Microgrid In Islanded Mode Research Articles

Optimal Load Frequency Control of Island Microgrids via a PID Controller in the Presence of Wind Turbine and PV

This article studied the load frequency control (LFC) of a multi-source microgrid with the presence of renewable energy sources. To maintain a sustainable power supply, the frequency of the system must be kept constant. A Proportional–Integral–Derivative (PID) controller is presented as a secondary controller to control the frequency of the microgrid in island mode, and the integral of squared time multiplied by error squared (ISTES) is used as a performance index. The use of the Craziness-Based Particle Swarm Optimization (CRPSO), which is an improved version of Particle Swarm Optimization (PSO), improves the convergence speed in optimizing the nonlinear problem of load and frequency controller design. The test microgrid is composed of the load and distributed generation units such as diesel generators, photovoltaics and wind turbines. The proposed controller provided the desired response to adjusting the microgrid frequency, achieving the final response after a short time and making it more stable and less oscillatory compared with the conventional system.

Research on fuzzy PID droop Control method for DC microgrid in island mode

In order to solve the problem of difficult power supply in remote mountainous areas and islands, based on the traditional drooping control of DC microgrid, this paper introduces fuzzy PID to realize the self-adjustment of drooping coefficient, which solves the problem of voltage instability of DC bus caused by external environmental conditions and load mutation. MATLAB/SIMULINK simulation results show that the fuzzy PID droop control can effectively reduce the voltage drop, maintain the bus voltage stability, and realize the current sharing of all distributed power sources in the DC microgrid under the load switching condition.

Design an Accurate Power Control Strategy of Parallel Connected Inverters in Islanded Microgrids

When the overall loads change periodically, the accurate real power and reactive power sharing considered is essential for the operation of an islanded microgrid. In this work, an accurate power control strategy of parallel-connected inverters in an AC microgrid in islanded mode has been designed. The examined microgrid involves four inverters linked to the load via different system impedance conditions is performed. Also, the control strategy for islanded microgrid operation with detailed switching models is simulated by using PSCAD/EMTDC software, and all dynamic and steady-state real and reactive power sharing is explained. Moreover, the real power sharing is achieved using only the decentralized control (droop control), while the reactive power sharing control approach is proposed as a central controller (secondary controller) in order to improve the precision of fundamental reactive power sharing, gives an efficient dynamic performance, and reduce the circulating current. The central controller in the microgrid requires communications. It is used as an external loop to accomplish identical reactive power sharing according to reactive power load especially when the mismatch in voltage drops through the feeders is occurred. The control approach will still work with traditional droop control when the central controller is a failure. The obtained results show that the proposed approach is immune to the time delay in the central controller. Additionally, the active power and the reactive power is shared accurately.

Parameter Optimization of Droop Controllers for Microgrids in Islanded Mode by the SQP Method with Gradient Sampling

For enhancing the stability of the microgrid operation, this paper proposes an optimization model considering the small-signal stability constraint. Due to the nonsmooth property of the spectral abscissa function, the droop controller parameters’ optimization is a nonsmooth optimization problem. The Sequential Quadratic Programming with Gradient Sampling (SQP-GS) is implemented to optimize the droop controller parameters for solving the nonsmooth problem. The SQP-GS method can guarantee the solution of the optimization problem globally and efficiently converges to stationary points with probability of one. In the current iteration, the gradient of the nonsmooth function can be evaluated on a set of randomly generated nearby points by computing closed-form sensitivities. A test on the microgrid system shows that the optimality and the efficiency of the SQP-GS are better than those of the heuristic algorithms.

Inertia emulation with incorporating the concept of virtual compounded DC machine and bidirectional DC–DC converter for DC microgrid in islanded mode

Inertia emulation with incorporating the concept of virtual compounded DC machine and bidirectional DC–DC converter for DC microgrid in islanded mode

An improved droop control with network adaptive ability in islanded mode of microgrid

When the microgrid operates in island mode, the droop control can be adopted to realize effective power distribution among distributed generations (DGs). Due to the different line impedancesof the various DGs, disproportional power distribution and circulating currents occur in conventional droop control, which make it difficult to maximize the efficiency of DGs and even may cause some DGs overloads. Therefore, an improved droop control strategy with an adjustable droop coefficient is proposed. The relationship between line impedance and reactive power is obtained by analysing conventional droop control. Then a new droop control algorithm is proposed. By introducing the output reactive power information of other DGs into the integral feedback link, the droop coefficient is modified to realize the accurate distribution of reactive power. The model is built on the Matlab/Simulink platform, and the effectiveness of the proposed control strategy is verified by comparing its reactive power distribution resultwith that of the conventional droop control.

