Conventional Droop Control Research Articles

Power Loss and Total Load Demand Coverage in Stand-Alone Microgrids: A Combined and Conventional Droop Control Perspectives

Conventional droop control plays an essential role in microgrids with distributed generators, DGs, and variable load demand. Despite its advantages, the conventional droop scheme does not take into account the minimization of power losses and the maximization of the total load coverage. Moreover, most of the nonconventional droop control studies that address power loss consider one variable load. This study proposes a combined droop scheme and inspects its effects on power loss minimization and total load coverage. The proposed scheme combines conventional and nonconventional droop schemes and constructs a 3D <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-f</i> droop characteristic that takes into account the presence of two variable loads. Four and six-bus microgrids are adopted to simulate and illustrate the system behavior in MatLab. Results are compared to the standard fully conventional droop approach; results showed that the combined scheme has higher performance in terms of power loss gain and total load coverage.

Decentralized Power Management of DC Microgrid Based on Adaptive Droop Control With Constant Voltage Regulation

An adaptive droop control with constant voltage regulation is proposed for the power and voltage management of a DC microgrid (DCMG) with multiple power sources, such as a utility grid (UG), a distributed generator (DG), an energy storage system (ESS), and an electric vehicle (EV). In the proposed scheme, the droop characteristics for the UG, ESS, and EV are adaptively changed according to electricity price conditions and state-of-charge (SOC) levels in order to optimize the DCMG operation flexibility as well as electricity cost. The proposed control method not only ensures the power-sharing of DCMG reliably without the use of a communication link, but also regulates the DC bus voltage stably at the nominal value. To achieve this, the proposed scheme consists of primary control and secondary control. The primary control is used to achieve power-sharing in the decentralized DCMG, while the secondary control is used to overcome the disadvantage of conventional droop control, i.e., to remove DC bus voltage deviations. Decentralized power management is also presented to enhance the DCMG system reliability in the presence of uncertainties such as DG generation power, the ESS and EV SOC levels, the grid and EV availabilities, the load demand, and electricity price conditions. The effectiveness of the proposed scheme is demonstrated in a comprehensive simulation and experiment under various conditions. The test results clearly confirm the control flexibility and overall performance of the proposed control scheme for decentralized DCMG system.

A Distributed Hierarchical Control Framework for Economic Dispatch and Frequency Regulation of Autonomous AC Microgrids

Motivated by the single point of failure and other drawbacks of the conventional centralized hierarchical control strategy, in this paper, a fully distributed hierarchical control framework is formulated for autonomous AC microgrids. The proposed control strategy operates with a distinct three-layer structure, where: a conventional droop control is adopted at the primary layer; a distributed leaderless consensus-based control is adopted at the secondary layer for active power and, hence, frequency regulation of distributed generating units (DGUs); and the tertiary layer is also based on the distributed leaderless consensus-based control for the optimal power dispatch. Under the proposed strategy, the three constituent control layers work in a coordinated manner. Not only is the load dispatched economically with a negligible power mismatch, but also the frequencies of all the DGUs are regulated to the reference value. However, the frequency regulation is achieved without requiring any central leader agent that has been reported in the contemporary distributed control articles. As compared to the conventional centralized hierarchical control, the proposed strategy only needs local inter-agent interaction with a sparse communication network; thus, it is fully distributed. The formulated strategy is tested under load perturbations, on an autonomous AC microgrid testbed comprising both low-inertia-type (inverter-interfaced) and high-inertia (rotating)-type DGUs with heterogeneous dynamics, and found to successfully meet its targets. Furthermore, it can offer the plug-and-play operation for the DGUs. Theoretical analysis and substantial simulation results, performed in the MATLAB/Simulink environment, are provided to validate the feasibility of the proposed control framework.

Distributed Control Scheme for Accurate Power Sharing and Fixed Frequency Operation in Islanded Microgrids

Global Positioning System (GPS) based control has recently been reported as a replacement of droop control to achieve fixed frequency operation in islanded microgrids. This article presents the perspective that the power sharing performance of a general microgrid synchronized by GPS is essentially determined by equivalent output impedance. A novel adaptive virtual impedance control approach, which implements different values of virtual resistance for d- and q-axis, is proposed accordingly. The virtual resistance concept is implemented comprising a basic local implementation for output impedance shaping, and a sparse resistance tuning network for the compensation of mismatched feeder impedance, which demands no knowledge of actual output impedance. The resistance tuning network utilizes the consensus protocol and only requires neighboring interactions among DG units. A complete tuning of output resistance for a given load condition results in accurate active and reactive power sharing even after communication is interrupted and will still outperform conventional droop control methods if load changes during the interruption. Small-signal analysis based on delay differential equations model of the overall microgrid is performed to investigate the adverse impact of communication delays on system stability. The efficacy of the proposed approach is validated by both simulation and experimentation.

