Conventional Droop Control Research Articles

A cooperative distributed droop gains adjustment in DC microgrids

Direct Current (DC) microgrids are a very promising solution to integrate renewable energy sources, energy storage systems and loads. The main goals of a microgrid controller are to achieve voltage regulation and load sharing among multiple distributed generators (DGs). Distributed control techniques are widely adopted to address these two objectives. Most of the existing distributed methods in the literature adjust the deviation caused by the conventional droop control by adding two additional terms. In this work, a new cooperative distributed controller to adjust the droop gains of DGs is proposed. The proposed method achieves average voltage regulation and proportional current sharing in a unified manner. It does not require additional corrective terms, resulting then in a simple control structure. Moreover, it relies only on exchanging one communication variable among neighboring DGs, which reduces the communication overhead. The stability of the proposed controller is analyzed using a small signal model of the system. The effectiveness of the proposed controller is evaluated under common load changes, local load variations, communication link failures and communication delays. The designed control scheme is verified through Hardware-In-the-Loop (HIL) tests using NI PXIe real-time simulator.

DUAL LOOP VOLTAGE DROOP REGULATED CONTROLLER FOR DC MICROGRID USING HYBRID PSO AND GGO ALGORITHMS

Abstract DC microgrids are effectively employed in distribution network integration with renewable energy sources. The droop control technique is commonly utilized for DC microgrid regulation during load sharing. It minimizes the potential instability caused by disturbances such as input voltage changes, uncertainty parameters, and constant power load (CPL). Nevertheless, conventional droop control is inadequate in achieving precise current and satisfactory voltage regulation distribution for load sharing. The proposed approach comprises two separate loops for the DC bus voltage regulation and load current distribution, delivering a CVL and CPL to overcome the single optimization voltage controller technique. The primary loop uses the PSO algorithm to precisely manage the current load sharing among two parallel converters. Due to this the instability problems arising from disturbances, and it mitigated by the proposed topology. The multiobjective optimization technique provides notable resilience, rapid dynamic response, and robust stability over considerable variations in load. The secondary loop utilises the GGO algorithm to optimize the parameter to minimize the DC bus voltage regulation, voltage management, reasonable distribution of load power, and suitable reliability. The performance of the voltage regulation enhance & settling time for output voltage has been mitigated more than 31ms times through the proposed controller compared to conventional controller under the variation of CVL, CPL, and input voltage disturbance demonstrated in simulation results.

A modified droop-based decentralized control strategy for accurate power sharing in a PV-based islanded AC microgrid

This paper introduces a novel droop-based decentralized control scheme to address the power-sharing challenges within a PV-fed islanded AC microgrid. This novel approach integrates both conventional (P-f/Q-V) and virtual impedance concepts to optimize and manage the precise distribution of active and reactive power among parallel operating inverters posing a significant research challenge. The conventional droop control methods encounter limitations such as voltage and frequency deviations and inaccuracies in power-sharing due to line impedance disparities. To overcome these limitations, the proposed solution integrates an enhanced virtual impedance control loop alongside the conventional control loop (P-f/Q-V). The efficacy of this approach is showcased through simulations conducted using the OPAL-RT OP4510 simulator within the MATLAB/Simulink platform. The Real-time simulation outcomes confirm the efficiency of the suggested control strategy, guaranteeing precise distribution of both active and reactive power while upholding stable voltage and frequency profiles within the system.

Augmented Two-Stage Hierarchical Controller for Distributed Power Generation System Powered by Renewable Energy: Development and Performance Analysis

The sustainable development of an area is highly reliant on a reliable electrical energy supply. Microgrids are important in integrating distributed energy resources (DERs) using power electronic converters. However, microgrid control becomes challenging with the increasing number of distributed generators and loads. With the conventional droop control method, power contributions from DER converters cannot be accurately shared due to a mismatch of line impedances. In this paper, an augmented hierarchical control mechanism is proposed to solve the issues mentioned above. This hierarchical control mechanism consists of primary and secondary controllers. The primary stage utilized the droop controller to improve optimal power flow, mainly for the resistive network. The secondary stage is based on an improved methodology to compensate for the voltage and frequency variations during small and large signal disturbances. Moreover, the modelling and analysis for PMSG-based wind energy conversion systems are also presented. The response of the primary controller for active and reactive power sharing is investigated. The analysis emphasizes the demonstration of optimal power-sharing under normal and abnormal conditions for the considered load. Finally, the suggested robust controller’s performance is evaluated in a MATLAB environment, and simulation results show the proposed scheme’s superiority under different operating conditions. The frequency is stable at 50 Hz after a 50 KW load is added.

