Dynamic Interactions of a Double-stage Photovoltaic Power Converter: Modelling and Control

Photovoltaic (PV) systems are a promising renewable source to achieve green energy targets and be part of the electricity generation. Lots of efforts have been devoted to increase the penetration level of PV systems and its share in the generated electricity. Power quality is one of the challenges that impact the penetration level of PV systems. It is important to ensure high power quality from PV systems to allow more installations to the grid. So, PV power quality issues have to be addressed properly. It was reported that the poor power quality of the PV systems might be caused by many reasons such as the large amount of PV power fluctuation, the low level of current from the PV system, and large populations of PV inverters. In addition to the aforementioned reasons, recently it was suggested that perturb and observe (P&O) controller is another source of harmonics which result in a deprived PV power quality. This newly reported problem is based on experimental observations without full understanding of the generation mechanism of these harmonics in the PV system, the relation between the P&O controller design and the generated harmonics, and the effect of these harmonics on the rest of the system. Thus, in-depth analysis of the harmonics in PV systems due to P&O controller and a solution to eliminate these harmonics are demanded. Therefore, in this research an investigation is carried out to explore P&O related harmonics in a double-stage grid-connected PV system. First, regarding the P&O related harmonics full explanation of how harmonics are generated due to the perturbing nature of the P&O controller is provided, a modelling approach is suggested to identify the frequency and the amplitude of the variations in the DC bus due to the P&O controller, the effect of different factors (e.g. weather conditions, system parameters, system operating point, and P&O architecture) on the induced harmonics are investigated. Secondly, regarding the effect of the P&O related harmonics on the rest of the system an intense simulation analysis is provided to explore the possible effect of the P&O related harmonics on increasing the interaction between the system power stages. This can help to set system design recommendations and guidelines such as sizing the dc-link capacitance and designing the system controllers. Finally, a novel mitigation solution is proposed to supress the P&O related harmonics. That can help to reduce the dynamic interaction between system power stages and improve the power quality of the PV system

Advanced Solutions for Renewable Energy Integration into the Grid Addressing Intermittencies, Harmonics and Inertial Response

Numerous countries are trying to reach almost 100\% renewable penetration. Variable renewable energy (VRE), for instance wind and PV, will be the main provider of the future grid. The efforts to decrease the greenhouse gasses are promising on the current remarkable growth of grid connected photovoltaic (PV) capacity. This thesis provides an overview of the presented techniques, standards and grid interface of the PV systems in distribution and transmission level. This thesis reviews the most-adopted grid codes which required by system operators on large-scale grid connected Photovoltaic systems. The adopted topologies of the converters, the control methodologies for active - reactive power, maximum power point tracking (MPPT), as well as their arrangement in solar farms are studied. The unique L(LCL)2 filter is designed, developed and introduced in this thesis. This study will help researchers and industry users to establish their research based on connection requirements and compare between different existing technologies. Another, major aspect of the work is the development of Virtual Inertia Emulator (VIE) in the combination of hybrid energy storage system addressing major challenges with VRE implementations. Operation of a photovoltaic (PV) generating system under intermittent solar radiation is a challenging task. Furthermore, with high-penetration levels of photovoltaic energy sources being integrated into the current electric power grid, the performance of the conventional synchronous generators is being changed and grid inertial response is deteriorating. From an engineering standpoint, additional technical measures by the grid operators will be done to confirm the increasingly strict supply criteria in the new inverter dominated grid conditions. This dissertation proposes a combined virtual inertia emulator (VIE) and a hybrid battery-supercapacitor-based energy storage system . VIE provides a method which is based on power devices (like inverters), which makes a compatible weak grid for integration of renewable generators of electricity. This method makes the power inverters behave more similar to synchronous machines. Consequently, the synchronous machine properties, which have described the attributes of the grid up to now, will remain active, although after integration of renewable energies. Examples of some of these properties are grid and generator interactions in the function of a remote power dispatch, transients reactions, and the electrical outcomes of a rotating bulk mass. The hybrid energy storage system (HESS) is implemented to smooth the short-term power fluctuations and main reserve that allows renewable electricity generators such as PV to be considered very closely like regular rotating power generators. The objective of utilizing the HESS is to add/subtract power to/from the PV output in order to smooth out the high frequency fluctuations of the PV power, which may occur due to shadows of passing cloud on the PV panels. A control system designed and challenged by providing a solution to reduce short-term PV output variability, stabilizing the DC link voltage and avoiding short term shocks to the battery in terms of capacity and ramp rate capability. Not only could the suggested system overcome the slow response of battery system (including dynamics of battery, controller, and converter operation) by redirecting the power surges to the supercapacitor system, but also enhance the inertial response by emulating the kinetic inertia of synchronous generator

