Robust distributed secondary voltage restoration control of ac microgrids under multiple communication delays

This paper focuses on the robust distributed secondary voltage restoration control of AC microgrids (MGs) under multiple communication delays and nonlinear model uncertainties. The problem is addressed in a multi-agent fashion where the generators’ local controllers play the role of cooperative agents communicating over a network and where electrical couplings among generators are interpreted as disturbances to be rejected. Communications are considered to be affected by heterogeneous network-induced time-varying delays with given upper-bounds and the MG is subjected to nonlinear model uncertainties and abrupt changes in the operating working condition. Robustness against uncertainties is achieved by means of an integral sliding mode control term embedded in the control protocol. Then, the global voltage restoration stability, despite the communication delays, is demonstrated through a Lyapunov-Krasovskii analysis. Given the delays’ bounds, and because the resulting stability conditions result in being non-convex with respect to the controller gain, then a relaxed linear matrix inequalities-based tuning criteria is developed to maximize the controller tuning, thus minimizing the restoration settling-time. By means of that, a criteria to estimate the maximal delay margin tolerated by the system is also provided. Finally, simulations on a faithful nonlinear MG model, showing the effectiveness of the proposed control strategy, are further discussed

On the Robust Control and Optimization Strategies for Islanded Inverter-Based Microgrids

In recent years, the concept of Microgrids (MGs) has become more popular due to a significant integration of renewable energy sources (RESs) into electric power systems. Microgrids are small-scale power grids consisting of localized grouping of heterogeneous Distributed Generators (DGs), storage systems, and loads. MGs may operate either in autonomous islanded mode or connected to the main power system. Despite the significant benefits of increasing RESs, many new challenges raise in controlling MGs. Hence, a three layered hierarchical architecture consisting of three control loops closed on the DGs dynamics has been introduced for MGs. The inner loop is called Primary Control (PC), and it provides the references for the DG’s DC-AC power converters. In general, the PC is implemented in a decentralized way with the aim to establish, by means of a droop control term, the desired sharing of power among DGs while preserving the MG stability. Then, because of inverterbased DGs have no inertia, a Secondary Control (SC) layer is needed to compensate the frequency and voltage deviations introduced by the PC’s droop control terms. Finally, an operation control is designed to optimize the operation of the MGs by providing power setpoints to the lower control layers. This thesis is mainly devoted to the design of robust distributed secondary frequency and voltage restoration control strategies for AC MGs to avoid central controllers and complexity of communication networks. Different distributed strategies are proposed in this work: (i) Robust Adaptive Distributed SC with Communication delays (ii) Robust Optimal Distributed Voltage SC with Communication Delays and (iii) Distributed Finite-Time SC by Coupled Sliding-Mode Technique. In all three proposed approaches, the problem is addressed in a multi-agent fashion where the generator plays the role of cooperative agents communicating over a network and physically coupled through the power system. The first approach provides an exponentially converging voltage and frequency restoration rate in the presence of both, model uncertainties, and multiple time-varying delays in the DGs’s communications. This approach consist of two terms: 1) a decentralized Integral Sliding Mode Control (ISMC) aimed to enforce each agent (DG) to behaves as reference unperturbed dynamic; 2) an ad-hoc designed distributed protocol aimed to globally, exponentially, achieves the frequency and voltage restoration while fulfilling the power-sharing constraints in spite of the communication delays. The second approach extends the first one by including an optimization algorithm to find the optimal control gains and estimate the corresponding maximum delay tolerated by the controlled system. In the third approach, the problem of voltage and frequency restoration as well as active power sharing are solved in finite-time by exploiting delay-free communications among DGs and considering model uncertainties. In this approach, for DGs with no direct access to their reference values, a finite-time distributed sliding mode estimator is implemented for both secondary frequency and voltage schemes. The estimator determines local estimates of the global reference values of the voltage and frequency for DGs in a finite time and provides this information for the distributed SC schemes. This dissertation also proposes a novel certainty Model Predictive Control (MPC) approach for the operation of islanded MG with very high share of renewable energy sources. To this aim, the conversion losses of storage units are formulated by quadratic functions to reduce the error in storage units state of charge prediction

