A survey on modeling of microgrids - from fundamental physics to phasors and voltage sources

Microgrids have been identified as key components of modern electrical systems to facilitate the integration of renewable distributed generation units. Their analysis and controller design requires the development of advanced (typically model-based) techniques naturally posing an interesting challenge to the control community. Although there are widely accepted reduced order models to describe the dynamic behavior of microgrids, they are typically presented without details about the reduction procedure---hampering the understanding of the physical phenomena behind them. Preceded by an introduction to basic notions and definitions in power systems, the present survey reviews key characteristics and main components of a microgrid. We introduce the reader to the basic functionality of DC/AC inverters, as well as to standard operating modes and control schemes of inverter-interfaced power sources in microgrid applications. Based on this exposition and starting from fundamental physics, we present detailed dynamical models of the main microgrid components. Furthermore, we clearly state the underlying assumptions which lead to the standard reduced model with inverters represented by controllable voltage sources, as well as static network and load representations, hence, providing a complete modular model derivation of a three-phase inverter-based microgrid

Frequency Control of Wind and PV Based Isolated Microgrid

The wind and solar PV based isolated microgrid can be economical and reliable for the remote area where the grid is not available. The frequency and voltage fluctuation are major problems in such remote microgrid due to intermittent nature of renewable resources like wind and solar PV. The frequency fluctuation should remain within acceptable limits for stable operation of the remote microgrid. This thesis work focuses on frequency control of wind and photovoltaic (PV) based isolated micro-grid using traditional droop control technique. The battery system acts as a master unit which is responsible for setting up the common AC bus voltage and frequency. The proportionalresonant (PR) voltage and current controllers are designed to control the current and voltage output of the voltage source inverter (VSI) connected with the DC battery. The better control performance can be obtained by using PR controller which easily tracks reference signal and eliminates steady state error in resonant frequency. The traditional droop control method is applied to generate voltage and the frequency reference set points for the VSI controller to control frequency of the AC micro-grid. The detailed analysis of the frequency control is performed by conducting two different case studies and load sharing between inverters is also presented to observe power shared by different inverters in isolated microgrid. In all the cases, system frequency is maintained within the acceptable range for the stable operation of wind and solar PV based isolated microgrid

Microgrid Control Strategy Study and Controller Design Based on Model Predictive Control

In the 21st century, because of the exhausting of oil, coal and other non-renewable energy, human beings enter a period of large-scale exploitation and utilization of renewable energy. Renewable energy generation become an important way for new energy utilization, however, as more and more distributed generation connect to power distribution network, the traditional distribution network structure will be changed. A large number of distributed generation applications of modern power electronic technology, have also produced a lot of harmonics to impact the power quality. It will threaten the safe operation of the distribution network and obstruct the utilization of renewable energy. The concept of microgrid provides a new thinking for the application of renewable energy. Microgrid can make full use of the characteristics of the renewable energy and it is the key of the future resources and environment for human beings. We can predict that, microgrid construction will be rapidly developed in 21st century, based on the utilization of renewable energy. In order to coordinate the contradiction between power grid and distributed generation, the concept of microgrid arises at the historic moment. Microgrid has two operation modes: islanded mode and grid-connected mode. By theoretically analyzing, simulation model construction and result analyzing, the microgrid coordinated control strategies will be studied in this paper. Firstly, this paper starts from the microgrid operation control mode, respectively establishing the traditional control strategy of simulation for the isolated and connected microgrid. The isolated grid control strategies is V/f control strategy based on droop characteristic and the connected grid control strategies is P/Q control strategy. Second, the model predictive control is introduced in chapter three including its principle and application. In the case study, the traditional PI controller is compared with model predictive control controller in single distributed generation system to introduce advantages of model predictive control method. Last, the model of microgrid with multiple distributed generations is built in MATLAB/Simulink. There are three cases in this model: working model switches between grid-connected and islanded mode; increase and decrease load in islanded mode; disconnect one PV system at certain time in islanded mode. By analyzing results of three cases, the MPC controller can achieve desirable efficiency of power control. Meanwhile, the voltage and frequency are working in the required range of the system. That proves the effectiveness of MPC controller

Uncovering Droop Control Laws Embedded Within the Nonlinear Dynamics of Van der Pol Oscillators

This paper examines the dynamics of power-electronic inverters in islanded microgrids that are controlled to emulate the dynamics of Van der Pol oscillators. The general strategy of controlling inverters to emulate the behavior of nonlinear oscillators presents a compelling time-domain alternative to ubiquitous droop control methods which presume the existence of a quasi-stationary sinusoidal steady state and operate on phasor quantities. We present two main results in this work. First, by leveraging the method of periodic averaging, we demonstrate that droop laws are intrinsically embedded within a slower time scale in the nonlinear dynamics of Van der Pol oscillators. Second, we establish the global convergence of amplitude and phase dynamics in a resistive network interconnecting inverters controlled as Van der Pol oscillators. Furthermore, under a set of non-restrictive decoupling approximations, we derive sufficient conditions for local exponential stability of desirable equilibria of the linearized amplitude and phase dynamics

