Islanded Microgrid Operation Research Articles

Two-Stage Load Shedding for Secondary Control in Hierarchical Operation of Islanded Microgrids

A two-stage load shedding scheme is presented to cope with potential power deficits caused by microgrid islanding. Coordinated with the fast response of inverter-based grid-forming distributed energy resources, the proposed approach estimates the load shedding at each stage and the resulting power flow redistribution. The first stage of load shedding will cease any rapid frequency decline in which the locally measured frequency deviation is employed to guide the load shedding level and process. Once a new steady state is reached at the end of first stage, the second stage is activated, which performs load shedding according to load priorities. The effectiveness of the proposed scheme is verified through time-domain simulations in PSCAD/EMTDC based on a scaled-down microgrid system.

Decentralised robust T‐S fuzzy controller for a parallel islanded AC microgrid

This study proposes a decentralised robust fuzzy control strategy for islanded operation of an AC microgrid with voltage source inverters. The objective is to design a robust controller for regulating the load voltage and sharing power among distributed generators (DGs) in the presence of uncertainties in the system and non-linear loads. The AC microgrid consists of parallel DGs connected to a main AC bus. A Takagi-Sugeno fuzzy approach is developed in this article to achieve stability and desired performance in dealing with non-linearities in the islanded microgrid and H ∞ criterion is used to obtain a robust control strategy in presence of uncertainties raised by unmodelled and high frequency dynamics. Therefore, a non-convex condition in H ∞ optimisation problem is converted to a convex linear matrix inequality (LMI) condition and is solved by MATLAB LMI toolbox. In order to develop a decentralized system, P–f and Q–V droop controllers are used to specify set-points for local inverter controllers in each DG. The effectiveness of the proposed control strategy in presence of constant power load and DG accidental outage is validated by the simulation of a microgrid test system in MATLAB SimPowerSystems toolbox. Comparison with cascaded proportional integral controller shows advantages of robust T-S controller.

Differential Frequency Protection Scheme Based on Off-Nominal Frequency Injections for Inverter-Based Islanded Microgrids

Protection is considered one of the major challenges associated with the operation of islanded microgrids with high penetration of inverter-based distributed generation (IBDG). As stated in the IEEE Std. 1547.4, due to the limited injected fault current levels by IBDG, conventional overcurrent relays might not be capable of detecting and clearing faults. Thus, this paper proposes a novel protection strategy for inverter-based islanded microgrids which relies on injecting an off-nominal frequency, through the DG interface, during fault conditions. A differential protection scheme that is based on the differences in the current frequency components, measured at both ends of a feeder section, is proposed to detect and isolate faults. This paper also presents the structure and control logic of the proposed relay for differential protection. The proposed protection scheme is implemented and tested on a distribution system equipped with IBDG, modeled to operate in an islanded mode. Simulation results are presented to highlight the effectiveness of the proposed protection scheme in detecting and isolating faults for inverter-based islanded microgrids.

Scenario‐based multi‐objective optimisation with loadability in islanded microgrids considering load and renewable generation uncertainties

Operation of microgrids in islanded mode either by intention or by involuntary action is of increasing concern nowadays. Optimal operation and control of such islanded microgrids considering various objectives are crucial for effective planning problem. This work proposes a new methodology for optimal operation of islanded microgrids considering economic and emission objectives together with loadability in droop-regulated islanded microgrid. Loadability being one of dominant aspects to be considered for effective microgrid planning and operation to ensure increased voltage-stability margin in the entire due course of operation. In conjunction with that islanded microgrid operational constraints and uncertainty in various system parameters need to be considered to interpret real-time operation of the microgrid. In the light of above, a multi-objective optimisation problem in droop-regulated islanded microgrid is formulated with economic–emission–loadability objectives by considering uncertainties in load demand and renewable generation along with system constraints. The formulated problem is solved using multi-objective antlion optimiser algorithm and validated on the test system. The performance of the proposed approach is compared with other optimisation algorithms to show its effectiveness. The importance of considering loadability along with economic–emission objectives to achieve compromised solution resulted in ensuring better operational planning of droop-regulated islanded microgrids.