A Mathematical Framework for Simultaneous Voltage and Frequency Regulation in Distributed Generator (DG) Grid-Interfaced Systems

Recent advancements in distributed generation (DG) systems interfaced with microgrids necessitate robust regulatory mechanisms to manage inherent power fluctuations, particularly from renewable sources like photovoltaics. These fluctuations can significantly impact the stability and efficiency of microgrids. This paper introduces a novel mathematical framework for simultaneous voltage and frequency regulation, aimed at addressing power quality and stability challenges in DG-grid interfaced systems. Utilizing a combination of algebraic topology and dynamical systems theory, we develop a model that incorporates an adaptive virtual frequency-impedance control loop. This mathematical approach allows for the analytical examination of the stability properties of the system and the design of control strategies that guarantee optimal operational thresholds. We extend the conventional droop control mechanisms with a rigorously defined Simultaneous Voltage and Frequency Correction Scheme (SVFCS), providing a theoretical underpinning that supports experimental observations. The efficacy of the proposed model is validated through numerical simulations that demonstrate adherence to the IEEE 519 standard, ensuring reduced harmonic distortion and enhanced system reliability. Our results highlight the potential for these mathematical methods to provide foundational insights into the control and optimization of microgrid operations.

A new adaptive droop control strategy for improved power sharing accuracy and voltage restoration in a DC microgrid

In a DC Microgrid, accurate power sharing and voltage restoration are two primary control goals to guarantee power quality and reliable operation. Inaccurate power sharing is a significant concern due to discrepancies in feeder line resistance, faulty equipment, lack of monitoring and control, and nonlinear load. Moreover, inaccurate power sharing may lead to overloading of converters and cause a cascade of failures throughout the entire system. This study proposes a new adaptive droop control strategy to address these challenges. To enhance accurate power sharing, error current sharing is formulated by considering bus current and total rated current. This is regulated by the adaptive controller to adjust droop resistance. Additionally, the impact of droop voltage because of feeder line resistance is considered in the proposed strategy. The primary control loop regulates the output voltage of converter utilizing the proposed observer-based optimum sliding mode control (OOSMC). The controller gain of the OOSMC is optimized using a gradient-based method to enhance transient and steady state response of the converter output. In the secondary loop, the twin-delayed deep deterministic policy gradient (TD3) for voltage restoration is employed with a new reward function to optimally tune the proportional-integral (PI) controller. Finally, the superiority of the proposed strategy is evaluated through rigorous testing scenarios, including the use of constant power load (CPL) and sudden failure in the converters. Moreover, the proposed strategy is validated by simulations and laboratory-based experiments. The results show that the OOSMC outperforms GA and PSO while the TD3 has better performance than traditional PI. Furthermore, when the equivalent power-rated converters are implemented in the system, the proposed strategy can transfer identical power sharing of 4 W to the load and provide accurate current sharing of 0.167 A compared to conventional droop control while the DC bus voltage is maintained at the 24 V reference value. In the case of different power-rated converters, the proposed strategy can achieve power sharing accuracy. The first, second and third converters supply 2 W, 4 W, and 6 W, respectively, to the load and provide accurate current sharing. Meanwhile the DC bus voltage can be kept at the reference voltage of 24 V. In the scenario of various kinds of disturbances, the proposed strategy can achieve accurate power and current sharing when the system is tested under load and input voltage variations. Meanwhile the DC bus voltage always returns to the voltage reference. Moreover, the proposed strategy can share power and current accurately when the converter occurs a sudden failure.