Enhacement of microgrid technologies using various algorithms

The electric power systems around the globe are gradually shifting from conventional fossil fuel-based generating units to green renewable energy sources. The motivation behind this change is the environmental and economic concerns. Furthermore, the existing power systems are being overloaded day by day due to the continuously increasing population, which consequently led to the overloading of transformers, transmission, and distribution lines. Despite the overwhelming advantages of renewable energy sources, there are few major issues associated with them. For example, the injection and detachment of DGs into the current power system causes disparity among produced power along with connected load, thus distracting system’s equilibrium and causes unwanted voltage and frequency oscillations and overshoots. These oscillations and overshoots may cause the failure of connected equipment or power system if not properly controlled. The investigation as such challenges to improve the frequency and voltage, the islanded’s power regulation and connected MG under source and load changes, which contain classic and artificial intelligence techniques. Moreover, these techniques are used also for economic analysis. To evaluate the exhibitions of microgrid (MG) operations and sizing economic analysis acts as a significant tool. Optimization method is obligatory for sizing and operating an MG as reasonably as feasible. Diverse optimization advances remain pertained to microgrid to get optimal power flow and management

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

Small-signal oscillatory stability of a grid-connected PV power generation farm affected by the increasing number of inverters in daisy-chain connection

The daisy-chain connection of inverters is one of the basic configurations of the power collecting network in a grid-connected photovoltaic (PV) power generation farm. In this study, the total impact of a cluster of M similar inverters in daisy-chain connection in the PV farm is examined in the following two aspects: 1) aggregated representation of the cluster of inverters is derived for stability study based on the dynamic equivalence. The derivation confirms the rationality of representing the cluster of inverters by an aggregated inverter connected to the external system via an equivalent reactance, which is the maximum eigenvalue of the matrix of daisy-chain connection defined in the article. 2) Analysis is conducted to indicate that the risk of oscillatory instability may be collectively induced by all the inverters in the daisy-chain connection in the cluster. This explains why the increasing number of inverters may imply the possible instability risk of a PV farm. An example of a power system with a grid-connected PV power generation farm is presented in the article to demonstrate and evaluate the analytical conclusion obtained

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

Electromagnetic fast-transients in LV networks with ubiquitous small-scale embedded generation

Small-scale embedded generation projects rated below 16A per phase are being integrated into low-voltage distribution networks in ever increasing numbers. Seen from the network operator's perspective as little more than negative load, the commissioning of such generators is subject to compliance with the Fit and Forget connection requirements of ENA Engineering Recommendation G83/1. This thesis has sought to quantify the electromagnetic switching transient implications of integrating very large volumes of embedded generation into the UK's low-voltage supply networks. Laboratory testing of a converter-interfaced PV source has been undertaken to characterise typical switching transient waveshapes, and equivalent representative source models have been constructed in EMTP-ATP. A detailed frequency-dependent travelling wave equivalent of the DNO-approved Generic UK LV Distribution network model has been developed and, by means of extensive statistical simulation studies, used to quantify the cumulative impact of geographically localised generators switching in response to common network conditions. It is found that the magnitude of generator-induced voltage and current transients is dependent on the number of concurrently switched generators, and on their relative locations within the network. A theoretical maximum overvoltage of 1.72pu is predicted at customer nodes remote from the LV transformer terminals, for a scenario in which all households have installed embedded generation. Latent diversity in switch pole closing and inrush inception times is found to reduce predicted peak transient voltages to around 25-40% of their theoretical maxima.EThOS - Electronic Theses Online ServiceGBUnited Kingdo