A novel energy scheduling framework for reliable and economic operation of islanded and grid-connected microgrids

This paper proposes a novel energy management framework by combining proactive and reactive approaches to efficiently address the uncertainties associated with generation and demand in islanded and interconnected operation of residential microgrid (MG). The MG considers renewable energy sources (RESs), a diesel generator (DG), and a storage device with the possibility of power exchanges with the grid. The proposed formulation is cast as stochastic mixed-integer linear programming (MILP) model with 24-hour rolling horizon, simulated periodically by updating input data and advancing on an hourly basis for a one year scheduling period. The objective is to minimize the total MG cost while preserving comfort priorities of individual households. The energy balance is potentiated by smart central management system of heating, ventilation and air-conditioning and electric water heater loads in conjunction with exploiting the building thermal inertia of well-insulated buildings. Random outages of MG components are modeled via a two-state Markov chain process, which is considered in the reactive approach. Extensive numerical results are presented to reveal the benefits obtained by coordinating the demand response and supply sources for a practical case of Helsinki, Finland. Finally, the impact of various system parameters on the optimal expected MG cost and risk indices is investigated.

A comprehensive review of energy management and planning of islanded microgrids: Part 1 — optimization formulations

This survey paper provides a critical overview of optimization formulations for planning and operation of islanded microgrids, including optimization objectives, constraints, and control variables. The optimization approaches reviewed address methods both for increasing the resiliency of advanced distribution systems and electrification of remote communities. This paper examines over 120 individual optimization studies and discovers that all optimizations studies of islanded microgrids are based on formulations selecting a combination of 16 possible objective functions, 14 constraints, and 13 control variables. Each of the objectives, constraints, and variables are discussed exhaustively both from the perspective of their importance to islanded microgrids and chronological trends in their popularity.

Techno-Economic Operation and Environmental Life-Cycle Assessment of a Solar PV-Driven Islanded Microgrid

Microgrids (MGs) are playing an important role in the maximum utilization of distributed energy resources. The optimal economic operation and low-carbon electricity generation can enhance MGs effectiveness. This paper presents the results of a solar-photovoltaic (PV)-driven islanded MG’s techno-economic optimization analysis and environmental life-cycle assessment (LCA) to achieve economical and environmentally superior performance. A net present cost (NPC)-based simulation for optimal sizing of the MG is proposed. A novel life-cycle inventory (LCI) is developed to evaluate the impacts of the MG under 21 midpoint indicators and three endpoint indicators by the ReCiPe 2016 method, metal particle releases by the Ecopoints approach, and the greenhouse-gas emissions by the IPCC method. The sensitivity analysis is carried out to verify the effects for three different batteries and five different PV modules for all of the considered impact indicators. The results reveal that the proposed MG offers a revenue of 29,520 US$/yr by routing excess energy to neighbors after fulfilling the prosumers’ demand at an optimal net present cost of 364,906 US$. Furthermore, the outcomes obtained from the LCA analysis show that, among the MG components, batteries have the highest impact on human health (74%) and the ecosystem (78%) due to greater greenhouse-gas emissions (CO2-48%, CH4-37%, and N20-48%).

Robust Line Hardening Strategies for Improving the Resilience of Distribution Systems With Variable Renewable Resources

Line hardening refers to strengthening certain distribution lines that will not be subject to outages during extreme conditions. Line hardening provides a resilient solution in the case of major faults in a radial or meshed distribution system. We propose a robust optimal line hardening method coupled with multiple provisional microgrids to improve the distribution system resilience against worst N-k contingencies. In this case, provisional microgrids, which are interconnected by hardened liens, do not have the islanding capability and depend on the supply provided by one or more electrically connected microgrids for islanding purposes. A trilevel optimal model is considered with the objectives of minimizing the costs of line hardening and the operation of multiple islanded provisional microgrids, which could include the cost of load shedding in each provisional microgrid considering the worst N-k contingencies. Specifically, a two-stage robust optimization model is formulated and Benders decomposition (BD) algorithm coupled with iterative relaxation procedure (IRP) is developed to enable the tractable computation. In our work, the master problem determines line hardening strategies and the subproblem discovers the impact of the worst N-k contingencies corresponding to the optimal operation of reformed provisional microgrids. The second-stage which includes the max–min subproblem is solved efficiently using linearization techniques, duality theory, and IRP. Our numerical results for the IEEE distribution test systems validate the effectiveness of the proposed model and reveal that distributed generation is critical in increasing the resilience of distribution systems corresponding to the worst N-k contingencies in provisional microgrids.

