Control strategies for improving power quality and PLL stability evaluation in microgrids

In recent years the interest in environmental protection and energy sustainability has steadily increased; this fact has promoted research activities, and projects focused on non-conventional renewable energy (NCRE) sources as a replacement for fossil fuels. In this context, NCRE-based technologies offer a solution for integrating distributed energy resources where the Microgrid (MG) concept has been introduced to facilitate the integration of a large number of micro-generators, energy storage units and loads. This integration is enabled by power electronic converters, and therefore there is an increase in the number of these devices in the modern electrical power systems. The proliferation of power electronic-based systems will inevitably affect both the power quality and the stability of electrical systems which have power electronic interfaces. In this context, two key issues that need to be addressed are the following: (i) the imbalances and harmonics inherently present in MG due to the random nature of loads and (ii) since MGs are weak systems, the stability between the power electronic-interface and the Point of Common Coupling (PCC). These two critical issues in MGs are studied in this thesis. The former is analysed considering 4-leg systems while the latter is studied considering a 3-leg system. The 3-wire system is used for developing a methodology to study the effects of the weak nature of MGs on a simple system. As future work, this methodology can be modified to analyse specific control schemes proposed in this thesis to manage imbalances and harmonic in MGs. To manage imbalance and harmonic issues in 4-leg MGs, two approaches are proposed. First, a novel methodology based on the application of a 4-leg shunt Active Power Filter (APF) is proposed. The control for the compensator is based on the Conservative Power Theory (CPT) augmented by resonant controllers. Experimental validation of the APF will be performed with a focus on the transient behaviour and on its performance when frequency variations occur. In the second approach, a cooperative control scheme based on the CPT is proposed for sharing imbalances and harmonics in three-phase four-wire droop-controlled systems. With this approach, it is demonstrated that the CPT can be an effective tool to develop imbalance sharing algorithms in isolated MGs.The behaviour of proposed control schemes is demonstrated using experimental prototypes deployed in a laboratory scale 4-leg MG, and using unbalanced and distorted conditions of a real isolated MG located in Canada. To study how the weak nature of MGs affect the stability of the interface between a converter and the PCC, a stability analysis focused on the phase-locked loop (PLL) has been undertaken based on a simple dq model proposed in this thesis. The aim of this proposal is to understand the effect of the weak nature of the MG (measured with the short circuit ratio) and the PLL bandwidth on the stability of the system. The system studied corresponds to a 3-leg converter connected to a balanced three-phase weak grid. The performance of this method is validated through simulation using Plexim PLECS software, and experimental validation using a laboratory-scale system is performed. The main contributions of this thesis can be summarised as follows: • In microgrids, relatively large variations in the electrical frequency may occur. The sensitivity to grid frequency variations is a known limitation of the CPT, and this work proposes and validates an implementation approach that shows the limited impact of frequency variation on the APF performance. • The CPT is applied to 4-wire systems considering full control of the 4 legs in the APF, to regulate the positive, negative and zero sequence voltages synthesised by the power converter. This includes the operation of the CPT compensator with single-phase non-linear loads. • The proposed sharing algorithm uses a novel approach based on the CPT methodology that can be used to implement a simple identification of the balanced, unbalanced and non-linear components of the currents and powers. Separation of the sequence components is not required. This produces a more robust imbalance sharing algorithm, particularly because most of the sequence separation algorithms are strongly affected by noise, harmonic distortion, small variations in the sampling time, etc. • A very simple method to share harmonic distortion between the generation sources is realised considering the application of the CPT transform. In this thesis, this methodology is discussed and experimentally validated. • A control algorithm is proposed based on the CPT which is very robust to issues such as distortion, noise, changes in the sampling time etc. As is demonstrated in this work, it is very simple to extend the proposed methodology to include harmonic distortion. • A systematic PLL design process is proposed to be used in weak grids to ensure system stability. This design process is based on a proposed low-complexity small signal model of the system in the dq reference frame. Based on this proposal, it could be possible (in future work), analyse the effects of the weak nature of MGs on the performance of the CPT-based controllers proposed in this thesis

Single-phase consensus-based control for regulating voltage and sharing unbalanced currents in 3-wire isolated AC microgrids