Hybrid dynamical control based on consensus algorithms for current sharing in DC-bus microgrids

The main objective of this work is to propose a novel paradigm for the design of two layers of control laws for DC-bus microgrids in islanded mode. An intensive attention will be paid to the inner control level for the regulation of DC–DC electronic power converters, where the use of Hybrid Dynamical System theory will be crucial to formulate and exploit switching control signals in view of reducing the dissipated energy and improving system performance. Indeed, this recent theory is well suited for analysing power electronic converters, since they combine continuous (voltage and currents) and discrete (on–off state of switches) signals avoiding, in this way, the use of averaged models. Likewise, an outer control level for controlling DC-bus microgrids will be developed to provide a distributed strategy that makes the microgrid scalable and robust with respect to blackouts of sources and/or loads, as well as, to provide a balance in the system of charge of the batteries, following the principle of Multi-Agent System theory. In this distributed strategy, they are several crucial and innovative aspects to be regarded such as the different converter architectures, the hybrid and nonlinear nature of these converters. Stability properties are guaranteed by using singular perturbation analysis.

Integration of sub-gradient based coordinate for multiple renewable generators in microgrid

Microgrids and smart grids are the future of the conventional grids. It will be an immense need to integrate the Distributed Generators (DGs) with smart grids and microgrids. DGs are mostly the solar energy, wind energy and small hydel power. These DGs has several advantages as high penetration of power will reduce the power loss, reduce the voltage drop and maintain the terminal voltage. These DGs are difficult to integrate with the microgrids in islanded mode as these causes severe fluctuation which disturbs the supply–demand balance. Renewable generators (RGs) working in MPPT mode cause high fluctuations when the RGs are working on islanded mode and generating more power than demand. Therefore, a control system is required to manage this issue and to maintain the balance of the system. Sub-gradient based distributed coordination technique is utilized for the control of renewable generators in which the supply–demand balance is maintained by coordinating the system with the utilization level. Simulation results will exhibit the operation of the controlled microgrid, its integrated DGs and will prove the efficiency and the effectiveness of the proposed methodology.

Dynamic reactive power optimization of hybrid micro grid in islanded mode using fuzzy tuned UPFC

This paper presents an adaptive management of voltage and reactive power requirement of non-conventional sources based microgrid. The hybrid microgrid is a combination of wind and diesel generation system which is tested for the study of changes in the reactive power and voltage stability. The proposed model of the test system using active and reactive power equations is presented. The Flexible AC Transmission System (FACTS) controller Unified power flow controller (UPFC) generally used in conventional energy system with the proportional Integral controller (PI) controller is investigated to analyze the performance of the test system In this paper UPFC is used to investigate the hybrid microgrid test system. Further the test system is analyzed with Fuzzy tuned parameters of PI controller of UPFC.

A New Adaptive Load-Shedding and Restoration Strategy for Autonomous Operation of Microgrids: A Real-Time Study

Islanding operation is one of the main features of a MicroGrid (MG), which is realized regarding the presence of distributed energy resources (DERs). However, in order to deal with the control challenges, which an MG faces during island operation, particularly when the transition is associated with certain excessive load, an efficient control strategy is required. This paper introduces a Central Management Agent (CMA) which maintains the stability of the MG, once it is islanded, by controlling an Energy Storage System (ESS) and a Central Synchronous Generator (CSG). Further, this paper proposes a new adaptive load-shedding/restoration schemes that calculates the amount of power imbalance based on frequency measurements combined with the mean value of the frequency gradient. The primacy of the proposed scheme over existing schemes, like instantaneous frequency gradient-based load shedding scheme, is its robustness against frequency oscillations. Moreover, the proposed method acts compatible with the control routine of DERs and the intermittent nature of the PV plant. As another salient feature of this paper, a Hardware In the Loop (HIL) testbed for real-time simulation is developed under which the proposed scheme and related communication with CMA along with other components are evaluated. The obtained results show that the control strategy can confidently conserve the stability of the MG in islanded mode and meet smooth reconnection to the grid-connected mode.