Distributed control of DC grids:Integrating prosumers’ motives

In this paper, a novel distributed control strategyaddressing a (feasible) psycho-social-physical welfare problem inislanded Direct Current (DC) smart grids is proposed. Firstly,we formulate a (convex) optimization problem that allows prosumersto share current with each other, taking into accountthe technical and physical aspects and constraints of the grid(e.g., stability, safety), as well as psycho-social factors (i.e.,prosumers’ personal values). Secondly, we design a controllerwhose (unforced) dynamics represent the continuous time primaldualdynamics of the considered optimization problem. Thirdly,a passive interconnection between the physical grid and thecontroller is presented. Global asymptotic convergence of theclosed-loop system to the desired steady-state is proved andsimulations based on collected data on psycho-social aspectsillustrate and confirm the theoretical results

Small signal modeling and analysis of microgrid systems

This dissertation focuses on small-signal modeling and analysis of inverter based microgrid systems. The proposed microgrid consists of two microsources placed on two different buses. The buses are connected using a distribution feeder with some impedance. The proposed microgrid can operate with the grid support, or without the grid support. When operated without the grid support, the standalone system’s microsources participate in controlling the system voltage and frequency. For a non-inertia source, such as the inverter, the load perturbations play an important role in system dynamics. In paper-I, such complex system was studied. In the grid-tied mode, the microsources share the load demand with other sources that are present in the main grid. The control algorithm for such system is much simpler than that of the islanded system. However, when aggregated in multi-bus system, prohibitively higher order state-space models are formed. In paper-II, a reduced order modeling of such systems was considered. Singular perturbation method was applied to identify the two time-scale property of the system. In paper-III, a similar approach was taken to develop a reduced order model of the islanded system that was developed in paper-I. Application of such reduced order models were illustrated by using them to simulate a modified IEEE-37 bus microgrid system. The islanded microgrids system’s stability is characterized in paper-IV by the Markov Jump Linear System Analysis. Conservative bounds on the expected value of the state were determined from a combination of the Markov process parameters, the dynamics of each linear system, and the magnitude of the impulses. The conclusions were verified with the simulation results. --Abstract, page iii

Dynamic modeling, stability analysis, and controller design for DC distribution systems

The dc distribution systems or dc microgrids are known to be best suited for integration of renewable energy sources into the current power grid and are considered to be the key enabling technology for the development of future smart grid. Dc microgrids also benefit from better current capabilities of dc power lines, better short circuit protection, and transformer-less conversion of voltage levels, which result in higher efficiency, flexibility, and lower cost. While the idea of using a dc microgrid to interface distributed energy sources and modern loads to the power grid seems appealing at first, several issues must be addressed before this idea can be implemented fully. The configuration, stability, protection, economic operation, active management, and control of future dc microgrids are among the topics of interest for many researchers. The purpose of this dissertation is to investigate the dynamic behavior and stability of a future dc microgrid and to introduce new controller design techniques for the Line Regulating Converters (LRC) in a dc distribution system. Paper I is devoted to dynamic modeling of power converters in a dc distribution system. The terminal characteristics of tightly regulated power converters which are an important factor for stability analysis and controller design are modeled in this paper. Paper II derives the simplified model of a dc distribution system and employs the model for analyzing stability of the system. Paper III introduces two controller design methods for stabilizing the operation of the LRC in presence of downstream constant power loads in a dc distribution system. Paper IV builds upon paper III and introduces another controller design method which uses an external feedback loop between converters to improve performance and stability of the dc grid. --Abstract, page iv

Non-linear model predictive energy management strategies for stand-alone DC microgrids

