Enhanced Droop Control Research Articles

An Adaptive Droop Control for Power Sharing Between Three Phase Parallel Inverter in Autonomous Microgrid

The development of an adaptive droop controller is for power sharing between three-phase parallel inverters. Traditionally, droop controllers have been employed in conventional power systems to allocate the load among generators with varying capacities. Our approach involves the utilization of an adaptive control technique. This method is designed for a setup involving three-phase parallel inverters linked to an AC microgrid, to enhance the proportional distribution of active power despite changes in feeder network characteristics and loads. The proposed control mechanism automatically tunes the frequency droop parameter to address mismatch frequencies. The system includes two sets of three-phase inverters regulated by a combination of P-F droop control, an enhanced droop control loop, and PI controllers. The adaptive droop coefficient control is capable of addressing problems where power distribution inaccuracies arise due to different line loads as well as different inverter capacities.

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.

Coordinated volt/VAR control for photovoltaic inverters: A soft actor-critic enhanced droop control approach

The reactive power capacity of photovoltaic inverters can be utilised to minimise power loss and mitigate rapid voltage fluctuations in an active distribution network. This paper proposes a coordinated volt/VAR control framework that simultaneously optimises the base reactive power output of photovoltaic inverters and the voltage intercept of each droop control function in the central hierarchy based on whole-system information to minimise the power loss. The photovoltaic inverters use the optimised droop function to eliminate the voltage violation based on the real-time local information in the local hierarchy. Subsequently, the optimisation problem is formed as a Markov decision process and solved using an improved deep reinforcement learning algorithm called a soft actor-critic to achieve fast control with full consideration of uncertainties. Thus, the proposed method can simultaneously guarantee the cooperation and rapidity of the control performance to minimise power loss and quickly respond to real-time voltage fluctuations. Case studies were implemented using IEEE 33-bus and 123-bus distribution networks. The comparison results demonstrate that the proposed method can reduce the power loss and voltage violation rate by up to 12.3% and 2.71%, respectively, compared with other benchmark methods, which demonstrates the superiority of the proposed method in terms of power loss minimisation and voltage fluctuation mitigation.

Power Sharing in Three-Level NPC Inverter Based Three-Phase Four-Wire Islanding Microgrids With Unbalanced Loads

The droop-based control schemes are widely used for power-sharing and control of the isolated microgrids. The conventional droop control scheme with inner current and voltage controllers can work effectively under ideal voltage. However, the existence of unbalance tends its performance to worsen. This paper aims to propose an enhanced droop control method to improve the power-sharing, and maintain the distributed generators’ (DGs) terminal voltages and the system frequency under the balanced as well as unbalanced loads. The enhancements of the proposed control scheme include the droop controller, inner current and voltage controller, and virtual impedance loop. The proposed virtual impedance deals with the accurate sharing of powers in the case of unequal line impedance and the control of zero sequence current. Moreover, to enable the proposed control to compensate for the unbalance, three-level neutral point clamped (NPC) inverters are used to form a three-phase four-wire microgrid. With this control scheme, the voltage unbalance factors (VUFs) of the DGs’ terminal voltages are reduced from 4% to less than 1%, negative sequence reactive powers, which are also an indication of unbalance, have been reduced significantly, and the system frequency is maintained within the standard permissible limit of 2.5%. Furthermore, a local load controller is also proposed to control the reactive loading of the DGs when they have local loads supplied from their terminals. The investigations have been carried out under simulation as well as lab-based environments to validate the efficacy of the proposed control method.

Consensus enhanced droop control strategy for islanding mode multi converter system

Under the gradual expansion of the new energy distributed generation, the islanding multi converter microgrid system plays a great role. The stability of voltage and frequency of multi converter systems in islanding mode and the coordination of power allocation among many converter systems have become vital research points. Focusing on the reactive power sharing problem of multi converter system for different line impedances and load disturbances, this paper proposes a consensus enhanced droop control strategy to achieve power sharing and voltage and frequency stability, and the mechanism and steady-state performance of the proposed control strategy are analyzed and studied. In this work, a multi converter system simulation model composed of six inverters is built in the software PLECS, which verifies that the consensus enhanced droop control scheme can make up for the shortcomings of the traditional droop control. Moreover, the hardware experimental setup of three-phase inverter system is further conducted to validate the simulation results. Under load disturbance, the control effects of consensus enhanced droop control proposed in this paper are analyzed and studied, and the good dynamic characteristics of consensus enhanced droop control strategy are verified. Finally, the high robustness of the proposed coordinated control strategy is further illustrated by the jump of unbalanced load. This paper clarifies the effectiveness of the proposed control strategy based on consensus algorithm in solving the reactive power sharing problem of different line impedances and load disturbances around the island mode multi converter system. Furthermore, the work in the paper can ensure effective reactive power sharing under various working conditions with high robustness.