A Life Cycle-Cost Analysis of Li-ion and Lead-Acid BESSs and Their Actively Hybridized ESSs With Supercapacitors for Islanded Microgrid Applications

The combination of supercapacitors (SCs) with Li-ion Batteries (LIBs) and Lead-Acid Batteries (LABs) as hybrid ESSs (HESSs) have widely been proposed for Microgrid (MG) applications. The SCs of HESSs eliminate the stress of surge currents on LIBs and LABs, which increases their life cycles, and decreases their life cycle costs and hence decreases the HESSs operational costs. However, the active topology of HESS, which is the most commonly used configuration, requires an extra SC and an extra DC/DC converter in comparison to the Battery Energy Storage (BESS) topology, which increases the HESS capital cost. This paper tries to investigate that the hybridization of LABs and LIBs with SCs is economically effective or not for applications in islanded MG. In this regard, an energy management and frequency control (EMFC) scheme is proposed for the operation of MG in islanded mode. Using the simulations of the proposed EMFC scheme for islanded MG, the size of main components of LIB ESS (LIBESS), LAB ESS (LABESS), LIB-SC HESS (LISHESS) and LAB-SC HESS (LASHESS) are calculated. The numerical results show that for a 10-year period operation in islanded MG, the LISHESS and LASHESS impose less cost than LIBESS and LABESS. Also, the LISHESS is cheaper (almost 11%) than LASHESS.

The Lyapunov-based stability analysis of reduced order micro-grid via uncertain LMI condition

This paper proposed a new three-step method for studying dynamic stability of an islanded micro-grid (MG). In conventional methods, state-space model linearization around an operating point and modal analysis are utilized. Due to lower inertia and operation of MG in islanding mode, MG is more sensitive to changes of parameters and contingencies. Therefore, small signal stability (sss) analysis has a very limited validity range and large signal studies should be also performed to get an appropriate perception of the system stability. In this paper, beside sss study to determine network stability status around a dominant operating point, an algorithm based on multi-time scale systems theory is suggested to detect slow and fast modes which can be used for system decomposition into fast and slow subsystems. Fast modes have large negative real values and/or damping ratios, which can be eliminated in large signal stability analysis with an appropriate approximation. Therefore, singular perturbation theory is used to reduce the model order and extract the network slow subsystem. Then, the Lyapunov function can be applied to reduced model. In addition, continuous load and the renewable distributed generators power variations in the system cause to wide range of changes in the operating point and state space model of the system. Therefore, the copula distribution is proposed to model the uncertainties. Then, the uncertain LMI condition based on scenarios extracted from the copula is used to determination of the Lyapunov function and the attraction domain. Finally, the attraction domain sensitivity to MG parameters change is studied to achieve sufficient insight about the system stability margins.

Coordinated Protection and Control Scheme for Smooth Transition from Grid-Connected to Islanded Mode of Microgrids

Microgrid (MG) architecture enables integration of different kinds of distributed energy resources (DERs) into existing electricity grid operation. The MG normally operates within the main utility grid and during grid outages is able to become isolated and sustain its local loads. The successful transfer and operation of MG in islanded mode is strongly influenced by the robust protection arrangement at the point of common coupling, which helps initiate islanding along with the associated MG control actions to counteract insecure and volatile dynamics arising after islanding. Therefore, the protection and control schemes should be coordinated with each other and also be compatible with dynamics of the DER; otherwise, the stability of the MG will be impacted during its autonomous islanded operation. In this paper, a new combined protection–control scheme is devised for a low-voltage MG through which the stability of the islanded MG is continuously ensured. Furthermore, structure of an intelligent electronic device (IED) which enables the proposed protection scheme is presented. The IED directly communicates to an intelligent central controller system for the purpose of triggering the associated control scheme. The performance of the proposed coordinated scheme is successfully validated through both numerical simulations in MATLAB/Simulink environment and experimental test bed. The obtained results show that the idea can confidently meet smooth transient from on grid to off grid and vice versa in MGs.

Robust Control Technique in an Autonomous Microgrid: A Multi-stage $$H\infty$$ Controller Based on Harmony Search Algorithm

Microgrid (MG) control in off-grid mode of operation is one of the main challenges in MGs. Under different loading conditions, voltage and frequency deviate from their nominal limits. The droop control method that applied with the current control and voltage control loops can restore the MG voltage and frequency to their nominal limits. However, it can’t regulate these parameters to their specified values. Additionally, the droop control method can’t do well when the system supplies either nonlinear or unbalanced loads. This paper proposes a control technique for adjusting the voltage and frequency of an MG in islanded mode under different loading conditions. The proposed method is based on applying the H-infinity ($$H\infty$$) robust control as a secondary controller with the droop control method to improve its performance. The proposed method is composed of four stages including; droop control loop, voltage control loop, current control loop and control loop for inductance–capacitance–inductance (LCL) filter and coupling circuit. $$H\infty$$ control has an internal model that can control the frequency and improve the system power quality by decreasing the disturbance frequencies. Moreover, it has two weighted parameters $$\xi$$ and $$\mu$$ that must be correctly adjusted to improve the stability of the system. For a proper adjusting of these parameters, the Harmony Search (HS) optimization method is applied. To prove the effectiveness and superiority of the proposed method, it is applied on a test system with three different loading scenarios using MATLAB/Simulink program. The results are compared with those of droop controller and they confirm that the proposed control method is more effective in controlling the microgrids under islanded mode operation.