The Modeling and Advanced Controller Design of Wind, PV and Battery Inverters

Renewable energies such as wind power and solar energy have become alternatives to fossil energy due to the improved energy security and sustainability. This trend leads to the rapid growth of wind and Photovoltaic (PV) farm installations worldwide. Power electronic equipments are commonly employed to interface the renewable energy generation with the grid. The intermittent nature of renewable and the large scale utilization of power electronic devices bring forth numerous challenges to system operation and design. Methods for studying and improving the operation of the interconnection of renewable energy such as wind and PV are proposed in this Ph.D. dissertation.;A multi-objective controller including is proposed for PV inverter to perform voltage flicker suppression, harmonic reduction and unbalance compensation. A novel supervisory control scheme is designed to coordinate PV and battery inverters to provide high quality power to the grid. This proposed control scheme provides a comprehensive solution to both active and reactive power issues caused by the intermittency of PV energy. A novel real-time experimental method for connecting physical PV panel and battery storage is proposed, and the proposed coordinated controller is tested in a Hardware in the Loop (HIL) experimental platform based on Real Time Digital Simulator (RTDS).;This work also explores the operation and controller design of a microgrid consisting of a direct drive wind generator and a battery storage system. A Model Predictive Control (MPC) strategy for the AC-DC-AC converter of wind system is derived and implemented to capture the maximum wind energy as well as provide desired reactive power. The MPC increases the accuracy of maximum wind energy capture as well as minimizes the power oscillations caused by varying wind speed. An advanced supervisory controller is presented and employed to ensure the power balance while regulating the PCC bus voltage within acceptable range in both grid-connected and islanded operation.;The high variability and uncertainty of renewable energies introduces unexpected fast power variation and hence the operation conditions continuously change in distribution networks. A three-layers advanced optimization and intelligent control algorithm for a microgrid with multiple renewable resources is proposed. A Dual Heuristic Programming (DHP) based system control layer is used to ensure the dynamic reliability and voltage stability of the entire microgrid as the system operation condition changes. A local layer maximizes the capability of the Photovoltaic (PV), wind power generators and battery systems, and a Model Predictive Control (MPC) based device layer increases the tracking accuracy of the converter control. The detail design of the proposed SWAPSC scheme are presented and tested on an IEEE 13 node feeder with a PV farm, a wind farm and two battery-based energy storage systems

Feedforward decoupling control method in grid-interfaced inverter

Recently, microgrid has been studied and applied widely all over the world. More and more experimental microgrids are being connected to the utility grid. This paper presents an improvement in the real and reactive power control of three-phase grid-interfaced inverter for microgrid applications. Based on the traditional PI feedback current control, the desirable values of P and Q can be achieved by controlling the currents in d-q stationary frame. Moreover, the feedforward control method also brings some advantages to the systems such as higher reliability and enhanced stability. One of the most important improvements is to decouple the real and reactive power, i.e. P and Q are controlled separately. In this paper, the controller with feedforward algorithm has been simulated and shows some promiscuous results. © 2013 Australasian Committee for Power Engineering (ACPE)

Modeling and Analyzing of Inverters for Controlling Voltage and Frequency in an Islanded Microgrid

Recently, the growth of inverter-based generations (IBGs) like solar photovoltaic (PV) and wind turbine generators in the form of microgrids (MGs) has been increasing. However, control of voltage and frequency becomes a challenging task for MGs especially when they operate in an islanded mode, due to the inherent low-inertia feature of IBGs compared to a grid-tied mode where there is a grid support. This thesis deals with modeling and analyzing of inverters capable of controlling voltage and frequency through external conventional droop control for the islanded operation of MG. In this thesis, a state space model is developed for a voltage source inverter with droop control. Moreover, non-linear models for the inverters in single and parallel cases with droop control are developed and the results are verified in different load scenarios. The results show a good performance of the projected control, both in the isolated operation of an inverter supplying a load, and in the parallel operation of isolated inverters, where there is power sharing to properly feed the load. MATLAB/Simulink environment is used for the work

Technical Challenges and Solutions of a three-phase bidirectional two stage Electric Vehicle charger

The sustainability of the power grid owing to the building strain of the ever-growing demand for electrical energy urges innovative and more practical solutions that enable active participation of end-users in stable and reliable management of power systems. One of the emerging projections of such a two-way exchange of electrical power between the grid and consumers is the developing field of bidirectional energy trade between power providers and electric vehicle owners. A bidirectional, three-phase, two-stage off-board electric vehicle EV charger design is proposed in this research. The first stage acts as alternating current AC to direct current DC converter during charging operation and behaves as three phase inverter and power factor corrector when energy exchange is from vehicle to grid. The second stage is a bidirectional DC-DC level converter linked to the first stage by a DC bus. The grid side filter is designed to enable the grid interfacing without any significant power quality problems. The proposed design, topology and the devised control infrastructure are tested through simulations on MATLAB/Simulink platform by interfacing the charger to a three-phase AC microgrid and the results approve the performance of the proposed charging topology