Development of a Smart Energy Community by Coupling Neighbouring Community Microgrids for Enhanced Power Sharing Using Customised Droop Control

This article aims to develop a smart isolated energy community (EC) by coupling the neighbouring rural community microgrids (CMGs) with enhanced droop control for efficient power sharing. This recommended solution employs a power management (PM) based droop-control to enable independent neighbouring CMGs to share power on an available basis by not constraining CMG inverters to equal power sharing. During the grid-connected mode, the droop control may have different power setpoints of each CMG. However, during the standalone mode of operation, the power setpoint should be defined according to their power rating and availability to maintain the system stability. In this article, a PM strategy is developed to maintain the power setpoints of the autonomous CMGs. An improper selection of power setpoints in autonomous CMG can raise the DC link voltage to an unmanageable value and can cause an inadvertent shutdown of CMG. The suggested PM-based droop control enables the CMG inverter not to restrict the inverter to equal power share but to distribute its active power as available in an asymmetric way, if required. The dynamic performance of the proposed coupled system incorporated with two remote isolated CMGs is investigated in a MATLAB environment. Further, a laboratory prototype of the proposed system has been developed using a LabVIEW-based sbRIO controller to verify the efficacy of the proposed approach.

Power management techniques for grid-connected DC microgrids: A comparative evaluation

DC microgrids are pointed out as a good alternative in distribution systems with integration of renewable energy sources. However, the management of the power flow in the DC microgrid is still an issue under investigation by the scientific community. This article presents a qualitative comparison analysis of power management systems (PMS) for grid-connected DC microgrids. The studied DC microgrid is a basic structure formed by the AC utility grid, a battery energy storage system (BESS), a distributed generator, and the customer loads. Due to the lack of control structures specifically designed to manage grid-connected DC microgrids, a master–slave and an enhanced droop control are proposed as power management techniques. These methods are compared with the conventional droop control, an adaptive droop control based on the battery bank state-of-charge, and a hierarchical control with voltage regulation adapted to operate as PMS. The autonomy of the energy storage system, necessity of communication line, presence of voltage deviation, and inclusion of a battery management system are the most relevant PMS characteristics under analysis. The contribution of this study is a deep comparative evaluation of the PMS control techniques; considering the impact over the DC link voltage, the power flow, and the BESS state-of-charge. Moreover, the performance of each method is evaluated for a distributed generation based on solar and wind renewable energy sources. The experimental results show that the proposed master–slave technique leads to a smooth transition of the power flow inside the DC microgrid, which returns a better performance when compared to the traditional methods. On the other hand, the enhanced droop strategy is a viable alternative to be used when the communication link between power electronic converters is not desired.

Advanced Current-Limiting and Power-Sharing Control in a PV-Based Grid-Forming Inverter Under Unbalanced Grid Conditions

Grid-forming (GF) inverter technology is a novel and evolving concept for systems with high penetration of renewables. The integration of these grid-independent units with the existing conventional generators and gradually replacing them should contribute to stabilizing the power grid. Several control strategies are proposed in order to design GF inverters to emulate the kinetic energy and the self-synchronization features of synchronous machines with robust control on voltage and frequency to deal with the dynamic issues of the grid. In this article, a photovoltaic (PV)-based GF inverter with a modified virtual synchronous machine control in parallel with a battery supported inverter with an enhanced droop control is considered to operate under non-ideal grid voltage conditions and in the isolated mode of operation. The control methodology encloses a PV synchronous generator, along with the nonlinear feedback linearization current-limiting control with voltage ride-through capabilities. They enable the GF PV inverter and the grid-following battery inverter to provide active and reactive power to the load during unbalanced grid conditions seamlessly. A hardware cosimulation environment in Simulink using the software tool of Piecewise Linear Electrical Circuit Simulation (PLECS) and System Generator from Xilinx is applied to evaluate the operation of the controller and verify its flexibility and effectiveness in different case studies.