Analysis of Consensus-Based Islanded Microgrids Subject to Unexpected Electrical and Communication Partitions

Microgrids (MGs) are power systems consisting of an electrical network composed by distributed loads and generation units that may include a communication network for improved operation. The considered MG in islanded mode is driven by voltage source inverters implementing decentralized droop control for active power sharing together with a communication-based consensus algorithm for frequency regulation. This paper analyzes the MG performance subject to network failures that provoke network partitions. It is considered that the electrical partition leads to several sub-MGs working in parallel where the power demand can be always guaranteed by the generation units, and the communication partition leads to several consensus algorithms also working in parallel. The double partitioning is analyzed through a closed-loop system model derived using the power flow equations that includes the electrical and communication connectivity. Analytical expressions for the steady-state values for both frequency and active power depending on the partitioning are derived. Selected experimental results on a low-scale laboratory MG illustrate the (undesirable) impact that unexpected partitions have in system performance.

Power Scheduling of Fuel Cell Cars in an Islanded Mode Microgrid With Private Driving Patterns

A microgrid in the islanded mode is considered where a fleet of fuel cell cars is used as a distributed power generation system. The objective of the proposed control system is to minimize the operational cost of the system, subject to the physical and operational constraints of the system. In order to deal with uncertainty in the prediction of the microgrid's load, two model predictive control methods, a min-max (MM) approach and disturbance feedback MM approach, are proposed. We develop three distributed control algorithms and we show that by using these algorithms, the driving patterns of the fuel cell cars can be kept private. In other words, no privacy sensitive data on the usage of the cars are collected by a central control agent. Numerical case studies are presented to demonstrate the excellent performance of the proposed control methods.

A methodology to prevent cascading contingencies using BESS in a renewable integrated microgrid

One of the key features of a microgrid is its capability to operate in an islanded mode when required. However, due to limited resources and bidirectional power flows, a microgrid in islanded mode is susceptible to different contingencies with extensive effects, e.g., cascading loss of distributed generators (also known as Distributed Energy Resources – DERs) due to unacceptable post-fault voltage recovery performance. In a renewable integrated microgrid, conventionally Battery Energy Storage System (BESS) is placed at the point of common coupling of distributed generators. However, such a placement approach does not prevent the cascading tripping of DERs following a fault, especially in the presence of a large share of induction motor loads. To address this challenge, this paper proposes a new placement methodology of BESS based on reactive power margin to prevent cascading contingencies and a subsequent blackout in a microgrid. The proposed methodology deploys an iterative technique. If the voltages at DERs terminals after a fault fail to recover within the stipulated time suggested by the grid code, BESS is placed at the bus with the lowest reactive power margin. It is known as the centralised placement approach of BESS. If this approach does not avert the DER tripping, the location of BESS is changed using the distributed placement scheme. In this scheme, several BESSs are placed on multiple buses, which have relatively lower values of reactive power margin. In a specific microgrid, either the centralised or the distributed BESS placement approach is eventually adopted. To explore the effectiveness of the proposed method, the IEEE 43 bus industrial test system integrated with renewable energy resources is considered as an islanded microgrid for simulations. It is found that the proposed approach results in the most appropriate placement of BESS to make the microgrid immune to cascading tripping of DERs.

Analysis Methodology for Evaluation of Time-Delay Impact on Network-Based System for Droop-Controlled AC Microgrid

Potential performance improvements can be obtained by introducing communication techniques in the power electronic systems. However, network-induced time-delay could bring negative consequences of degrading performance or destabilizing the system in many cases. To investigate and handle the impacts of time-delay, a suit of analytical methodology is proposed, where both delay-insensitive and delay-sensitive control strategies of network-based system have their theoretical methods and different problem-solving paths. The former is to predict the maximum allowable boundaries of time-delay for releasing more network resources, and the latter is to use the controller-altering method for changing its original instability, respectively. The proposed methodology is concretely applied in network-based system of droop-controlled AC (Alternating Current) microgrid in islanded mode. Two different current-sharing strategies are mathematically analyzed and given to verify their validity. Experimental results also show the effectiveness of the proposed methodology in droop-controlled AC microgrid system, which provides theoretical guidance on how to use network-based control for the other power electronic systems.

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 dual control strategy for power sharing improvement in islanded mode of AC microgrid

Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.