Optimizing power sharing and voltage control in DC microgrids using a novel adaptive droop control strategy based on current consensus algorithm

In DC microgrids, a contradiction between power equalization and bus voltage control exists under conventional droop control. To address this issue, this study proposes a current consensus algorithm-based adaptive droop control for hierarchical controlled DC microgrids. The strategy includes primary, secondary, and current-consensus algorithms. In the primary and secondary control layers, an improved droop control strategy is realized, where the secondary layer provides adaptive parameters for primary control. Also, a current consensus algorithm is suggested to ensure equal current sharing, where the iterative calculation results are sent as the input for two control layers. Based on the novel strategy, a small signal analysis is carried out to investigate the relationship between system stability and controller parameters. Finally, simulation and experiments are conducted to assess the effectiveness of the proposed control strategy in complex operating conditions such as resistive load step-change and plug-and-play situation. Experiment and simulation results indicate that the novel control strategy guarantees accurate bus voltage control with a maximum steady-state error of 1.58 % and simultaneously achieves power equalization regardless of line impedance differences and the abrupt change of system status. Moreover, the comparative experiment between the traditional droop control and the proposed strategy verifies the superiority of this novel method.

An Improved Secondary Control Strategy for Dynamic Boundary Microgrids toward Resilient Distribution Systems

In order to achieve the flexible and efficient utilization of distributed energy resources, microgrids (MGs) can enhance the self-healing capability of distribution systems. Conventional primary droop control in microgrids exhibits deviations in voltage and frequency and lacks research on voltage–frequency control during network reconfiguration. Therefore, this paper investigates the control strategy of secondary control for voltage and frequency during the process of reconstructing distribution networks to operate in the form of microgrids after faults. Firstly, the mathematical model of three-phase voltage-source inverters with droop control is analyzed. Then, inspired by the generator’s secondary frequency control strategy, an improved secondary control method based on droop control is proposed, and the effectiveness of the improved droop control strategy is verified. Finally, simulation examples of distribution networks with various dynamic boundary topology changes are designed to simulate a relatively stable control result, validating the proposed strategy.

Improved Power Sharing Strategy for Parallel Connected Inverters in Standalone Micro-grid

The primary goal of integrating alternative energy systems such as solar and wind turbines into the power grid using power electronic devices is to meet the growing energy demands. Connecting inverters in parallel effectively enhance power capacity, reliability, and overall system efficiency. However, an uneven power distribution among the inverters is a significant limitation in these parallel connected inverters (PCI). This study focuses on a distributed generation (DG) unit comprising a solar photovoltaic system (SPV) and a battery energy storage system (BESS) connected to voltage source inverters (VSI) 1 and 2. The proposed approach aims to achieve uniform load/power distribution among the inverters with power management, maintaining a constant DC link voltage despite variations in solar irradiation and temperature. Additionally, the strategy targets the reduction of total harmonic distortion (THD) in the load current. The conventional droop control method is restricted in its ability to achieve accurate and uniform power distribution during load changes. An enhanced P-f and Q-E-based droop control technique (EDCT) and a fuzzy logic controller (FLC) have been proposed to address these challenges. Performance analysis using MATLAB/Simulink was conducted with two test cases, and a comparative assessment was carried out with a conventional controller, such as a Proportional Integral Controller (PIC) and Sliding Mode Controller (SMC), to showcase the effectiveness of the developed control technique. The proposed control method ensures equal sharing of active power of 2.5 and 4kW, 2.5 and 7.5kW and reactive powers of 2.5 and 4Vars, 2.5 and 7.5Vars among the inverters for the two cases even during the load change at t=0.5seconds. The settling time for DC link voltage and THD is effectively diminished to 0.03 and 0.25 seconds and 1.18% and 2.87% for the two test case studies of the proposed method, which are much lower than those of existing methods available in the literature.