Rapid Power Compensation-Based VSC-HVDC Control Strategy for Low-Frequency Oscillation Suppression of the Island Power System

Owing to the increased proportion of new energy power generation, such as wind power and photovoltaics, connected to the island grid, the system powered by the voltage source converter-based HVDC (VSC-HVDC) is prone to oscillate or even lose stability when disrupted. In this study, considering the rapid power compensation (RPC), an active power control strategy has been suggested for the receiving-end converter station of VSC-HVDC that could efficiently suppress the low-frequency oscillation of the island power system. First, the mechanism of VSC-HVDC inhibiting low-frequency oscillation of the island grid is analyzed in this study, and then, it theoretically determines that the damping capacity and inertia level of the rapid power compensation control strategy are stronger than those of conventional droop control and inertia control. Second, the receiving-end converter station switches from the RPC mode to droop control in order to allow the system to have a smooth recovery from the steady-state operation in the later stage of oscillation suppression. Moreover, detailed control logic and state-switching strategies have been designed. Finally, the simulation reflects that the proposed control strategy has a stronger oscillation suppression ability, allowing it to obtain rapid suppression of low-frequency oscillation.

Adaptive Droop Control Using Adaptive Virtual Impedance for Microgrids With Variable PV Outputs and Load Demands

In microgrids, intermittency of renewable energy sources (RES) and uncertain state-of-charge (SoC) of energy storage systems (ESS) can cause power deficiency to some distributed generation units (DGs). In this case, DGs with power deficiency may not meet the power demand, resulting in voltage collapse or frequency divergence. Unfortunately, this is seldom considered in inverter control design in existing literature. Thus, in-depth investigation into the microgrid performance under renewable energy resource fluctuations and appropriate control methods are urgently needed. In this article, an adaptive droop and adaptive virtual impedance control strategy is proposed. Unlike conventional droop control where the droop coefficients are fixed by assuming the DGs can always meet the load demand, the droop coefficients here are adjusted according to actual solar PV power output. In this way, proper power sharing among DGs can be achieved under renewable energy variation. Furthermore, the impact of varying DG capacities on system stability is mathematically investigated. An adaptive virtual impedance is then incorporated into the adaptive droop method to deal with the system instability caused by renewable energy variations. The proposed strategy is analyzed theoretically and validated in MATLAB/Simulink simulation and laboratory experiments. The results demonstrate the advantages of the proposed method over conventional approaches under various scenarios.

A multi-state control strategy for battery energy storage based on the state-of-charge and frequency disturbance conditions

With the large-scale penetration of renewable generation, many power systems face increasing frequency deterioration issues. Because of the fast response capability, battery energy storage (BES) has become an essential flexible resource to maintain the system frequency stability. This paper investigates how the state of charge (SOC) of a BES could be effectively managed while guaranteeing the frequency control service. A novel multi-state control strategy is proposed to adaptively control the active power of a BES for various frequency disturbance scenarios. This control strategy explicitly takes into account the interdependency between the BES's SOC, lifetime, and frequency regulation performance. A finite state machine is implemented to divide the frequency and the SOC into multiple states to set diverse control objectives and constraints for a BES under different operating conditions. By simultaneously considering the SOC/frequency states, the initial SOC-recovery power, and the overload capacity of inverters, three sub-strategies are designed in the multi-state framework to control the BES in any feasible state. Case studies on a modified IEEE RTS-79 system verify the effectiveness of the proposed control strategy. Leveraging the proposed strategy, the BES can provide better frequency control effect as compared to the conventional droop control.