Active and Reactive Power Joint Optimization Dispatch of islanded Microgrid Based on SCCSA Algorithm

Based on the requirements of the economic reliability of the operation of islanded micro grid, a joint optimal dispatch model of the active power and reactive power of the islanded micro grid is proposed in this paper, which considers the constraints of upper and lower limits of output force and power balance of each micro source. In order to minimize the cost of operation and the penalty cost of voltage deviation of each node, an improvement is made on the basis of the original crow search algorithm, and combined with the advantages of the sine cosine algorithm, the improved sine cosine crow search algorithm is used to solve the model and the results of the active reactive power output of the island are obtained. The simulation results show that the proposed method can effectively control the voltage of each node in the micro grid within the allowable range, and is more beneficial to the economic operation of micro grid.

Optimum Interconnected Islanded Microgrids Operation with High Levels of Renewable Energy

ABSTRACTThis paper aims to study the optimal probabilistic energy efficiency horizon of active distribution networks with high penetration of wind power generation considering a variety of possible islanded microgrid (IMG) zones. The penetration of renewable power sources such as wind turbines (WTs) and the load variations can create challenges in the active distribution system that will significantly affect system modeling, power flow algorithm, voltage stability index and system planning. In this regard, an appropriate model for IMGs is suggested, which aims to estimate the voltage stability margin of a two-bus system based on both saddle-node and limited induced bifurcations. Furthermore, a new concept, named reduced IMG network (RIN), is well explored to generalize the proposed index to n-bus islanded MGs using a developed power flow algorithm. The simulated frame work indicates a two-stage probabilistic planning algorithm that considers the variability in load and generation. The proposed algorithm improve voltage stability index and minimize the IMG customers’ interruption. This improvement allow for a time delay in renewing MGs. The performance and effectiveness of the proposed method are tested and verified through the IEEE 33-bus test system across different case studies.

Driven Primary Regulation for Minimum Power Losses Operation in Islanded Microgrids

The paper proposes an improved primary regulation method for inverter-interfaced generating units in islanded microgrids. The considered approach employs an off-line minimum losses optimal power flow (OPF) to devise the primary frequency regulation curve’s set-points while satisfying the power balance, frequency and current constraints. In this way, generators will reach an optimized operating point corresponding to a given and unique power flow distribution presenting the minimum power losses. The proposed approach can be particularly interesting for diesel-based islanded microgrids that face, constantly, the issue of reducing their dependency from fossil fuels and of enhancing their generation and distribution efficiency. The Glow-worm Swarm Optimization (GSO) algorithm is selected as a key heuristic tool for solving the optimization problem. The main program is carried out in Matlab environment. A case study with a parametric analysis is implemented and all results are assessed and compared with the conventional droop control method to show the effectiveness of the proposed method as well as the improved reliability of the system.

Optimal Operation of Droop-Controlled Islanded Microgrids

Recently researchers have focused on the optimal operation of droop-controlled islanded microgrids (DCIMG). However, most of the papers consider only the economic objective. But, with increasing focus on environmental compliance, it has also become necessary to reduce emission from generation activities. Furthermore, most of the previous papers do not consider the heat demand in the microgrid and the uncertainties involved in load and renewable generation forecasting. This paper bridges all these gap areas by presenting a method for determining optimal droop settings of dispatchable distributed generation units in a DCIMG. The objectives are to minimize the operational cost and minimize the emission in the islanded droop-controlled microgrid while meeting all the operational constraints. The proposed formulation takes into account the electricity demand, the heat demand, load uncertainties, and renewable power uncertainties in the microgrid. Load and renewable power uncertainties are modeled by a stochastic scenario based approach. The multiobjective optimization problem is solved using fuzzified particle swarm optimization. The proposed method is validated on a 33-bus DCIMG test system. The results show the effectiveness of the proposed method.