A distributed control strategy is proposed to share unbalanced currents in three-phase threewire isolated AC Microgrids (MGs). It is based on a novel approach where, rather than analysing the MG as a three-phase system, it is analysed as three single-phase subsystems. The proposal uses a modified single-phase Q - E droop scheme where two additional secondary control actions are introduced per phase. The first control action performs voltage regulation, while the second one achieves the sharing of negative sequence current components between the 3-legs power converters located in the MG. These secondary control actions are calculated online using a consensus-based distributed control scheme to share negative sequence current components, voltage regulation, and regulating the imbalance at the converters' output voltage to meet the IEEE power quality standards. The proposed methodology has the following advantages over other distributed control solutions, such as those based on the symmetrical components or those based on the Conservative Power Theory: (i) it achieves sharing of unbalanced currents, inducing smaller imbalances in the converters' output voltages than those of other methods, and (ii) the sharing of the unbalanced currents is simultaneously realised in both the sequence domain and the a-b-c domain. The latter is difficult to achieve using other solutions, as will be demonstrated in this work. Extensive experimental validation of the proposed distributed approach is provided using a laboratory-scale 3-wire MG

Distributed Control Strategies for Microgrids: An Overview

There is an increasing interest and research effort focused on the analysis, design and implementation of distributed control systems for AC, DC and hybrid AC/DC microgrids. It is claimed that distributed controllers have several advantages over centralised control schemes, e.g., improved reliability, flexibility, controllability, black start operation, robustness to failure in the communication links, etc. In this work, an overview of the state-of-the-art of distributed cooperative control systems for isolated microgrids is presented. Protocols for cooperative control such as linear consensus, heterogeneous consensus and finite-time consensus are discussed and reviewed in this paper. Distributed cooperative algorithms for primary and secondary control systems, including (among others issues) virtual impedance, synthetic inertia, droop-free control, stability analysis, imbalance sharing, total harmonic distortion regulation, are also reviewed and discussed in this survey. Tertiary control systems, e.g., for economic dispatch of electric energy, based on cooperative control approaches, are also addressed in this work. This review also highlights existing issues, research challenges and future trends in distributed cooperative control of microgrids and their future applications

Experimental evaluation of a CPT-based 4-leg active power compensator For distributed generation

Four-wire microgrids (MGs) and distribution systems are inherently unbalanced with the presence of negative and zero sequence components in voltages and currents. In small autonomous systems, the imbalance, in addition to the harmonic distortion produced by nonlinear loads, can significantly affect the power quality, loadability, and stability of the system. Furthermore, in isolated networks with significant generation from intermittent renewable energy sources, the stiffness of the system is reduced and this could amplify the effects of imbalance on the stability and power quality. To mitigate some of these problems, a novel methodology based on the application of a four-leg active power filter is proposed in this paper. The control of the compensator is based on the conservative power theory augmented by resonant controllers. The behavior of the proposed system is demonstrated using an experimental prototype deployed in a laboratory scale MG

A Control Algorithm Based on the Conservative Power Theory for Cooperative Sharing of Imbalances in 4-Wire Systems

© 1986-2012 IEEE. A cooperative control scheme based on the conservative power theory (CPT) is proposed, which can share imbalances in three-phase four-wire droop-controlled systems. By utilizing the CPT, the balanced, unbalanced, and distorted components of the currents and powers in a microgrid can be identified. Using control loops based on virtual impedances and implemented in the stationary a-b-c frame, the imbalances and harmonics are shared between the different four-leg inverters in the microgrid. A secondary control loop is implemented to regulate the maximum voltage imbalance/distortion at the point of common coupling or any other point in the microgrid. The theoretical background of the method is presented, and experimental validation is demonstrated using a laboratory-scale microgrid with two inverters operating at 5 kW

Distributed Control Strategy Based on a Consensus Algorithm and on the Conservative Power Theory for Imbalance and Harmonic Sharing in 4-Wire Microgrids

A distributed control system is proposed which uses the Conservative Power Theory (CPT) and a consensus algorithm to share imbalance and harmonics between different converters in three-phase four-wire droop-controlled Microgrids (MGs). The CPT is used to identify the balanced, unbalanced and distorted components of the currents and powers in the system. Control loops based on virtual impedance and implemented in the stationary a-b-c frame are then used to distribute these components between the various converters in the MG. The magnitudes of the virtual impedances are adaptively calculated using a novel consensus-based distributed control scheme with the aim of sharing imbalances and harmonics according to the residual VA capacity of each converter whilst regulating the imbalance and distortion at their output to meet the appropriate IEEE power quality standards. Extensive simulations show that the proposed distributed control has excellent performance, and experimental validation is provided using a laboratory-scale 4-wire MG