Enhancing the performance of multi-microgrid with high penetration of renewable energy using modified droop control

Currently, Microgrids (MG) have given high attention due to their ability to increase power systems reliability and controllability. Also, small-scale renewable units are increasing according to the distribution of renewable energy. In this paper, a hybrid MG (H-MG) is proposed. This H-MG includes AC and DC buses, two voltage levels, linear and nonlinear loads, and different distributed generators such as diesel, wind, and PV besides storage units. Modified angle droop control is utilized in the coupling inverters between DC-MG and AC-MG because it leads to a much smaller standard frequency deviation than conventional droop control. Also, to enhance the intermitted sources of energy sharing and the system characteristics of damping, an additional control loop is introduced. Where, both the voltage angle and the voltage magnitude are corrected by adjusting the output of the droop controller. Moreover, due to the high R/X ratio of the study system as in rural area, the proposed angle droop control is enhanced by using some transformation to amend both the feedback gains and the droop equations to account for the high coupling between active and reactive power and enhance power-sharing in such case. Various simulation studies are executed and compared with the related works to show the efficiency and quality of the proposed MG operation.

An efficient dynamic power management model for a stand-alone DC Microgrid using CPIHC technique

The power generation using solar photovoltaic (PV) system in microgrid requires energy storage system due to their dilute and intermittent nature. The system requires efficient control techniques to ensure the reliable operation of the microgrid. This work presents dynamic power management using a decentralized approach. The control techniques in microgrid including droop controllers in cascade with proportional-integral (PI) controllers for voltage stability and power balance have few limitations. PI controllers alone will not ensure microgrid’s stability. Their parameters cannot be optimized for varying demand and have a slow transient response which increases the settling time. The droop controllers have lower efficiency. The load power variation and steady-state voltage error make the droop control ineffective. This paper presents a control scheme for dynamic power management by incorporating the combined PI and hysteresis controller (CPIHC) technique. The system becomes robust, performs well under varying demand conditions, and shows a faster dynamic response. The proposed DC microgrid has solar PV as an energy source, a lead-acid battery as the energy storage system, constant and dynamic loads. The simulation results show the proposed CPIHC technique efficiently manages the dynamic power, regulates DC link voltage and battery’s state of charge (SoC) compared to conventional combined PI and droop controller (CPIDC).

Assessing the Impact of Reactive Power Droop on Inverter Based Microgrid Stability

Droop control is the most common approach for controlling inverter-based micro-grids. The active power droop gain has always been considered as the main parameter for identifying the micro-grid stability margin. Increasing this margin improves the transient performance and provides robustness to the micro-grid for a wide range of operations. Previous work on droop control focused on the active power droop gain, which is required for accurate power sharing as well as for micro-grid stability assessment. This paper utilizes small-signal stability analysis to analyze the impact of the reactive power droop gain on micro-grid stability, which is ignored in previous work. Consequently, a micro-grid domain of stability chart is proposed and defined in the mp <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -nq plane, which represents the zone within which the micro-grid will maintain stable operation. The proposed domain of stability chart is utilized to assess and compare the impact of the conventional and proportional derivative (PD) reactive power droop controller on the micro-grid stability margin. The results show that there exists a reactive power droop gain at which the stability margin is minimum. Furthermore, it has been shown, through the domain of stability chart, that the PD reactive power droop controller is capable and sufficient to significantly increase the micro-grid stability margin while maintaining equal load sharing. Further, the domain of stability chart can serve as a useful tool for defining the micro-grid droop gain operational boundaries and for assessing and comparing inverter-based micro-grid control schemes.

Investigation of Adaptive Droop Control Applied to Low-Voltage DC Microgrid

In a DC microgrid, droop control is the most common and widely used strategy for managing the power flow from sources to loads. Conventional droop control has some limitations such as poor voltage regulation and improper load sharing between converters during unequal source voltages, different cable resistances, and load variations. This paper addressed the limitations of conventional droop control by proposing a simple adaptive droop control technique. The proposed adaptive droop control method was designed based on mathematical calculations, adjusting the droop parameters accordingly. The primary objective of the proposed adaptive droop controller was to improve the performance of the low-voltage DC microgrid by maintaining proper load sharing, reduced circulating current, and better voltage regulation. The effectiveness of the proposed methodology was verified by conducting simulation and experimental studies.

