Distributed Energy Resources Units Research Articles

MAS-Based Distributed Cooperative Control for DC Microgrid Through Switching Topology Communication Network With Time-Varying Delays

A multiagent system based distributed cooperative control is proposed for a dc microgrid. First, the multiagent system based control scheme is built, where each distributed energy resource unit is associated with a first-level unit control agent to deal with primary control and a secondary-level distributed cooperative control agents to implement secondary control. For the secondary control purposes of global voltage regulation and proportional current sharing, both voltage and current regulators are installed in each secondary-level agent. Their consensus secondary control protocols are designed by using the state information exchanged among only adjacent secondary-level control agents based on a switching topology communication network with time-vary delays. Then these protocols are applied into the associated first-level agent to structure an improved outer-loop droop controller. According to the reference set points corrected by the improved droop controller, an inner-loop voltage/current robust controller is designed in place of the conventional PI control to regulate the voltage and current of distributed energy resource unit. Finally, the validity of the proposed control is testified by means of simulation results.

Distributed control based power sharing strategy for an islanded AC microgrid

This study presents a novel secondary control strategy for a radial microgrid with end-to-end distributed energy resource (DER) interconnections, operating under islanded conditions. The control strategy utilises sparse communication and precludes the presence of a centralised controller. The distributed algorithm has a twofold objective. The first task is to determine the desired power injection at each DER bus for a load change occurring anywhere in the microgrid. The remaining task includes obtaining the bus angle set points at all the DER units such that the actual power injections converge to the desired power injections at each DER bus. The algorithm enables accurate system wide power injection in proportion to the respective DER base ratings. The control strategy is tested on a three-bus, three-DER system with local loads. The controller is shown to work for two different sets of microgrid ratings. The bounds are evaluated for each design parameter specified according to the system nominal ratings to maintain the stability of the distributed algorithm. Proofs are stated to show the convergence of algorithms to the correct power injection solution for all the nodes. Real-time simulation results are presented to corroborate accurate power sharing through the algorithm.

Sliding-Mode Control of AC Voltages and Currents of Dispatchable Distributed Energy Resources in Master-Slave-Organized Inverter-Based Microgrids

This paper proposes a current-controlled voltage-mode control scheme for dispatchable electronically coupled distributed energy resource (DER) units based on sliding-mode control (SMC) strategy. The proposed control strategy provides fast and stable control on the terminal voltage and frequency of DER unit and ensures protection of the power-electronic interface to external faults. In addition it maintains the quality of the output voltage of the host DER unit in spite of unbalanced and/or distorted load currents. Performance of the proposed SMC strategy is demonstrated through time-domain simulation of a single islanded DER unit, starting from black-start and subjected to various operating scenarios, and it is compared to the performance of a proportional-integral (PI) based control strategy. Also a current-mode control strategy is proposed for DER units based on SMC. The performance of both voltage- and current-mode control strategies are demonstrated in a sample master-slave organized three-unit microgrid.

Decentralized Unified Control for Inverter-Based AC Microgrids Subject to Voltage Constraints

The control law of electronically-interfaced distributed energy resources (DERs) must be able to maintain the stability and voltage regulation of the host microgrid in the two modes of operation. Ideally, this should be achieved by a decentralized primary control strategy that is independent of any communication infrastructure in order to increase the resilience of the microgrid. This is challenging as the primary control objectives in islanded and grid-connected modes of operation are conflicting. This paper proposes a decentralized control law for DER units based on state feedback and disturbance rejection. The controller provides an integral action which enables output current reference tracking. An ad-hoc partial input saturation technique is also proposed in order to prevent the integral action from having an adverse impact on the voltage amplitude and frequency regulation in the islanded mode of operation. The effectiveness of the proposed control strategy is demonstrated via a time-domain simulation of a medium-voltage distribution network with three embedded DER units, as well as through an experimental three-bus microgrid with two DER units. The results demonstrate the robustness of the proposed control strategy to transitions between the modes of operation and other network topological changes.