PWM Common Mode Reference Generation for Maximizing the Linear Modulation Region of CHB Converters in Islanded Microgrids

Microgrid-interactive converters are increasingly expected to provide auxiliary services, particularly during islanded microgrid operation. Unbalanced, non-sinusoidal pulse width modulation (PWM) references may be required when converters provide auxiliary services, such as negative-sequence compensation. Since PWM references cannot exceed dc-bus voltage defined limits without causing overmodulation, the maximum positive-sequence voltage producible by microgrid-interactive converters may be diminished unless reference waveforms are adjusted. In this paper, a technique for maximizing the linear modulation region by injecting a common mode component into PWM references is proposed for microgrid-interactive cascaded H-bridge (CHB) converters. The technique presented can be implemented in real-time without lookup tables, is controller independent, is compatible with any carrier-based PWM method, enabling its use for symmetric and asymmetric CHB, and enables continued operation after internal fault events when utilized with fault tolerant PWM schemes. Experimental data verify the common mode injection technique.

Power Flow of Islanded AC Microgrids: Revisited

In this letter, a new methodology is proposed to extend the applicability of conventional power flow methods to islanded microgrids (MGs), operating with the droop control method. A slack bus is considered to be connected at a random MG node. The operational conditions of this slack bus as well as of the rest MG nodes are determined through an iterative procedure. The objective of this procedure is to force the active and reactive power flowing through the slack bus to zero, representing the islanded MG operation. The proposed approach has a similar accuracy with time-domain simulations, while it reduces significantly the required computational burden.

A Communication-Supported Comprehensive Protection Strategy for Converter-Interfaced Islanded Microgrids

The deployment of distributed generators (DGs) gives rise to several challenges for a microgrid or conventional distribution feeder, regarding control and protection issues. The major ones are: bi-directional flow of power, changes in fault current magnitude, and continuous changes in operational configuration due to both the plug-and-play of DGs and loads, and the intermittency of the renewable DGs. This issue is exacerbated when the microgrid contains several converter-interfaced DGs and operates in the islanded mode of operation. Hence, conventional protection strategies and relaying techniques will no longer be sufficient to protect islanded microgrids against network faults and disturbance conditions. This paper proposes a fast and reliable communication-supported protection strategy for ensuring the safe operation of converter-interfaced islanded microgrids. The strategy is implementable using commercially accessible microprocessor based digital relays, and is applicable for the protection of low voltage islanded microgrids. It provides backup protection to handle communication failures and malfunctions of protective devices. The paper also presents the detailed structural layout of the digital relay, which executes the proposed protection strategy. A number of improvements are proposed to find an alternative method for conventional overcurrent relays to reliably detect small-magnitude fault currents and high impedance faults, commonly encountered in converter-interfaced islanded microgrids. A simple and economical bus protection method is also proposed. Several simulations are conducted on a comprehensive model of a realistic operational industrial microgrid (Goldwind Smart Microgrid System) using PSCAD/EMTDC software environment—for different case studies and fault scenarios—to verify the effectiveness of the present strategy and its digital relay.

Oscillatory stability assessment of microgrid in autonomous operation with uncertainties

One of the main challenges of the microgrid (MG) operation in autonomous mode is the uncertain output due to the fluctuating nature of renewable energy resources (RES). This study investigates the effects of RES uncertainties to the oscillatory stability of a hybrid MG in islanded operation. A comprehensive model of Wind Energy Conversion System (WECS), a two-stage Photovoltaic (PV) and bio-diesel engine (BDE) based distributed generation (DG) units are considered to capture a complete dynamic response of the hybrid MG. Trajectories and distribution of damping ratios and oscillatory frequencies of the critical modes were thoroughly investigated through Monte Carlo simulation considering wind speed and solar irradiance uncertainties. From the probabilistic study, it was observed that the presence of RES variations results in a dynamic change of power-sharing strategies and introduce an adverse effect on small signal stability. Uncertain condition of wind speed brings more deterioration in system damping than solar irradiation variation. From time domain simulation, it was confirmed that at higher wind speed, damping on the critical modes reduced. As a consequence, the hybrid MG experienced more oscillatory conditions and even lead to unstable situation at high wind speed conditions. While with solar irradiance change, the investigated MG system can maintain its stable operation.