Optimal fractional‐order adaptive neuro fuzzy inference system‐based droop controller for DFIG‐wind turbine to control load frequency of hybrid microgrid

AbstractThis paper aims to ameliorate the contribution capability of doubly fed induction generator to damp the frequency and output power deviation of stand‐alone hybrid microgrid system. Skyrocketing progress in converter‐interfaced renewable energy sources leads to weakness of islanded microgrids' inertia which imposes some problems in the frequency control and hybrid system stability. This problem can be effectively solved by utilizing renewable resources with high harvestable energy such as doubly fed induction generator‐wind turbine. Furthermore, the alternative energy resources have been taken into account to participate in strength of weak inertia of the microgrid. Since the conventional controller like proportional integral derivative droop controller cannot present efficient damping term because of their slow controlling action in order to diminish the frequency and output power deviation. In this regard, a fractional‐order adaptive neuro fuzzy inference system droop is here introduced to stabilize the frequency within a satisfactory secure range with low fluctuation over the conventional droop controllers. To attain an accurate control action, multi‐objective sine cosine algorithm is used to optimize the parameters of based on multi‐criterion objective functions. To verify the performance of proposed droop controller, it has been tested under different scenarios. At last, the simulation results reveal that sine cosine algorithm‐based proposed droop controller can significantly enhance the microgrids' inertia and diminish the frequency and output power deviation.

Multi-level model predictive control for all-electric ships with hybrid power generation

Power availability to preserve propulsion is a vital issue in the shipping industry which relies on persistent power generation and maintaining the stability of the power and propulsion system. Since the introduction of on-board all-electric Direct Current Power and Propulsion Systems (DC-PPS) with hybrid power generation, which are more efficient compared to direct-diesel and Alternating Current (AC) all-electric configurations, there have been extensive investigations on stabilization and power generation control to enable robust and reliable performance of DC-PPS during different ship operations. In this paper, a multi-level approach is proposed for hybrid power generation control. For this goal, first, a mathematical model is proposed for each power system component and then, the overall on-board power system is modeled in a state space format. Then, a multi-level Model Predictive Control (MPC) approach is proposed for the DC voltage control which unlike conventional droop control approaches, takes the DC current generated by power sources into account explicitly. The performance of the proposed approach is evaluated via several simulation experiments with a high fidelity model of a high voltage DC-PPS. The results of this paper lead to enabling more effective approaches for power generation and stability control of constant power loaded microgrids.

Improved Droop Control Strategy of Multiple Energy Storage Applications in an AC Microgrid Based on the State of Charge

Distributed energy storage technology is used to stabilize the frequency and voltage of the microgrid operating in islanded mode. However, due to the inconsistent state of charge (SoC) of the energy storage unit (ESU), the active power output of the ESU cannot be shared reasonably. On the basis of stabilizing voltage and frequency, this paper presents a power exponential function droop control (PEFDC) strategy considering the SoC. In this control strategy, the ESU is allowed to adjust the output power adaptively according to its own SoC level during discharge and reaches SoC equilibrium. Simulation models are built to compare the PEFDC strategy with conventional droop control (CDC) and power function droop control (PFDC) approaches. The simulation results illustrate the superiority of the proposed control strategy over the other two methods. Finally, the hardware-in-the-loop experiment is conducted to verify the effectiveness of the PEFDC strategy.

Intelligent Transition Control between Grid-Connected and Standalone Modes of Three-Phase Grid-Integrated Distributed Generation Systems

This paper proposes an intelligent seamless transition controller for smooth transition between grid-connected (GC) and standalone modes of distributed generation (DG) units in the grid. The development of this seamless controller contributes to two main processes in the transition modes: the synchronization process and an islanding process. For the synchronization process, the stationary reference frame phase-locked loop (SRF-PLL) associated with the voltage source inverter (VSI) is modified using the frequency, voltage deviation, and phase angle information. Furthermore, the islanding process is classified as intentional and unintentional islanding scenarios for achieving efficient transition control. Here, the intentional islanding process is achieved with the information that is available in the system due to the planned disconnection. For the unintentional islanding process, a fuzzy inference system (FIS) is used to modify the conventional droop control using the information of change in active power, voltage, and frequency. To identify the action of the proposed approach during the transition process, numerical simulations are conducted with the hardware-in-loop (HIL) simulator by developing a 10kWp three-phase grid-connected DG system. The results identified the efficient control of the VSI for both islanding and grid connection processes. In the islanding conditions, the proposed controller provides advantage with less detection and disconnection time, and during synchronization, it instantly minimizes the phase-angle deviation to achieve efficient control.