Due to substantial generation and demand fluctuations in stand-alone green micro-grids, energy management strategies (EMSs) are becoming essential for the power sharing purpose and regulating the microgrids voltage. The classical EMSs track the maximum power points (MPPs) of wind and PV branches independently and rely on batteries, as slack terminals, to absorb any possible excess energy. However, in order to protect batteries from being overcharged by realizing the constant current-constant voltage (IU) charging regime as well as to consider the wind turbine operational constraints, more flexible multivariable and non-linear strategies, equipped with a power curtailment feature, are necessary to control microgrids. This dissertation work comprises developing an EMS that dynamically optimises the operation of stand-alone dc microgrids, consisting of wind, photovoltaic (PV), and battery branches, and coordinately manage all energy flows in order to achieve four control objectives: i) regulating dc bus voltage level of microgrids; ii) proportional power sharing between generators as a local droop control realization; iii) charging batteries as close to IU regime as possible; and iv) tracking MPPs of wind and PV branches during their normal operations. Non-linear model predictive control (NMPC) strategies are inherently multivariable and handle constraints and delays. In this thesis, the above mentioned EMS is developed as a NMPC strategy to extract the optimal control signals, which are duty cycles of three DC-DC converters and pitch angle of a wind turbine. Due to bimodal operation and discontinuous differential states of batteries, microgrids belong to the class of hybrid dynamical systems of non-Filippov type. This dissertation work involves a mathematical approximation of stand-alone dc microgrids as complementarity systems (CSs) of Filippov type. The proposed model is used to develop NMPC strategies and to simulate microgrids using Modelica. As part of the modelling efforts, this dissertation work also proposes a novel algorithm to identify an accurate equivalent electrical circuit of PV modules using both standard test condition (STC) and nominal operating cell temperature (NOCT) information provided by manufacturers. Moreover, two separate stochastic models are presented for hourly wind speed and solar irradiance levels

Reduced-order models for representing converters in power system studies

A reduced-order model that preserves physical meaning is important for generating insight in large-scale power system studies. The conventional model-order reduction for a multiple-timescale system is based on discarding states with fast (short timescale) dynamics. It has been successfully applied to synchronous machines, but is inaccurate when applied to power converters because the timescales of fast and slow states are not sufficiently separated. In the method proposed here, several fast states are at first discarded but a representation of their interaction with the slow states is added back. Recognizing that the fast states of many converters are linear allows well-developed linear system theories to be used to implement this concept. All the information of the original system relevant to system-wide dynamics, including nonlinearity, is preserved, which facilitates judgments on system stability and insight into control design. The method is tested on a converter-supplied mini power system and the comparison of analytical and experiment results confirms high preciseness in a broad range of conditions

Protection and Control of Active Distribution Networks and Microgrids

This thesis is mainly focused on (i) modeling and control of Electronically Coupled Distributed Energy Resources (EC-DERs) under severe network imbalances and transient incidents, and (ii) protection of active distribution networks and microgrids against different types of faults. In the first part, an enhanced control strategy is proposed to improve the performance of EC-DERs under faults and transient disturbances, in a multi-unit microgrid setting. With the use of proposed control strategy, the host microgrid can ride through network faults, irrespective of whether they take place within the microgrid jurisdiction or impact the upstream grid, and quickly reclaim its pre-fault operating conditions to improve post-fault recovery. Further, the proposed control scheme enables the host microgrid to retain its power quality for the duration of the faults, in both modes of operation, which is a desirable property for detection of certain classes of faults, as well as for sensitive loads. In the second part of the thesis, appropriate strategies are proposed for protection of low- and medium-voltage microgrids in the islanded mode as well as the grid-connected mode of operation. The proposed protection strategies aim to detect and isolate the faults that impact the microgrid, in a selective manner. The proposed strategies can be implemented through programmable microprocessor-based relays which are commercially available; hence, the structure of new relays that enable the proposed protection strategies are also discussed in the thesis. In addition, the thesis investigates the operation of an existing distribution network as a microgrid. Thus, practical control and protection strategies that enable off-grid operation of the distribution network (considering the system constraints) are discussed. The effectiveness of the proposed control and protection strategies are demonstrated through time-domain simulation studies conducted in the PSCAD/EMTDC software environment