Enhanced droop controller for operating parallel-connected distributed-generation inverters in a microgrid

Distributed generation (DG) is gaining popularity because of prevalent environmental issues and the need for better power quality. To address these concerns, the microgrid was introduced to utilize DG. Inverter controls are essential for improving power quality because DG units are connected to a grid through an inverter. In this paper, an enhanced droop control technique is proposed to properly operate parallel-connected inverters in an AC microgrid. The proposed control strategy is based on the droop control technique, which utilizes locally measurable feedback signals. The droop control method is used for power sharing because of the absence of critical communication links among parallel-connected inverters. The transient response of the conventional droop control of power sharing is slow, and its frequency and voltage deviations are high. These factors limit accurate power sharing among parallel-connected inverters. The enhanced droop control is introduced to mitigate this problem, through which the trade-off between voltage regulation and reactive power sharing decreases significantly. In addition, the proposed method improves the transient response and achieves higher output power without voltage and frequency deviations. The results indicate that the desired transient response and accurate reactive power sharing with the line impedance mismatch are achieved. The simulation results are analyzed using MATLAB/Simlink to verify the effectiveness of the proposed control strategy.

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.

An Enhanced Droop Control Method for Accurate Load Sharing and Voltage Improvement of Isolated and Interconnected DC Microgrids

In this paper, a distributed local control scheme for dc microgrid is proposed along with the basic droop control. It eliminates the limitations of droop control when the distributed generators are geographically distributed, for which, the line resistances cannot be neglected. Effects of line inductance and constant power loading (CPL) are investigated by analyzing the voltage tracking transfer function for single source system. Stability of two sources single load microgrid with proposed controller is investigated. Simulated responses are presented for two sources single load microgrid (for the sake of simplicity) to depict the proper load sharing and voltage improvement capability of the proposed control method with the consideration of line resistances. However, this can be extended to multiple-source multiple-load configuration connected to the dc bus. A comparison of the result is presented to show the better performance of the proposed control scheme as compared to the conventional droop control and hierarchical secondary control. The interconnected operation of the microgrid is also investigated to show the applicability of the proposed control in the interconnected mode. A centralized controller in each area is used to make the tie-line power flow zero at steady state.

An Enhanced Islanding Microgrid Reactive Power, Imbalance Power, and Harmonic Power Sharing Scheme

To address inaccurate power sharing problems in autonomous islanding microgrids, an enhanced droop control method through online virtual impedance adjustment is proposed. First, a term associated with DG reactive power, imbalance power, or harmonic power is added to the conventional real power-frequency droop control. The transient real power variations caused by this term are captured to realize DG series virtual impedance tuning. With the regulation of DG virtual impedance at fundamental positive sequence, fundamental negative sequence, and harmonic frequencies, an accurate power sharing can be realized at the steady state. In order to activate the compensation scheme in multiple DG units in a synchronized manner, a low-bandwidth communication bus is adopted to send the compensation command from a microgrid central controller to DG unit local controllers, without involving any information from DG unit local controllers. The feasibility of the proposed method is verified by simulated and experimental results from a low-power three-phase microgrid prototype.

An Enhanced Droop Control Scheme for Islanded Microgrids

Due to the line impedance impact on islanded low-voltage microgrids, traditional droop control cannot guarantee the proper power sharing between distributed generation (DG) units. In the meanwhile, droop control would cause voltage and frequency deviation, which will influence the power quality of customers. This paper proposed an enhanced droop control scheme with autonomous frequency restoration control. Reactive power sharing problem is solved by adding a redesigned virtual impedance and voltage compensation loop, which ensures PCC voltage in nominal range. To the problem of frequency deviation, autonomous frequency restoration loop is added. Small signal model of frequency control is established to analyze the stability. In circumstance of frequent load switching, the frequency can properly restore to nominal value, with smooth and stable transient process. The simulation results demonstrate the effectiveness of the proposed control strategy.

A new method of voltage and frequency control in isolated microgrids using enhanced droop controller optimized by frog algorithm

This paper presents a new method for optimal frequency and voltage control in microgrids (MG) considering the mutual effect of line parameters on power transfer. In MGs each distributed generation (DG) has its special droop characteristics. When a load consumption change occurs in the MG, an imbalance between generation and consumption rises, thus a change occurs in the voltage and frequency of DGs according to their droop characteristics. In order to minimize oscillations of voltage and frequency, we have used optimal droop controller which is based on considering the mutual effects of transfer parameters on active and reactive powers. Since line parameters heavily affect the droop characteristics, these parameters are obtained optimally by the frog algorithm such that oscillations in voltage and frequency are minimized. In order to enhance the controlling scheme, effects of line resistance on reactive power changes and effects of line reactance on active power changes are also considered. Since enhanced droop control depends on line parameters, it cannot be implemented in large MGs; therefore, it is modeled by multi-layer perceptron neural networks. We have used the frog algorithm in order to train the perceptron network. The obtained results show satisfactory voltage and frequency control in various load level. In this paper, MGs are studied and simulated in an isolated state.