Conventional Droop Control Scheme Research Articles

Accurate Active and Reactive Power Sharing Based on a Modified Droop Control Method for Islanded Microgrids.

When multiple paralleled distributed generation (DG) units operate in an islanded microgrid, accurate power sharing of each DG unit cannot be achieved with a conventional droop control strategy due to mismatched feeder impedance. In this paper, a small AC signal (SACS)-injection-based modified droop control method is presented for accurate active and reactive power sharing among DG units. The proposed control method adjusts the voltage amplitude of each DG unit by injecting small AC signals to form a reactive power control loop. This strategy does not need communication links or to specifically obtain the physical parameter of the feeder impedance and only requires the local information. Moreover, the parameter design procedure and stability analysis are given full consideration. Finally, simulation and experimental results verify the effectiveness of the proposed control scheme, and accurate active and reactive power sharing can be achieved.

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.

Enhancing dynamic stability performance of hybrid ac/dc power grids using decentralised model predictive control

High voltage direct current (HVDC) networks are transforming ac power grids to hybrid ac/dc power grids; hence hybrid ac/dc power grids are facing new stability challenges. In this paper, a decentralised control scheme based on model predictive control (MPC) is proposed to enhance the stability of hybrid ac/dc power grids under disturbances. The proposed scheme controls the dc voltage, frequency, active and reactive power simultaneously with minimum deviations from steady-state values. A communication network is used to share the sampled grid data with the local MPC of each voltage source converter (VSC). Hence, the central controller can be avoided. Moreover, the active power-sharing between converters can be pre-designed. Thus, fast and accurate active power control can be realised. Finally, the proposed control scheme is validated using a five-terminal VSCHVDC test system with MATLAB. The VSCHVDC network with the proposed control scheme shows a smaller deviation and a faster response compared with the conventional droop control scheme and centralised MPC scheme under small disturbances. Under large power variations, the proposed control scheme assists to reach the steady-state rapidly with a minimum impact on the entire power grid. The proposed control scheme has also proved its robustness under communication delays and outages.

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.

A Consensus-Based Cooperative Control for DC Microgrids Interlinked via Multiple Converters

In the clustered dc microgrids (MGs), two dc MGs can be connected through multiple interlinking converters (ICs) in order to increase system power capacity as well as to achieve a better efficiency, reliability, and stability. This is more common for high-power dc MGs. The conventional droop control strategy is not a practical choice for providing proportional power sharing among two dc MGs with more than two ICs. This is mainly due to either differences in transmission line impedances or different output impedances of the ICs which can result in an unwanted circulating current among them. In this article, a two-level control scheme in a fully distributed manner is proposed for interconnecting two dc MGs by multiple ICs. The proposed scheme is based on the consensus protocol which only requires local measurement and neighbor information through a sparse communication structure. This control scheme proportionally shares power not only between two dc MGs but also among the multiple ICs and suppresses the circulating currents as well. A complete set of simulation studies is carried out in PLECS (Plexim) software to verify the effectiveness of the proposed control scheme.

Efficiency-Prioritized Droop Control Strategy of AC Microgrid

Operation efficiency is one of important concerns for power conversion system. Efficiency issue of microgrids with multiple paralleled inverters has been paid slightly concerns. Therefore, this article presents an efficiency analysis method of microgrid in system level and an efficiency-prioritized droop control strategy to improve operation efficiency of microgrids under different load profiles. Efficiency model of microgrid is first established to reveal time-varying efficiency characteristic under different load profiles. Then, optimum solution of efficiency model is derived by Lagrange multiplier method, and optimum conditions to capture maximum efficiency are obtained. Furthermore, an efficiency-prioritized droop control strategy is proposed to improve operation efficiency according to the established efficiency model. In addition, stability analysis of the proposed droop controller is performed, from which the guideline for controller design is given. The simulation and experimental results are provided to validate the proposed efficiency-prioritized droop control strategy. The proposed droop control strategy is able to improve overall efficiency of microgrid under different load profiles, which also preserves advantages of the conventional droop control strategy.

An adaptive compensation droop control strategy for reactive power sharing in islanded microgrid

In a parallel distributed generation system, the conventional droop control strategy makes it difficult for the inverter to output reactive power precisely due to the line impedance uncertainty and load fluctuation, which leads to a voltage deviation of the microgrid system. In order to precisely distribute reactive power, this paper advances an adaptive compensation strategy based on fuzzy control. The fuzzy controller detects the load power of AC bus terminal and output adaptive power coefficient to adjust amplitude voltage to generate compensation voltage. The compensation voltage is fed back to the d-axis voltage of the power loop for reactive power redistribution. Because the fuzzy control membership function value is difficult to determine accurately, this paper puts forward a method which combines neural network with fuzzy control to determine membership function values and fuzzy rules. Simulations and experimental results show that the adaptive compensation control strategy can share power accurately with good dynamic characteristics and robustness.