Cyber-attacks in modular multilevel converters

Distributed control of modular multilevel converter (MMC) submodules offers several potential benefits such as flexibility and modularity. In this approach, low-level control tasks, such as capacitor voltage balancing, can be distributed amongst controllers placed in the SMs. This decreases the computational burden for the central control system that performs high-level control tasks. Distributed control architecture requires a cyber-physical network (CFN) through which local controllers share all the information necessary to perform their respective control loops. To date, none of the reported works in this field have paid attention to potential imperfections in the CFN. Indeed, previous works are based on the assumption that the network always provides correct information to the local controllers. However, erroneous measurements in the CFN may degrade the distributed control scheme operation, leading to suboptimal or even unstable operation. These events can occur in the presence of cyber-attacks. This paper is the first to investigate the impacts of cyber-attacks in MMCs. The effects of a specific cyber-attack, named false data injection attack (FDIA), on a consensus-based distributed control strategy are studied. Additionally, a method for detecting FDIAs is proposed, along with a countermeasure strategy, to ensure the safe operation of the MMC. The proposals reported in this paper are validated using simulation and experimental results

Distributed Predictive Secondary Control for Imbalance Sharing in AC Microgrids

This paper proposes a distributed predictive secondary control strategy to share imbalance in three-phase, three-wire isolated AC Microgrids. The control is based on a novel approach where the imbalance sharing among distributed generators is controlled through the control of single-phase reactive power. The main characteristic of the proposed methodology is the inclusion of an objective function and dynamic models as constraints in the formulation. The controller relies on local measurements and information from neighboring distributed generators, and it performs the desired control action based on a constrained cost function minimization. The proposed distributed model predictive control scheme has several advantages over solutions based on virtual impedance loops or based on the inclusion of extra power converters for managing single-phase reactive power among distributed generators. In fact, with the proposed technique the sharing of imbalance is performed directly in terms of single-phase reactive power and without the need for adding extra power converters into the microgrid. Contrary to almost all reported works in this area, the proposed approach enables the control of various microgrid parameters within predefined bands, providing a more flexible control system. Extensive simulation and Hardware in the Loop studies verify the performance of the proposed control scheme. Moreover, the controller’s scalability and a comparison study, in terms of performance, with the virtual impedance approach were carried out

Control strategies for improving power quality and PLL stability evaluation in microgrids

Tesis para optar al grado de Doctor en Ingeniería Eléctrica en cotutela con la Universidad de Nottingham, U.K.En los últimos años, el interés por la protección del medio ambiente y la sostenibilidad energética ha aumentado de manera constante; este hecho ha promovido actividades de investigación y proyectos centrados en fuentes de energía renovable no convencional (ERNC) como reemplazo de los combustibles fósiles. En este contexto, las tecnologías basadas en ERNC ofrecen una solución para integrar recursos energéticos distribuidos donde se ha introducido el concepto Microred (MR) para facilitar la integración de un gran número de microgeneradores, unidades de almacenamiento de energía y cargas. Esta integración se logra utilizando convertidores basados en electrónica de potencia, y por lo tanto, hay un aumento en estos dispositivos en los sistemas modernos de energía eléctrica. La proliferación de convertidores de potencia afectará inevitablemente tanto la calidad de la energía como la estabilidad de los sistemas eléctricos que tienen interfaces basadas en estos dispositivos. En este contexto, dos cuestiones clave que deben abordarse son las siguientes: (i) los desequilibrios y armónicos inherentemente presentes en la MR debido a la naturaleza aleatoria de las cargas y (ii) dado que las microredes son sistemas débiles, la estabilidad entre el convertidor de potencia y el punto de acoplamiento común (PCC). Estas dos cuestiones críticas en microredes se estudian en esta tesis. El primero se analiza considerando sistemas de 4 hilos mientras que el último se estudia considerando un sistema balanceado de 3 hilos. Para gestionar los problemas de desequilibrio y armónicos en microredes de 4 hilos, se proponen dos enfoques. Primero, se propone una metodología novedosa basada en la aplicación de un filtro de potencia activa (APF, por sus siglas en inglés) de 4 hilos. El control del compensador se basa en la Teoría de la Potencia Conservativa (CPT). En el segundo enfoque, se propone un esquema de control cooperativo basado en la CPT para compartir desequilibrios y armónicos en sistemas trifásicos de cuatro hilos controlados mediante control droop. El comportamiento de los dos esquemas de control propuestos se demuestra mediante el uso de prototipos y utilizando las condiciones de operacion una microred real ubicada en Canadá. Para estudiar cómo la naturaleza débil de las MG afecta la estabilidad de la interfaz entre un convertidor y el PCC, se ha realizado un análisis de estabilidad centrado en el bucle de bloqueo de fase (PLL, por sus siglas en inglés), basado en una modelacion del sistema en coordenadas dq. El sistema estudiado corresponde a un convertidor de 3 hilos conectado a una red débil trifásica equilibrada. El rendimiento de este método se valida mediante la simulación con el software Plexim PLECS y se realiza una validación experimental