Multi-agent deep reinforcement learning for joint dynamic conservation voltage reduction and Q-sharing in inverter-based autonomous microgrids

Conservation voltage reduction (CVR) is implemented in power systems to mitigate power consumption in steady-state time scales. We propose dynamic-scale CVR (DCVR) as a potent solution to provide cost-effective frequency support in inverter-interfaced (micro) grids. The proposed DCVR reduces the voltage profile in dynamics and consequently power reduction helps to maintain instant production-consumption balance for dynamic frequency support. However, DCVR implementation in autonomous MGs (AMGs) faces critical control and stability problems. DCVR is implemented by controlling the voltage, which is a local variable, and makes it difficult to be realized through a decentralized structure. Besides, preserving accurate reactive power sharing (Q-sharing) through conventional droop controllers while employing the DCVR is another critical concern. In this light, this paper proposes a novel artificial intelligence (AI)-based decentralized control structure to implement the DCVR in AMGs for tackling the existing issues. The multi-agent deep reinforcement learning (DRL) model, with a deep Q-network (DQN) algorithm, is adopted to address the grid instabilities and inaccurate Q-sharing issues that arise due to incorporating the DCVR. The proposed method is able to handle the nonlinearity and complexity of the system while maintaining proper dynamic performance and AMG stability. Simulation results in MATLAB/Simulink prove the effectiveness of the proposed control method.

An improved design for virtual synchronous generator control loop based on synergy theory

Virtual synchronous generator (VSG) technology has achieved some results in distributed generation networks and enhanced system stability. For problems that may occur in the system due to ignoring parameter linkages, this paper designs a synergistic controller that establishes the connection between the active frequency and the reactive voltage control loop based on synergy theory. As the output of the synergetic controller, the derived control law u is added to the power frequency control in the form of negative feedback. Two disturbance forms, a step disturbance, and a three-phase short-circuit fault, are set up. The conventional droop control and synergetic droop control results are compared using a MATLAB simulation system. Then, the system parameter variations were studied and analyzed, and the sensitivity of the control system with different values of parameters K1, K2, and K3 in the case of a three-phase short circuit was analyzed, which provides a reference guide for selecting parameters in later control optimization. Finally, the control effectiveness of a multi-machine VSG system was tested. The simulation results show that the proposed method has a pronounced effect on making the system stabilize quickly, which illustrates the effectiveness of this technique.

Stabilization of constant power loads and dynamic current sharing in DC microgrid using robust control technique

DC microgrids are appealing in distribution networks due to their high efficacy, stability, and ease of incorporation with renewable energy resources. In order to maintain system reliability, load sharing is crucial, because disturbances such as the constant power load (CPL), constant voltage load (CVL), uncertainty parameters, and variations in input voltage may result in instability. The conventional droop control method has been frequently employed to regulate the DC microgrid. However, under significant disturbances, conventional droop control is not suitable for achieving both accurate current sharing and acceptable voltage regulation. To tackle these limitations, this paper introduces a robust control for sharing load current and regulating DC bus voltage in a DC microgrid feeding a CPL and CVL. The proposed approach is split into two loops. The main loop, which is based on an RST controller, is used to regulate the current load sharing between two parallel boost DC/DC converters, handle the instability issue caused by disturbances, and improve system reliability. The DC bus voltage is adjusted using the secondary loop built using the proportional-integral (PI) controller. The key benefits are precise voltage regulation, load power sharing, and good reliability. Aside from that, the RST controller has great resilience, quick dynamic response, and strong stability for significant load fluctuations. The feasibility and efficacy of the proposed approach are confirmed by time-domain simulation results, and hardware-in-the-loop (HIL) testing under various scenarios. The obtained findings show that the suggested control strategy is capable of ensuring proportionate power-sharing while dealing with DC voltage instability under CPL, CVL, uncertainty parameters, and input voltage fluctuations.

Improved coordinated control strategy for VSC-MTDC system with DC voltage secondary regulation

Droop control is widely used in multi-terminal flexible DC (VSC-MTDC) transmission systems by virtue of the advantage of multi-station cooperative unbalanced power dissipation, however, the essence of the droop control strategy is to change the DC current to realize the unbalanced power dissipation, and the resulting DC voltage deviation will affect the normal operation of the system. Firstly, this paper theoretically analyses the working characteristics of the conventional droop control and proposes a control method to realize the quasi-differential-free regulation of DC voltage by translating the droop curve. Second, according to the power margin of the converter station, the feedforward compensation amount of each converter station is reasonably set to avoid the power impact on the converter station. Finally, for the problem that the actual value of DC voltage still deviates from the rated value, a control strategy containing secondary regulation of DC voltage is proposed to further restore the DC voltage to the initial value on the basis of ensuring the effect of power regulation, which improves the stability of the operation of the VSC-MTDC system. The final simulation results verify the effectiveness of the proposed method.