Adaptive Routing Algorithm for Information Management in Distributed Microgrids in Smart Grid

Abstract The increasing affordability of renewable energy resources is inspiring new interest in smart management techniques for the power grid. This paper presents an adaptive routing algorithm technique based on distributed dual-gradient update for facilitating communication among distributed energy resources (DER) units in smart grid. In particular, we consider the optimal message routing as a minimum cost flow problem among DER units in the network. The proposed technique employs a multi-commodity flow optimization, with the aim of minimizing the communication cost against network performances indicators such as throughput and delay. The results show that the proposed technique is able to route messages across the network in fewer processing steps than minimum spanning tree algorithm without any message drop. The algorithm is investigated for mesh, partial mesh and ring networks, with the result consistently showing potential cost savings and reduced complexity which can save significant amount of time when applied to a larger system.

A Microgrid Multilayer Control Concept for Optimal Power Scheduling and Voltage Control

In this paper, a novel multilayer control structure for microgrids is proposed. A scheduling layer comprehends the minimization of the microgrid operating costs together with the CO2 emissions produced and provides a sequence of power references for the next 24 h. Subsequently, within the executive layer, an off line ac power flow calculation will be performed to obtain the initial values of the voltage magnitudes of the different microgrid buses. The adjustment layer, which is the scope of this paper, includes a control strategy to maintain the voltage in the network. The purpose of this third layer is to keep the voltage within a pre-specified tolerance band by adjusting the power provided by the microgrid distributed energy resources (DER). Depending on the voltage deviation, the location of the DER units in the network and their distance from the voltage deviation, an appropriate dynamic gain will be provided to the relevant DER units. The renewed settings are then fed back to the first layer, which performs a new optimization and redistributes the adjusted reference set points among the DER units. The performance and effectiveness of the proposed hierarchical multilayer control structure were evaluated and demonstrated by several case studies.

Modeling Using Colored Petri Net of Communication Networks Based on IEC 61850 in a Microgrid Context

The increasing penetration of distributed energy resources (DERs) and local/small-scale power systems, named microgrids (MG), becomes a growing challenge for distribution system operators (DSOs). In general, the DER units and MG operation are based on a decentralized and time-critical decisions, which performs a complex behavior. To deal with this complex distributed system, the information and communication schemes are essential. Considering this, the IEC 61580 standard shows up as an appropriate solution, as it fulfills all related specifications of distributed systems. Since building a test bench becomes impractical due to the complexity and costs, the chosen solution is simulation. Therefore, this work presents a methodology that can aid the DSOs in the process of design and performance analysis of IEC 61850 networks applied to microgrid context. For this, the colored Petri net is proposed as the formal tool in order to have an easy and comprehensive modeling/simulation strategy. To validate the proposed methodology, a scenario considering the islanding detection control has been investigated with some numerical results.

Fuzzy-IP Controller for Voltage Regulation in a Stand-Alone Microgrid System

This paper presents a method that combines fuzzy inference system and Integral-Proportional named Fuzzy-IP controller to regulate an independent microgrid voltage with a distributed energy resource unit. The unit employed photovoltaic (PV) array with DC voltage converted to three-phase AC voltage. The control design was carried out through modeling and simulation using Matlab software environment. Transfer function with the 2×2 structure using system identification has estimated the non-linear plant model. Two controllers transformed the a-b-c to the d-q axis coordinates of voltage to simplify linear control design. An oscillator is applied to set frequency according to the recommendation. The results show that the compensated case-study system using Fuzzy-IP with disturbance tracked the setpoint excellently and it regulated the voltage properly including frequency control using internal oscillator. The paper presents performance superiority of the proposed method over PI control by comparing the transient response, the mean squared error, and the root mean squared error.