Control strategies for improving power quality and PLL stability evaluation in microgrids

In recent years the interest in environmental protection and energy sustainability has steadily increased; this fact has promoted research activities, and projects focused on non-conventional renewable energy (NCRE) sources as a replacement for fossil fuels. In this context, NCRE-based technologies offer a solution for integrating distributed energy resources where the Microgrid (MG) concept has been introduced to facilitate the integration of a large number of micro-generators, energy storage units and loads. This integration is enabled by power electronic converters, and therefore there is an increase in the number of these devices in the modern electrical power systems. The proliferation of power electronic-based systems will inevitably affect both the power quality and the stability of electrical systems which have power electronic interfaces. In this context, two key issues that need to be addressed are the following: (i) the imbalances and harmonics inherently present in MG due to the random nature of loads and (ii) since MGs are weak systems, the stability between the power electronic-interface and the Point of Common Coupling (PCC). These two critical issues in MGs are studied in this thesis. The former is analysed considering 4-leg systems while the latter is studied considering a 3-leg system. The 3-wire system is used for developing a methodology to study the effects of the weak nature of MGs on a simple system. As future work, this methodology can be modified to analyse specific control schemes proposed in this thesis to manage imbalances and harmonic in MGs. To manage imbalance and harmonic issues in 4-leg MGs, two approaches are proposed. First, a novel methodology based on the application of a 4-leg shunt Active Power Filter (APF) is proposed. The control for the compensator is based on the Conservative Power Theory (CPT) augmented by resonant controllers. Experimental validation of the APF will be performed with a focus on the transient behaviour and on its performance when frequency variations occur. In the second approach, a cooperative control scheme based on the CPT is proposed for sharing imbalances and harmonics in three-phase four-wire droop-controlled systems. With this approach, it is demonstrated that the CPT can be an effective tool to develop imbalance sharing algorithms in isolated MGs.The behaviour of proposed control schemes is demonstrated using experimental prototypes deployed in a laboratory scale 4-leg MG, and using unbalanced and distorted conditions of a real isolated MG located in Canada. To study how the weak nature of MGs affect the stability of the interface between a converter and the PCC, a stability analysis focused on the phase-locked loop (PLL) has been undertaken based on a simple dq model proposed in this thesis. The aim of this proposal is to understand the effect of the weak nature of the MG (measured with the short circuit ratio) and the PLL bandwidth on the stability of the system. The system studied corresponds to a 3-leg converter connected to a balanced three-phase weak grid. The performance of this method is validated through simulation using Plexim PLECS software, and experimental validation using a laboratory-scale system is performed. The main contributions of this thesis can be summarised as follows: • In microgrids, relatively large variations in the electrical frequency may occur. The sensitivity to grid frequency variations is a known limitation of the CPT, and this work proposes and validates an implementation approach that shows the limited impact of frequency variation on the APF performance. • The CPT is applied to 4-wire systems considering full control of the 4 legs in the APF, to regulate the positive, negative and zero sequence voltages synthesised by the power converter. This includes the operation of the CPT compensator with single-phase non-linear loads. • The proposed sharing algorithm uses a novel approach based on the CPT methodology that can be used to implement a simple identification of the balanced, unbalanced and non-linear components of the currents and powers. Separation of the sequence components is not required. This produces a more robust imbalance sharing algorithm, particularly because most of the sequence separation algorithms are strongly affected by noise, harmonic distortion, small variations in the sampling time, etc. • A very simple method to share harmonic distortion between the generation sources is realised considering the application of the CPT transform. In this thesis, this methodology is discussed and experimentally validated. • A control algorithm is proposed based on the CPT which is very robust to issues such as distortion, noise, changes in the sampling time etc. As is demonstrated in this work, it is very simple to extend the proposed methodology to include harmonic distortion. • A systematic PLL design process is proposed to be used in weak grids to ensure system stability. This design process is based on a proposed low-complexity small signal model of the system in the dq reference frame. Based on this proposal, it could be possible (in future work), analyse the effects of the weak nature of MGs on the performance of the CPT-based controllers proposed in this thesis