A distributed economic control and transition between economic and non-economic operation in islanded microgrids

In islanded microgrids, traditional droop control tends to make the total operating costs higher as the power is distributed by capacity ratios of distributed energy resources (DERs). According to equal increment rate criteria, to minimize the whole expenses, an interesting marginal costs-based economic droop control is proposed in this paper, and the active power can be distributed by identical marginal costs among DERs under economic droop control. If some DERs have arrived at the maximum or minimum power, consensus-based “virtual” controller is applied to make marginal costs still be the same among the rest of DERs. Moreover, distributed secondary frequency controller (DSFC) is proposed to rapidly restore system frequency to the nominal value. As an ancillary service, the whole design can easily realize the transition between economic and non-economic operation only by altering the type of droop control. The above controllers only need to interact with neighbor DERs by sparse communication network, so that eliminating the necessity of a central controller. An islanded microgrid incorporating renewable generators, storage devices and conventional generators are presented to verify the effectiveness of the above controllers. Simulation results show that the proposed controllers can successfully make islanded microgrids realize economic operation and the above transition.

Impact of Clock Drifts on Communication-Free Secondary Control Schemes for Inverter-Based Islanded Microgrids

In inverter-based microgrids, individual inverters operate usually with their own digital processor. The clocks used to generate the time signals of these processors differ from each other due to clock drifts. This paper analyzes the impact that the drifts of the processors clocks produce on the operation of inverter-based islanded microgrids. Several communication-free secondary control schemes, which avoid well-known problems caused by digital communication networks, are considered. Active power sharing and frequency regulation are the metrics used to evaluate the performance of the control schemes. The study reveals advantages, drawbacks, and practical limitations of the control schemes caused by the clocks drifts. Therefore, it facilitates the selection of the most suitable control scheme for the practical deployment of microgrids. The theoretical results are validated by experimental tests in a laboratory microgrid equipped with three inverters and three digital signal processors driven by autonomous internal clocks.

Optimal Switched Mode Control to Synthesize Dynamic Frequency Response from Virtual Synchronous Machine in Islanded Microgrid Operation

Abstract Islanded microgrids can experience severe frequency variation due to their inherently low inertia. Virtual inertia has been proposed in the literature to overcome this shortage. Virtual Synchronous Machine (VSM) concept has been the cornerstone of realizing virtual inertia from inverter connected energy sources. In this paper we propose and solve an optimal control problem to identify the optimal switching controls that achieve best dynamic frequency response from VSM. The results show that the optimal number of switching stages is independent of system parameters, and the optimal timing for switching between two control modes can be determined using local measurements alone, without having to perform complex computations.

Multi-Objective Coordinated Planning of Active-Reactive Power Resources for Decentralized Droop-Controlled Islanded Microgrids Based on Probabilistic Load Flow

The intermittency of distributed generation and the fluctuation of load will derive uncertainties to the decentralized droop-controlled islanded microgrids (IMGs) operation. In order to describe and analyse the probabilistic characteristics of the operating state of an IMG, this paper proposes an analytical method based on cumulants to solve the probabilistic load flow (PLF) for decentralized droop-controlled IMGs, considering the correlation of input variables. Nataf transform is used to deal with the correlation, and the PLF is solved using the cumulant method and the Gram-Charlier series expansion. On this basis, a multi-objective coordinated planning model of active-reactive power resources is presented, considering the annual comprehensive cost and operating risk simultaneously. The Pareto optimal solution set is found using non-dominated sorting genetic algorithm-II to provide a set of alternative planning schemes. The proposed PLF based on cumulant method is compared with Monte Carlo simulation to verify its accuracy and efficiency. Besides, the simulation results also demonstrate that the proposed coordinated planning model of active-reactive power resources can coordinate the security and economy of the IMG operation.