Microgrids/Nanogrids Implementation, Planning, and Operation

Today’s power system is facing the challenges of increasing global demand for electricity, high-reliability requirements, the need for clean energy and environmental protection, and planning restrictions. To move towards a green and smart electric power system, centralized generation facilities are being transformed into smaller and more distributed ones. As a result, the microgrid concept is emerging, where a microgrid can operate as a single controllable system and can be viewed as a group of distributed energy loads and resources, which can include many renewable energy sources and energy storage systems. The energy management of a large number of distributed energy resources is required for the reliable operation of the microgrid. Microgrids and nanogrids can allow for better integration of distributed energy storage capacity and renewable energy sources into the power grid, therefore increasing its efficiency and resilience to natural and technical disruptive events. Microgrid networking with optimal energy management will lead to a sort of smart grid with numerous benefits such as reduced cost and enhanced reliability and resiliency. They include small-scale renewable energy harvesters and fixed energy storage units typically installed in commercial and residential buildings. In this challenging context, the objective of this book is to address and disseminate state-of-the-art research and development results on the implementation, planning, and operation of microgrids/nanogrids, where energy management is one of the core issues

Real-Time Load Frequency Control for an Isolated Microgrid System

Microgrids are small power grids with distinct operation characteristics; they can operate either independently or connected to larger grids, and usually a significant proportion of their generation capacity is comprised from intermittent resources such as solar and wind power generations. Power grids, in general, must operate such that the power generation and power demand are balanced at all times. Such balance is attained by implementing a Load Frequency Control (LFC) mechanism. The goal of LFC in a microgrid system is to maintain the system\u27s frequency within acceptable limits around nominal value under various conditions, such as fluctuating power demand and/or contingency situation such as unexpected loss of one or more of the system\u27s generating units, in order to ensure system\u27s stable operation. In case of small and isolated microgrid systems, however, the stability of the microgrid system is an issue of much greater significance as there are no means of connecting to primary grid power. The objective of this thesis is to design a Load Frequency Control (LFC) mechanism using Battery Storage System (BSS) and Diesel Generation (DG) units for an isolated microgrid system. The microgrid system under consideration is comprised from two DG units, a BSS unit, and two solar panels. The proposed LFC mechanism is implemented in a decentralized fashion. It was tested under different operation conditions; fluctuating power demand which represents the normal operation of power systems, and emergency situations where one of the system\u27s generation units was lost in each case. Results show that the proposed control systems were robust and successful to regulate the system\u27s frequency under all conditions. The microgrid model as well as the proposed control strategy is developed within the Simulink and SimPowerSystems environments

TOWARDS OPTIMAL OPERATION AND CONTROL OF EMERGING ELECTRIC DISTRIBUTION NETWORKS

The growing integration of power-electronics converters enabled components causes low inertia in the evolving electric distribution networks, which also suffer from uncertainties due to renewable energy sources, electric demands, and anomalies caused by physical or cyber attacks, etc. These issues are addressed in this dissertation. First, a virtual synchronous generator (VSG) solution is provided for solar photovoltaics (PVs) to address the issues of low inertia and system uncertainties. Furthermore, for a campus AC microgrid, coordinated control of the PV-VSG and a combined heat and power (CHP) unit is proposed and validated. Second, for islanded AC microgrids composed of SGs and PVs, an improved three-layer predictive hierarchical power management framework is presented to provide economic operation and cyber-physical security while reducing uncertainties. This scheme providessuperior frequency regulation capability and maintains low system operating costs. Third, a decentralized strategy for coordinating adaptive controls of PVs and battery energy storage systems (BESSs) in islanded DC nanogrids is presented. Finally, for transient stability evaluation (TSE) of emerging electric distribution networks dominated by EV supercharging stations, a data-driven region of attraction (ROA) estimation approach is presented. The proposed data-driven method is more computationally efficient than traditional model-based methods, and it also allows for real-time ROA estimation for emerging electric distribution networks with complex dynamics

Optimal operation of hybrid AC/DC microgrids under uncertainty of renewable energy resources : A comprehensive review

The hybrid AC/DC microgrids have become considerably popular as they are reliable, accessible and robust. They are utilized for solving environmental, economic, operational and power-related political issues. Having this increased necessity taken into consideration, this paper performs a comprehensive review of the fundamentals of hybrid AC/DC microgrids and describes their components. Mathematical models and valid comparisons among different renewable energy sources’ generations are discussed. Subsequently, various operational zones, control and optimization methods, power flow calculations in the presence of uncertainties related to renewable energy resources are reviewed.fi=vertaisarvioitu|en=peerReviewed

Issues on Analysis and Design of Single-Phase Grid-Connected Photovoltaic Inverters