Small-signal stability improvement of an islanded microgrid with electronically-interfaced distributed energy resources in the presence of parametric uncertainties

This paper deals with the concept of small signal stability (SSS) issue in an electronically-interfaced microgrid with large penetration of distributed energy resources (DERs) subjected to the parametric uncertainties. Utilizing a method to enhance SSS of the system stemmed from industrial experiences is the first achievement of this study, which is realized by employing two extra feed-forward loops in the power controller section of DER unit. This control structure damps the active and reactive power oscillations by shifting the dominant eigenvalues to the left half-plane (LHP) in case of any changes in the operating point or droop coefficients. For investigating the impacts of parametric uncertainties on the overall stability, the basic theorem of the proposed robust control strategy, frequency-domain modeling of the microgrid, robustness and close-loop stability analyses are outlined. This method shows that improving the power control section topology results in higher stability margins in case of uncertain load perturbations. Furthermore, it demonstrates how much load variations are allowed to preserve SSS of the system in the frequency-domain which is a more convenient approach and takes less time than time-domain simulations. Finally, time-domain simulations conducted in MATLAB/Simulink on the sample test system will show proper results of the frequency-domain findings.

Control Strategy for Dispatchable Distributed Energy Resources in Islanded Microgrids

This paper proposes a multi-input muti-output control strategy for dispatchable power electronic based distributed energy resources (DERs) in islanded mode operation of microgrids. The controller is designed based on the dq model of the DER unit. Cascade, feedforward, and internal model control principle, incorporating the theory of integral control and repetitive controller, are used. The proposed controller regulates the voltage at the output of DERs under any load (balanced, unbalanced, and harmonic) variation and/or fault condition. The stability of the closed-loop system has been investigated. Enhanced phase-locked loop (EPLL) is used to determine the system variables in the dq frame under unbalanced (and balanced) operating conditions. The structure of the EPLL is further modified to regulate the microgrid frequency and a stability analysis of proposed frequency control loop is presented. The performance of the proposed controller is verified through test case studies, carried out on an autonomous microgrid with single and multiple DER units.

Fault tolerant control of electronically coupled distributed energy resources in microgrid systems

This paper proposes a sensor fault tolerant control (FTC) strategy for electronically-coupled distributed energy resource (DER) units in grid-connected microgrid systems. First, a fault dependent dynamical model of the DER unit incorporating sensor faults is proposed. The FTC strategy is then developed using a sliding mode observer (SMO) which has an inherent robustness property. The rational for using a SMO is the need to detect and reconstruct any fault that may occur in output measurements of the system as a result of sensor failure. The fault signals are reconstructed by using a robust approach that encompasses uncertainties in the system, such as frequency variation. This paper also represents the cyber attack as an unknown sensor fault, and consequently the DER current sensor measurements manipulated by a data integrity attack can be detected and reconstructed. The reconstructed fault signals are then used to modify the faulty sensor measurements, which are fed to the voltage source converter (VSC) controller. This ensures accurate generation of pulse width modulation (PWM) signals and consequently accurate tracking of real and reactive power references. Different case studies are tested on a detailed nonlinear model of the DER system. The impact of faults on the DER performance and the effectiveness of the proposed FTC scheme are evaluated through extensive simulation studies. Experimental verification of the method is provided to further validate the proposed approach.

Multiagent System-Based Distributed Coordinated Control for Radial DC Microgrid Considering Transmission Time Delays

This paper focuses on a multiagent-based distributed coordinated control for radial dc microgrid considering transmission time delays. First, a two-level multiagent system is constructed, where local control is formulated based on local states and executed by means of the first-level agent, and distributed coordinated control law is formulated based on wide-area information and executed by means of the secondary-level agent in order to improve the voltage control performances. Afterward, the research mainly focuses on designing the local controller and the distributed coordinated controller. For the purpose of robust stability, the local control is designed as local state-feedback-based $\text{H}_{\boldsymbol \infty }$ robust controller. It is worth mentioning that the distributed coordinated control consists of local state feedback control and decoupling coordinated control law that only come from adjacent distributed energy resource units. Moreover, the distributed coordinated controller is designed by means of delay-dependent $\text{H}_{\boldsymbol \infty }$ robust control method taking into account transmission time delays. Finally, the validity of the proposed control scheme is demonstrated by means of simulation results.

