distribution grid. Conventional distribution grids were passive systems, in which the power was flowing unidirectionally from upstream to downstream. Nowadays, and increasingly in the future, the penetration of distributed generation (DG), with its stochastic nature and lack of controllability, represents a major challenge for the stability of the network, especially at the distribution level. In particular, the power flow reversals produced by DG cause voltage excursions, which must be compensated. This poses an obstacle to the energy transition towards a more sustainable energy mix, which can however be mitigated by using a more active approach towards the control of the distribution networks. Demand side management (DSM) offers a possible solution to the problem, allowing to actively control the balance between generation, consumption and storage, close to the point of generation. An active energy management implies not only the capability to react promptly in case of disturbances, but also to ability to anticipate future events and take control actions accordingly. This is usually achieved through model predictive control (MPC), which requires a prediction of the future disturbances acting on the system. This thesis treat challenges of distributed DSM, with a particular focus on the case of a high penetration of PV power plants. The first subject of the thesis is the evaluation of the performance of models for forecasting and control with low computational requirements, of distributed electrical batteries. The proposed methods are compared by means of closed loop deterministic and stochastic MPC performance. The second subject of the thesis is the development of model based forecasting for PV power plants, and methods to estimate these models without the use of dedicated sensors. The third subject of the thesis concerns strategies for increasing forecasting accuracy when dealing with multiple signals linked by hierarchical relations. Hierarchical forecasting methods are introduced and a distributed algorithm for reconciling base forecasters is presented. At the same time, a new methodology for generating aggregate consistent probabilistic forecasts is proposed. This method can be applied to distributed stochastic DSM, in the presence of high penetration of rooftop installed PV systems. In this case, the forecasts' errors become mutually dependent, raising difficulties in the control problem due to the nontrivial summation of dependent random variables. The benefits of considering dependent forecasting errors over considering them as independent and uncorrelated, are investigated. The last part of the thesis concerns models for distributed energy markets, relying on hierarchical aggregators. To be effective, DSM requires a considerable amount of flexible load and storage to be controllable. This generates the need to be able to pool and coordinate several units, in order to reach a critical mass. In a real case scenario, flexible units will have different owners, who will have different and possibly conflicting interests. In order to recruit as much flexibility as possible, it is therefore importan

Alternative Sources of Energy Modeling, Automation, Optimal Planning and Operation

An economic development model analyzes the adoption of alternative strategy capable of leveraging the economy, based essentially on RES. The combination of wind turbine, PV installation with new technology battery energy storage, DSM network and RES forecasting algorithms maximizes RES integration in isolated islands. An innovative model of power system (PS) imbalances is presented, which aims to capture various features of the stochastic behavior of imbalances and to reduce in average reserve requirements and PS risk. Deep learning techniques for medium-term wind speed and solar irradiance forecasting are presented, using for first time a specific cloud index. Scalability-replicability of the FLEXITRANSTORE technology innovations integrates hardware-software solutions in all areas of the transmission system and the wholesale markets, promoting increased RES. A deep learning and GIS approach are combined for the optimal positioning of wave energy converters. An innovative methodology to hybridize battery-based energy storage using supercapacitors for smoother power profile, a new control scheme and battery degradation mechanism and their economic viability are presented. An innovative module-level photovoltaic (PV) architecture in parallel configuration is introduced maximizing power extraction under partial shading. A new method for detecting demagnetization faults in axial flux permanent magnet synchronous wind generators is presented. The stochastic operating temperature (OT) optimization integrated with Markov Chain simulation ascertains a more accurate OT for guiding the coal gasification practice