This thesis provides a comprehensive study of the problems in single-phase grid-connected photovoltaic (PV) systems. The main objective is to provide an explicit formulation of the dynamic properties of the power-electronic-based PV inverter in the frequency domain. Such a model is used as the main tool to trace the origins of the observed problems that cannot be studied with the conventional time-domain analyses. The dynamic model also provides the tool for deterministic control-system design. Grid-connected PV inverters have been reported to reduce damping in the grid, excite harmonic resonances and cause harmonic distortion. These phenomena can lead to instability or production outages and are expected to increase in the future, because the installed capacity of the grid-connected PV energy is rapidly growing. A PV generator (PVG) itself is a peculiar source affecting the inverter dynamic behavior. The PVG is internally a current-source with limited output voltage and power. The nonlinear behavior yields distinguishable operating regions: the constant-current region at the voltages lower than the maximum-power-point (MPP) voltage and constant-voltage region at the voltages higher than the MPP voltage. Such a behavior is quite well known but not really understood to need special attention. Vast majority of the photovoltaic inverters originate from the voltage-source inverter (VSI) with a capacitor connected at the input terminal for power-decoupling purposes. It is convenient to assume that the inverter is supplied by a voltage source and perform the analyses based on the existing modeling and design teqhniques of the voltage-fed (VF) VSI. However, the input voltage of a PV inverter must be controlled for MPP-tracking purposes, which implies that the inverter has to be analyzed as a current-fed (CF) inverter. Such analysis reveals that the CF VSI has second-order dynamics compared to the first-order dynamics of the VF VSI. The control dynamics of the CF VSI is also shown to incorporate right-half-plane zero and pole introducing control-system-design constraints. This thesis presents the dynamic modeling procedure of PV inverter both at open and closed loop taking into account the type of the input source. The dynamic behavior of the PVG is modeled as an operating-point-dependent dynamic resistance, which is shown to shift the operating point dependent zero and pole in the inverter control dynamics between the right and left halves of the complex plane. It is also shown that the negative-incremental-resistance behavior of the inverter output impedance makes the inverter prone to instability and the grid to harmonic resonance problems, and that such a behavior can originate e.g. from the grid synchronization and the cascaded control scheme. It is important to recognize such a behavior in order to enable reliable largescale utilization of the PV energy

Microgrid Control and Protection: Stability and Security

When the microgrid disconnects from the main grid in response to, say, upstream disturbance or voltage fluctuation and goes to islanding mode, both voltage and frequency at all locations in the microgrid have to be regulated to nominal values in a short amount of time before the operation of protective relays. Motivated by this, we studied the application of intelligent pinning of distributed cooperative secondary control of distributed generators in islanded microgrid operation in a power system. In the first part, the problem of single and multi-pinning of distributed cooperative secondary control of DGs in a microgrid is formulated. It is shown that the intelligent selection of a pinning set based on the number of its connections and distance of leader DG/DGs from the rest of the network, i.e., degree of connectivity, strengthens microgrid voltage and frequency regulation performance both in transient and steady state. The proposed control strategy and algorithm are validated by simulation in MATLAB/SIMULINK using different microgrid topologies. It is shown that it is much easier to stabilize the microgrid voltage and frequency in islanding mode operation by specifically placing the pinning node on the DGs with high degrees of connectivity than by randomly placing pinning nodes into the network. In all of these research study cases, DGs are only required to communicate with their neighboring units which facilitates the distributed control strategy. Historically, the models for primary control are developed for power grids with centralized power generation, in which the transmission lines are assumed to be primarily inductive. However, for distributed power generation, this assumption does not hold since the network has significant resistive impedance as well. Hence, it is of utmost importance to generalize the droop equations, i.e., primary control, to arrive at a proper model for microgrid systems. Motivated by this, we proposed the secondary adaptive voltage and frequency control of distributed generators for low and medium voltage microgrid in autonomous mode to overcome the drawback of existing classical droop based control techniques. Our proposed secondary control strategy is adaptive with line parameters and can be applied to all types of microgrids to address the simultaneous impacts of active and reactive power on the microgrids voltage and frequency. Also, since the parameters in the network model are unknown or uncertain, the second part of our research studies adaptive distributed estimation/compensation. It is shown that this is an effective method to robustly regulate the microgrid variables to their desired values. The security of power systems against malicious cyberphysical data attacks is the third topic of this dissertation. The adversary always attempts to manipulate the information structure of the power system and inject malicious data to deviate state variables while evading the existing detection techniques based on residual test. The solutions proposed in the literature are capable of immunizing the power system against false data injection but they might be too costly and physically not practical in the expansive distribution network. To this end, we define an algebraic condition for trustworthy power system to evade malicious data injection. The proposed protection scheme secures the power system by deterministically reconfiguring the information structure and corresponding residual test. More importantly, it does not require any physical effort in either microgrid or network level. The identification scheme of finding meters being attacked is proposed as well. Eventually, a well-known IEEE 30-bus system is adopted to demonstrate the effectiveness of the proposed schemes