Droop coefficient correction control for power sharing and voltage restoration in hierarchical controlled DC microgrids

With the widely application of the DC microgrid (MG), the utilization of renewable resources is improved by interconnecting nearby DC MGs. Since MGs are connected by different line impedances, the intuitive tradeoff exists between the conflicting goals of current sharing and voltage regulation in the conventional droop control scheme. A hierarchical control structure with a dynamic droop coefficient correction control (DCCC) is presented to achieve better power sharing in DC MGs. In the proposed hierarchical control scheme, the droop coefficient correction method is employed in the primary level, which corrects the droop coefficient automatically and the proportional current has been shared. Moreover, the droop coefficient of each converter is also dynamically adjusted to deal with the impact of uncertain line impedances. The robustness of performance to system uncertainties is also demonstrated. Meanwhile, the different current ratio of the microgrids can be provided to the DC bus by using the tertiary control level, which achieves an arbitrary power sharing among the microgrids. In addition, a small-signal model is developed to investigate the effects of coupling relationship in the control parameters in the voltage loop, voltage restoration controller and droop coefficients. Finally, the experimental results are presented to evaluate the effectiveness of droop coefficient correction control approach and hierarchical control scheme for the DC MG under resistive and constant power load (CPL) conditions.

Power Sharing and Control by Droop Controller with Advanced Filter Design: A Case Study in Lock-down Periods

In the lock-down period, the islanding mode of operation with droop controllers has several advantages in the alternating current grid. This study focuses on an improvised droop controller. It consists of an advanced filtering segment embedded with a conventional droop controller, which overcomes the drawback of droop controllers of the non-handling of non-linear loads in an ordinary situation. A selective harmonic elimination technique in grid-connected mode and lock-down mode and an advanced filter embedded with droop control are used so that the proposed controller can also work as an Active Harmonic Filter (AHF). The simulation results in different cases show that the proposed controller can control the active and reactive power in the lock-down period as well as the harmonics in the normal period up to an extent.

Control strategy of hybrid energy storage system based on virtual DC generator to suppress DC bus voltage fluctuation

In order to eliminate the impact of sudden changes in light intensity and load on the DC microgrid system, based on the analysis of the principles and advantages of the virtual DC generator (VDG), the application of the virtual DC generator to the battery of the hybrid energy storage system is adopted. In the control strategy, it improve the ability of the hybrid energy storage system to suppress DC bus fluctuations. The Matlab/Simulink simulation results show that when the light intensity and load change suddenly, the DC microgrid bus voltage fluctuation amplitude of the virtual motor droop control is 6.1V, 39V, which is significantly smaller than the conventional droop control DC bus voltage fluctuation amplitude 8.7 V, 50.6V. The simulation results show that the control strategy proposed in this paper can improve the ability of the hybrid energy storage system to suppress DC microgrid bus voltage fluctuations.

Novel Passive Controller Design for Enhancing Boost Converter Stability in DC Microgrid Applications

Boost converters have nonminimum phase (NMP) characteristics, which makes the stable closed-loop control design difficult. Based on the passive theory, this article proposes to add a feedforward loop in the conventional double-loop closed loop to compensate for the NMP effect. Therefore, the potential instability caused by NMP can be avoided. Such compensation is especially useful for dc microgrid applications where boost converters are widely used as interface converters. Two types of feedforward gain are discussed and compared. The gain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> with a high-pass filter is used eventually for integrating with conventional droop control because it does not change the low-frequency quiescent operation point. The stability of the proposed controller is analyzed through loop gain on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$s$ </tex-math></inline-formula> plane. Besides, how the output impedance of the converter is shaped by the passive controller is analyzed. The experimental results validate the effectiveness of the proposed passive controller.

An Improved Reactive Power Sharing Method for an Islanded Microgrid

Conventional droop control is widely used for achieving proportional power sharing in islanded microgrids. The active power sharing becomes proportional to the ratings of the inverters by applying the conventional droop control. However, reactive power sharing is not proportional because of resistive feeder impedance, unequal feeder lengths, and asymmetric load distribution in the network. In this article, an existing control strategy that has been proposed for improving reactive power sharing is enhanced by removing its dependency on communication. The required synchronization for executing the correction is generated by each inverter locally using a load change detection algorithm. This modification also makes the original control strategy immune to load changes happening during the correction process. An almost accurate reactive power sharing is achieved by applying the proposed modification even for highly resistive networks. Also, a tradeoff required in tuning the controller gain is discussed. The efficacy of the proposed modification has been validated by performing detailed simulation studies and by carrying out exhaustive experimental studies on a laboratory prototype.