Virtual-battery based droop control and energy storage system size optimization of a DC microgrid for electric vehicle fast charging station

DC microgrid is supposed to be a feasible solution to reduce the negative impact of electric vehicle (EV) fast charging on the electric grid and improve the penetration of photovoltaics (PV) generation. In this paper, an improved decentralized Virtual-battery based droop control with the capability of bus voltage maintenance, load power dispatch and SOC balance of the energy storage system (ESS) is proposed to ensure the autonomous and stable operation of the DC microgrid. The reference output voltage and virtual resistance in the droop control loop are altered dynamically based on the Virtual-battery model of the ESS. The coordinated control among the PV-ESS-Grid integrated system is realized through the primary Bus-Signaling control, where the reference voltages at which the control modes of the PV array and the grid are switched are designed based on the VirtualOCV of the ESS. The effectiveness of the proposed control strategy is validated in MATLAB/Simulink environment with an equivalent bus capacitance-based model where the EV charging profile is obtained from real-world charging data of a fast charging station. The merits of the control strategy including higher PV utilization, less frequent connection of the grid and more precise voltage tracking are highlighted in comparison with the conventional droop control strategy. Finally, the sizing of the ESSs is optimized based on the total cost of the DC microgrid, including the daily electricity cost purchased from the grid and the depreciation cost of the ESSs based on the expanded capacity degradation model of Li-ion batteries.

Research on the Coordinated Control of the True Bipolar VSC-HVdc Grid Based on Operating Point Optimization

The voltage source converter based high voltage direct current transmission system with the true bipolar wiring mode features higher flexibility and reliability, yet it cannot be without a proper and effective coordinated control strategy, which is the key to the safe and reliable operation of the system as well as the effective and full utilization of renewable energy on a large scale. A step further from the conventional droop control strategy, the coordinated control strategy based on operating point optimization is proposed in this paper. This paper first illustrates and examines the coordinated control method at the system level in view of special wiring and control modes; it then looks into the method of operating point optimization seeking to achieve both safe and cost-effective operation; and the paper finally demonstrates the effectiveness of the coordinated control strategy verified by MATLAB and PSCAD simulations from the perspective of system operating point deviation, security, and cost-effectiveness.

Damping Performance Analysis and Control of Hybrid AC/Multi-Terminal DC Power Grids

The voltage-source converter (VSC) based multi-terminal direct-current (MTDC) networks are currently being developed in large-scale power grids for efficient and economical transmission of electrical energy generated from remotely sited renewable energy sources (RESs). As the MTDC network is developed alongside the conventional AC grid, it has transformed the power grid to a hybrid AC/MTDC power network. However, since the MTDC network is controlled via the power electronic converters (PECs), it significantly affects the AC power grid damping and synchronising performance as it decouples the natural dynamics between various electro-mechanical systems in the power grid. This paper proposes a supplementary control scheme to improve the damping performance of the entire hybrid AC/MTDC power grid. The damping torque analysis (DTA) technique is used as the primary technique to develop the supplementary control scheme. The preliminary analysis is carried out using a two-terminal hybrid AC/DC power grid and has suggested a supplementary control loop based on the rotor speed deviation for the DC voltage controller of the VSC to improve the damping performance. With the state-space model, the synchronising and damping torque coefficients are calculated and accordingly, the feedback gain is determined to provide optimal synchronising and damping torque components. Subsequently, the fidelity of the supplementary controller is verified using a four-terminal hybrid AC/MTDC grid. Simulation studies prove that the proposed supplementary controller can improve the hybrid AC/MTDC network damping performance, and it performs very effectively with the master-slave control and the conventional droop control scheme.

Dynamic Droop Control Method for Islanded Photovoltaic Based Microgrid for Active and Reactive Power Control with Effective Utilization of Distributed Generators

Conventional droop-control scheme shares the load amongst energy sources in proportion to their ratings. The scheme suffers from the issue of ineffective utilization of the sources when performance of some of the sources is dependent on environmental conditions. Hence, a modified droop- control strategy is proposed for a microgrid comprising of photovoltaic (PV) based distributed generators (DG) operating in parallel with other DGs. Dynamic nature applied to the droop characteristic by the primary control unit (PCU) sets the frequency reference such that the PV sources operate at their maximum power point and the energy demanded from the auxiliary source is the minimum. The margin available after supplying the active power is used to allocate the references for reactive power sharing. The reactive power sharing algorithm employed in secondary control unit (SCU) ensures that the standard deviation of the percentage utilization of the inverters is kept the minimum. Even in case of the failure of the communication between the PCU and SCU, a reasonably good performance is ensured as the control shifts to the master-slave control having dynamic droop adjustment feature. The effectiveness of the proposed strategy against other approaches is justified through the simulation results obtained in MATLAB/Simulink.