Consensus based distributed secondary control for current sharing and voltage restoration considering local loads and constant power loads in dc microgrids

ABSTRACT Ensuring accurate current sharing and regulating output voltage is challenging in DC microgrids. Conventional droop control has limitations in achieving voltage regulation and accurate current sharing. Unequal transmission line resistances, local loads, and constant power load (CPL) add to the complexity. To tackle this issue, this paper presents a novel consensus based distributed secondary control that determines the resistance of transmission lines by injecting an external pulse to the reference voltage of each converter, and then measuring the resulting changes in output voltage and current. The proposed secondary control signal, denoted as ψi , enables simultaneous proportional current sharing and voltage restoration in the DC microgrid. To guarantee the stability of the proposed method, a comprehensive stability analysis of the system is performed. In contrast to similar methods, the proposed method enables achieving precise proportional current sharing and voltage restoration, even when faced with varying resistive loads and CPLs. Furthermore, the proposed method exhibits remarkable features including, high convergence speed, the capability to determine transmission line resistances instead of assuming them to be known, and robustness in plug-and-play scenarios and communication system failures. The proposed method is validated through simulations in MATLAB and experimental tests, affirming its effectiveness.

Multi-objective based Hybrid Artificial Intelligence Controlled Parallel Inverter in Islanded and Grid Connected Operations

Background: The Integration of non-conventional energy systems (NCES), like solar, wind, etc., into the grid with power electronic devices is adapted to meet the demand. Parallel connection of inverters (PCI) is an efficient method to boost power handling capacity, reliability, and system efficiency. However, the main drawback is the unequal power sharing among the inverters while using the conventional droop control technique (CDC). In addition, the circulating currents (CC) flow between these PCI, leading to common mode voltage (CMV), current waveform distortion, and reduction in the system's overall performance. Objectives: This work consists of a photovoltaic system (PVS) and battery energy storage system (BES) as the distributed generation (DG) unit to voltage source inverter (VSI) 1 and 2. The multi- objectives of the suggested work are (a) the equal power/load sharing among two inverters, (b) effective minimization of CC and the CMV, (c) maintaining constant DC-link (DCL) voltage during different solar irradiation and constant temperature, and (d) the reduction in total harmonic distortion (THD) of load current. Methods: A novel approach related to the enhanced droop control (EDC) method with an adaptive neuro-fuzzy hybrid controller (ANFHC) was suggested here to overcome the above issues. The performance analysis of the suggested technique was done in the Matlab/ Simulink platform with islanded and grid-connected modes for different loads. Results: A comparative analysis with the available methods like the proportional integral controller (PIC) and sliding mode controller (SMC) was carried out to exhibit the viability of the developed control technique. Conclusion: This study focused on the operation and control of a PCI with ANFHC in islanded and grid-connected modes to address issues such as uniform power sharing and reduction of CC and CMV.

Adaptive Control Approach for Accurate Current Sharing and Voltage Regulation in DC Microgrid Applications

A DC microgrid is an efficient way to combine diverse sources; conventional droop control is unable to achieve both accurate current sharing and required voltage regulation. This paper provides a new adaptive control approach for DC microgrid applications that satisfies both accurate current sharing and appropriate voltage regulation depending on the loading state. As the load increases in parallel, so do the output currents of the distributed generating units, and correct current sharing is necessary under severe load conditions. The suggested control approach raises the equivalent droop gains as the load level increases in parallel and provides accurate current sharing. The droop parameters were checked online and changed using the principal current sharing loops to reduce the variation in load current sharing, and the second loop also transferred the droop lines to eliminate DC microgrid bus voltage fluctuation in the adaptive droop controller, which is different and inventive. The proposed algorithm is tested using a variety of input voltages and load resistances. This work assesses the performance and stability of the suggested method using a linearized model and verifies the results using an acceptable model created in MATLAB/SIMULINK Software Version 9.3 and using Real-Time Simulation Fundamentals and hardware-based experimentation.