Robust multiobjective control method for power sharing among distributed energy resources in islanded microgrids with unbalanced and nonlinear loads

This paper proposes a robust control method for power sharing among dispatchable electronically-coupled distributed energy resources (DERs) in an autonomous microgrid. The DER units operate in parallel to meet the total aggregated load demand. The aggregated load is assumed to be uncertain, unbalanced and/or nonlinear, thus it is considered as a source of unmodeled dynamics. The main objective of this paper is to design an H∞ robust controller to enable tracking reference commands of the voltages at the output of the DERs in the presence of unmodeled dynamics. In addition, the H∞ control design problem is combined with other control methods, such as constrained optimal control and regional pole placement, to achieve an optimal control objective, and to improve transient response. The optimal controller is achieved by minimizing an upper bound on H2 performance. The regional pole placement is achieved by defining a linear matrix inequalities (LMI) region. This multiobjective H2/H∞ problem, combined with regional pole placement are expressed as a set of LMIs, and solved by using standard convex optimization algorithms. To show the effectiveness of the proposed method in maintaining the desired voltage at the DER outputs, in presence of unbalanced and nonlinear loads, simulation results are provided for a single DER unit, and the performance of the proposed control strategy is compared with three different controller strategies. Power sharing among multi DER units in autonomous networks is then demonstrated through digital time-domain simulation studies using MATLAB/SimPowerSystems Toolbox.

A generalized descriptor-system robust H∞ control of autonomous microgrids to improve small and large signal stability considering communication delays and load nonlinearities

This paper presents a descriptor system H∞ approach to enhance performance of robust H∞ controller and previously-reported decentralized robust servo-mechanism (DRSM) control scheme for autonomous voltage sourced converter (VSC)-based microgrids including multiple distributed energy resources (DERs). The power management system specifies voltage set points for local controllers and the frequency of each DER unit is specified by a hierarchical droop-based control structure. A descriptor system H∞ robust controller is designed based on a closed-loop representation of microgrid either with H∞ or DRSM controller for set point tracking, disturbance rejection and improving performance of microgrid for small/large-signal disturbances and nonlinear loads. Here, unlike some of the previous researches, the load current is modeled as disturbance and also communication time-delay is considered. The theoretical concepts of proposed control strategy, including mathematical modeling of microgrid, basic theorems, and design procedure are outlines. Then, design problem is formulated by a set of linear/bilinear matrix inequalities and then solved using a new iterative algorithm in the form of convex optimization problem. To demonstrate effectiveness of the proposed control scheme, offline time-domain simulation studies are performed on a multi-DER microgrid in MATLAB/Simulink environment and also the results are experimentally verified by OPAL-RT real time digital simulator.

Electric vehicle charging in stochastic smart microgrid operation with fuel cell and RES units

Plug-in electric vehicles increasingly augment their share in the global market as they appear to be an economic and emission-free alternative to modern means of transportation. As their presence strengthens, ways that will ensure economic charge along with uninterrupted grid operation are necessary to be found. This paper aims to approach the economic optimization problem that includes several Electric Vehicles (EVs) within a Low Voltage (LV) network comprising various Distributed Energy Resources (DER) as fuel cell, Renewable Energy Sources (RES), (photovoltaics, wind turbine) etc. via a scenario based simulation. The purpose is to investigate the main variables of the grid, such as its operating cost, charging patterns, power injection from the upstream network, resulting from the coordinated control of DER in Smart Microgrid operation in conjunction to the flexible load the controlled EV charging introduces. The base case study is that of absence of EVs, and therefore the demand is met only by the upstream network and the DER units. Subsequently, EVs are introduced as controllable loads and finally as dispatchable storage units incorporating a Vehicle to Grid (V2G) capability to the Smart Microgrid. Furthermore, the problem is not tackled deterministically and although forecasts for all network parameters are assumed to be known, forecasting errors and stochastic driver patterns cannot be ignored. Thus, for each imposed policy, a scenario based approach is implemented to determine operating cost in various cases along to DER utilization and the effect EVs bear on these results.

