Ali Bidram

Hierarchical Structure of Microgrids Control System

Advanced control strategies are vital components for realization of microgrids. This paper reviews the status of hierarchical control strategies applied to microgrids and discusses the future trends. This hierarchical control structure consists of primary, secondary, and tertiary levels, and is a versatile tool in managing stationary and dynamic performance of microgrids while incorporating economical aspects. Various control approaches are compared and their respective advantages are highlighted. In addition, the coordination among different control hierarchies is discussed.

Control and Circuit Techniques to Mitigate Partial Shading Effects in Photovoltaic Arrays

Partial shading in photovoltaic (PV) arrays renders conventional maximum power point tracking (MPPT) techniques ineffective. The reduced efficiency of shaded PV arrays is a significant obstacle in the rapid growth of the solar power systems. Thus, addressing the output power mismatch and partial shading effects is of paramount value. Extracting the maximum power of partially shaded PV arrays has been widely investigated in the literature. The proposed solutions can be categorized into four main groups. The first group includes modified MPPT techniques that properly detect the global MPP. They include power curve slope, load-line MPPT, dividing rectangles techniques, the power increment technique, instantaneous operating power optimization, Fibonacci search, neural networks, and particle swarm optimization. The second category includes different array configurations for interconnecting PV modules, namely series-parallel, total-cross-tie, and bridge-link configurations. The third category includes different PV system architectures, namely centralized architecture, series-connected microconverters, parallel-connected microconverters, and microinverters. The fourth category includes different converter topologies, namely multilevel converters, voltage injection circuits, generation control circuits, module-integrated converters, and multiple-input converters. This paper surveys the proposed approaches in each category and provides a brief discussion of their characteristics.

Distributed Control Systems for Small-Scale Power Networks: Using Multiagent Cooperative Control Theory

Existing electric power distribution networks are operating near full capacity and facing rapid changes to address environmental concerns and improve their reliability and sustainability. These concerns are satisfied through the effective integration and coordination of distributed generators (DGs), which facilitate the exploitation of renewable energy resources, including wind power, photovoltaics, and fuel cells [1]. Although DGs can be of rotating machinery type, more recently, DGs have been designed to support renewable energy resources by electronic interfacing through voltage source inverters (VSI). Each DG corresponds to one energy source, and its control inputs are given to the interface VSI [1]-[5]. The successful coordination of DGs can be realized through microgrids, which are small-scale power systems consisting of local generation, local loads, and energy storage systems. Microgrids are autonomous subsystems with dedicated control systems that provide guaranteed power quality for local loads such as hospitals, economic centers, apartments, and universities. The microgrid concept, with its local control and power quality support, allows for the scalable integration of local power resources and loads into the existing power grid and enables a high penetration of distributed generation [5]-[10].

A Multiobjective Distributed Control Framework for Islanded AC Microgrids

This paper proposes a distributed two-layer control structure for ac microgrids. Inverter-based distributed generators (DGs) can operate either as voltage-controlled voltage source inverters (VCVSI) or current-controlled voltage source inverters (CCVSI). VCVSIs provide the voltage and frequency support, whereas CCVSIs regulate the generated active and reactive powers. The proposed control structure has two main layers. The first layer deals with the voltage and frequency control of VCVSIs. The second layer regulates the active and reactive powers of CCVSIs. These controllers are implemented through two communication networks with one-way communication links and are fully distributed; each DG only requires its own information and the information of its neighbors on the communication network graph. The proposed control framework is verified on a microgrid test system and IEEE 34 test feeder.

Distributed Adaptive Voltage Control of Inverter-Based Microgrids

This paper proposes an adaptive and distributed secondary voltage control for microgrids with inverter-based distributed generators (DG). The proposed control is fully adaptive and does not require the information of DG parameters. Neural networks are used to compensate for the uncertainties caused by the unknown dynamics of DGs. The controller structure is fully distributed such that each DG only requires its own information and the information of its neighbors on the communication network. Therefore, this secondary control is associated with a sparse communication network. The effectiveness of the proposed methodology is verified for different loading, outage, and islanding scenarios, as well as variable communication structures in a microgrid setup.

Multi-Resolution Modeling of Power Electronics Circuits Using Model-Order Reduction Techniques

Highly detailed models of power-electronic converter circuits can be slow to simulate due to the wide disparity in transient time scales. This paper presents a framework for multi-resolution simulation of switching converter circuits by providing an appropriate amount of detail based on the time scale and phenomenon being considered. In this approach, a detailed full-order model that accounts for the higher-order effects of components, parasitics, switching nonlinearity (e.g., saturated inductors), switching event detection, etc., is constructed first. Efficient order-reduction techniques are then used to extract several lower order models for the desired resolution of the simulation. The simulation resolution can be adjusted as needed even during a simulation run time. The state continuity across different resolutions and switching events is ensured using appropriate similarity transforms. The proposed high-fidelity model of converter is verified with hardware measurement and is used to verify different simulation resolutions. The proposed methodology is demonstrated to achieve orders of magnitudes improvement in simulation speed.

Synchronization of nonlinear heterogeneous cooperative systems using input-output feedback linearization

In this paper, input-output feedback linearization is used to design distributed controls for multi-agent systems with nonlinear and heterogeneous non-identical dynamics. Using feedback linearization, the nonlinear and heterogeneous dynamics of agents are transformed to identical linear dynamics and non-identical internal dynamics. Based on the dependence of agent outputs on agent inputs, feedback linearization may lead to a first-order or high-order tracking synchronization problem. The controller for each agent is designed to be fully distributed such that each agent only requires its own information and the information of its neighbors. The effectiveness of the proposed control protocols are verified by simulation on a microgrid test system.

Frequency control of electric power microgrids using distributed cooperative control of multi-agent systems

Distributed cooperative control of multi-agent systems is used to implement the secondary frequency control of microgrids. The proposed control synchronizes the frequency of distributed generators (DG) to the nominal frequency and shares the active power among DGs based on their ratings. This frequency control is implemented through a communication network with one-way communication links, and is fully distributed such that each DG only requires its own information and the information of its neighbors on the communication network graph. Due to the distributed structure of the communication network, the requirements for a central controller and complex communication network are obviated, and the system reliability is improved. Simulation results verify the effectiveness of the proposed secondary control for a microgrid test system.

Distributed cooperative control of nonlinear and non-identical multi-agent systems

This paper exploits input-output feedback linearization technique to implement distributed cooperative control of multi-agent systems with nonlinear and non-identical dynamics. Feedback linearization transforms the synchronization problem for a nonlinear and heterogeneous multi-agent system to the synchronization problem for an identical linear multiagent system. The controller for each agent is designed to be fully distributed, such that each agent only requires its own information and the information of its neighbors. The proposed control method is exploited to implement the secondary voltage control for electric power microgrids. The effectiveness of the proposed control is verified by simulating a microgrid test system.

Finite-time frequency synchronization in microgrids

This paper proposes a finite time frequency controller that synchronizes the microgrid frequency to the nominal frequency and shares the active power among distributed generators (DG) based on their active power ratings. The finite-time control accelerates the synchronization speed and provides the synchronization for microgrid frequency and DG active powers in a finite period of time. In addition, the proposed control is distributed; i.e., each DG only requires its own information and the information of its neighbors on the communication network graph. The efficacy of the proposed finite-time frequency control subsequent to islanding process and load changes is verified for an islanded microgrid test system.