Improved Droop Controller for Distributed Generation Inverters in Islanded AC Microgrids

Stability in island microgrids is crucial for efficient power distribution among distributed generation (DG) inverters. Conventional droop control, while effective in power sharing, poses challenges with voltage stability due to frequency and voltage deviations resulting from changing load power. Such deviations can lead to system instability, impacting power flows within each inverter. Therefore, this paper introduces a proposed droop control approach that effectively tackles the issues of frequency and voltage deviation, aiming to restore them to their rated values and significantly enhance transient response in power flows among inverters. The novel method incorporates integrating controllers for frequency and voltage, coupled with the utilization of virtual impedances. These virtual impedances, comprising virtual positive/negative-sequence impedance (VPI/VNI) loops at the fundamental frequency and a virtual harmonic impedance (VHI) loop at harmonic frequencies, play a crucial role in overcoming mismatched line impedance conditions, ultimately improving overall system performance. Simulation results demonstrate the effectiveness and outstanding performance of inverters operating in parallel within an island AC microgrid. The proposed approach ensures stable voltage and frequency levels in all operational states, regardless of varying load conditions.

Experimental Validation of Virtual Inductance for Synchronization of Grid-Forming Converters

Grid-forming converters provide a voltage and frequency to the grid and can thus be used to enable the temporary islanded operation of distribution grid structures, e.g. during contingencies. When operating several grid-forming converters in parallel, they maintain synchronization by a supplementary controller, as e.g. a conventional droop controller. However, the droop control is designed for grids with a high line reactance to resistance ratio, which is not valid at the low voltage level. As a consequence, grid-forming converters might not be able to synchronize or maintain synchronization to the grid. This paper presents test results from a laboratory setup with a real low voltage microgrid validating the inability of two grid-forming converters to synchronize. To overcome this issue, a virtual output inductance, which effectively increases the line inductance, is implemented in the converter controllers and validated in the test setup. The measurement results prove that the additional inductance diminishes the synchronizing interactions and enables the successful synchronization of the converters. Furthermore, it is shown that the tuning of the inner converter control loops can significantly impact the effectiveness of the virtual inductance during transients.

Improving power sharing and enhancing the stability of an isolated AC microgrid by implementing droop control and virtual impedance

The stability and accuracy of power sharing between DGs in isolated microgrids are key points for favouring renewable energy sources within a wider operating margin to extract the maximum available power. The conventional droop control method CDC and the improved virtual impedance droop control CDC-VI are proposed in this work to enhance the stability, improve the speed, robustness and accuracy of power distribution between two parallel DGs feeding three linear loads of different types connected to the AC bus within an isolated AC microgrid. A Matlab/Simulink simulation is carried out to validate our proposal, robustness, accuracy of power sharing between DGs and stability are well improved by the implementation of the proposed CDC and CDC-VI controllers. The main improvements brought by these controllers include improved power distribution with greater precision, enhanced stability even in the presence of disturbances, and faster response in dynamic scenarios. These improvements are crucial to ensuring efficient, reliable operation of microgrids, particularly in isolated environments where integration of renewable energies and management of critical loads are paramount for uninterrupted service continuity.

New Mesh Configurations With Decentralized Droop Control Method for DC Microgrids

This paper proposes new, practical, and scalable mesh configurations for DC microgrids. The new mesh configurations are inspired by the concepts of graphs in graph theory. A decentralized secondary droop control method without involving communication complexity is also presented in this paper. The proposed control method eliminates the limitations of the conventional droop control method in the scenarios where the cable resistances in a DC microgrid system cannot be neglected. The new mesh configurations with the proposed control method achieve accurate current sharing among all the converters. The effectiveness and performance of the proposed control method are validated using circuit simulations and hardware-based experiments on existing and proposed configurations of multi-converter DC microgrid systems.