I-PD CONTROL DESIGN AND ANALYSIS IN AN ISLANDED MICROGRID SYSTEM

Abstract Voltage and frequency control is very important especially to face the migration from conventional to smart grid. In conventional way, voltage and frequency are regulated from the main power plant. However, in a smart grid system, the controller can be distributed into sub-system. A microgrid as a key sub-system must have independent control especially in islanded or stand-alone mode. This paper presents an approach named Integral-Proportional Derivative (I-PD) to control the three-phase voltage in a microgrid. In the simulation using MATLAB, a distributed energy resource unit applying voltage-source converter in order to have three-phase voltage from a DC-source is taken into account. Using system identification to simplify the controller design generates a linear model of the system. The compensated system shows a very good reference tracking capability during set point and load changes. It also reduces the coupling effect due to active and reactive power.

Review on the Coordination and Energy Management of Microgrids Broad Based on PQ Controller and Droop Control. Some Useful Information Is Given in This Paper

The integration of distributed Energy Resources (DER) is challenging electrical network quality which is the main vital and ubiquitous all over the world. Coordination and energy management of microgrids in grid connected and in islanded mode is one of solutions of distribution power quality with best reliability, and stability to ensure power system resilience. In this review, the coordination and energy management of microgrids broad based on PQ controller and droop control some useful information is given in details and analyzed. Additionally, possible structures, options and control methods of DER units are presented which is followed by the descriptions of system controls and energy management strategies. Eventually, future trends of microgrids are discuss, point out how this concept can be a key to select a best method of coordination and managing microgrids for future researchers in order to tackle some challenges based on power quality.

An Optimal Secondary Voltage Control Strategy for an Islanded Multibus Microgrid

In this paper, an optimal secondary voltage control (SVC) strategy has been proposed for an islanded multibus microgrid (MG). The proposed strategy consists of a multiobjective function, power flow equality constraints, and four sets of inequality constraints. The multiobjective function aims to achieve a compromise between multibus voltage regulation and reactive power distribution among voltage modulation units by taking full advantage of both droop-controlled and PQ-controlled distributed energy resource (DER) units in the MG. Power flow equations for an islanded MG with or without secondary frequency control were considered as equality constraints. The limits of DER capacities, bus voltages, power flows on distribution lines, and system frequency must be accommodated during the process of solving the proposed SVC control strategy iteratively using the primal-dual interior-point method. Finally, simulation results have been presented to validate the feasibility of the proposed optimal SVC strategy.

New Requirements of the Voltage/VAR Function for Smart Inverter in Distributed Generation Control

International Electronical Committee (IEC) 61850-90-7 is a part of the IEC 61850 series which specifies the advanced functions and object models for power converter based Distributed Energy Resources (DERs). One of its functions, the Voltage/VAR (V/V) control function, is used to enhance the stability and the reliability of the voltage in the distribution system. The conventional V/V function acts mainly for flattening the voltage profile as for a basic grid support function. Currently, other objectives such as the minimization of line loss and the operational costs reduction are coming into the spotlight. In order to attain these objectives, the V/V function and hence the DER units shall actively respond to the change of distribution system conditions. In this paper, the modification of V/V function and new requirements are proposed. To derive new requirements of V/V function, loss minimization is applied to a particle swarm optimization (PSO) algorithm where the condition of voltage constraint is considered not to deteriorate the voltage stability of the distribution system.

Adaptive coordinated feeder flow control in distribution system with the support of distributed energy resources

This paper aims to minimize the feeder power flow deviation from scheduled value by balancing generation and load demand within the distribution system under various local disturbances. The available distributed energy resources (DER) can be properly utilized to control the feeder flow which gains significant advantages (i) to support load frequency control (LFC) problem at transmission level (ii) reduces unscheduled interchange in feeder which is priced higher (iii) increased reliability of the grid. The deviation in feeder flow is reduced by properly coordinating the power output from DER such as fuel cell, battery storage system and responsive loads (demand response) through an adaptive fuzzy controller with optimally tuned gains using particle swarm optimization. The level of participation from each DER units are decided using fuzzy inference system based on the availability of resources and level of disturbance. The photovoltaic array and wind turbine are also considered in the system and always operated at the maximum power point. The proposed method is examined on IEEE 37 bus system implemented in the MATLAB based Power Analysis Toolbox (PAT) for performing time-domain dynamic analysis. It is shown that the proposed adaptive controller exhibits better performance for various disturbance conditions.