A novel Decoupled Trigonometric Saturated droop controller for power sharing in islanded low-voltage microgrids

This paper proposes a novel droop control based on Decoupled Trigonometric Saturated (DTS) controller for stable power sharing applied to meshed parallel inverter systems in islanded microgrids. The novel DTS control scheme is introduced to improve the power-sharing accuracy with a better stability and to provide a proper dynamic decoupling of active and reactive power in the presence of different impedances. Moreover, this method not only achieves the aforementioned decoupling; but also, guarantees both voltage and frequency stability. The theoretical concept of the proposed novel droop control strategy is presented in detail. The DTS controller is applied to a common AC bus microgrid structure and a meshed parallel inverter system structure in islanded microgrids with mainly inductive or resistive line impedances. An offline time-domain simulation is conducted in MATLAB®/SimPowerSystems environment using RT-EVENTS toolbox from OPAL-RT to model the inverters. Resulting waveforms from a three-phase microgrid with four distributed generators are presented along with a comparison against the conventional droop control strategy and show the effectiveness of the proposed method in allocating both real and reactive power.

Adaptive complex virtual impedance control scheme for accurate reactive power sharing of inverter interfaced autonomous microgrids

The mismatched feeder impedance of the voltage-sourced converter (VSC) units may result in reactive power sharing error in the islanded microgrids. This study presents a reactive power sharing strategy for single-phase inverter-based microgrids that eliminates the circulating currents and reduces the coupling between real and reactive powers. The proposed controller is based on an adaptive complex virtual impedance that aims to equalise the E - Q characteristic output of VSCs. The resistive term regulates the virtual voltage drop using the proposed ΔR v - P droop characteristic, while the inductive one controls the phase angle of VSC current and increases the X/R ratio of the microgrid. The proposed strategy does not require a central controller and knowledge of the microgrid lines and loads data and is applicable for the various microgrid topologies. Furthermore, it employs the unidirectional low bandwidth communication link and is robust against the delay and failure of communication channels. The proposed method is implemented using the conventional droop control scheme and presents the medium plug and play capability. Both simulation and experimental case studies using various microgrid topologies are conducted to demonstrate the merits of the proposed scheme.

Evaluation of MPC-based arctan droop control strategy in islanded microgrid

The microgrid (MG) is an emerging technology for supplying energy from renewable energy sources (RES). Due to their intermittent nature, RES require an inverter and smooth operation control system when integrated into an MG. Conventional droop control is commonly used in microgrid control but cannot achieve adequate power-sharing when output impedances of inverters are different; thus, enhancing its performance is required. This paper’s main objective is to design a finite control set model predictive control (FSC-MPC) based arctan droop control and evaluate its performance against the conventional droop control strategy. The proposed control technique guarantees a strong transient response when there is a dynamic load change during MG operation. The results showed that the proposed control strategy ensures proportionate power-sharing of paralleled voltage source inverters with unequal line impedances than the traditional droop controller, which can vary.

A Need-Based Distributed Coordination Strategy for EV Storages in a Commercial Hybrid AC/DC Microgrid With an Improved Interlinking Converter Control Topology

This paper presents a need-based distributed coordination strategy (NDCS) for multiple electric vehicle (EV) storages in an islanded commercial hybrid alternating-current (AC)/direct-current (DC) microgrid. The control capacity of the interlinking converter is enhanced by incorporating combined power-droop and voltage-droop strategies to leverage the coupling of AC and DC voltages. Therefore, the AC bus voltage can be regulated simultaneously by regulating only the DC bus voltage without affecting the power-sharing capabilities of the converter. The NDCS is proposed to coordinate the EV storages to regulate the DC bus voltage. The main objective of the NDCS is to decide whether the coordination of the available EV storages is to be done in a decentralised or a distributed manner. The mathematical model and the algorithm to deploy NDCS are developed to realise its application to a real system. The effectiveness of the control system is verified in a commercial hybrid AC/DC microgrid comprising one photovoltaic unit and four EV storages directly connected to the DC bus via DC/DC converters and four distributed-generation (DG) units connected to the AC bus using the conventional droop-control scheme. The performance of both controllers is tested under variable irradiation, commercial loading, and various fault conditions. The results of the case studies demonstrate the efficacy of the overall system in terms of robustness for a variable generation-demand scenario, time delay, EV plug-and-play, and fault conditions.

A Modified Droop Controller for Parallel Operation of Single-Phase Inverters in Islanded Microgrid

Conventional droop control is a basic control strategy for power sharing in islanded microgrid applications. This strategy has several limitations, such as low transient response, frequency and voltage deviations. This paper presents a modified droop control method for the proper operation of parallel-connected inverters in islanded microgrid. The proposed method is able to improve transient response and achieved higher output power without voltage and frequency deviations by introducing a power derivative term into a conventional droop method. A new method of average power computation is proposed which significantly reduces the delay encountered in conventional approaches and provides the power signal without DC components. The simulation was developed in MATLAB/Simulink to verify the effectiveness of the modified droop control scheme. Based on the results, it can be concluded that the modified droop control strategy improves the performance of parallel-connected power electronic inverters in an islanded microgrid as compared to conventional droop control strategy.

An Adaptive Virtual Impedance Based Droop Control Scheme for Parallel Inverter Operation in Low Voltage Microgrid

This paper presents an adaptive virtual impedance based droop control scheme for parallel inverter operation in low voltage microgrid. Because it is essential to achieve power sharing between inverters in microgrid , various droop control scheme s ha ve been proposed . In practice, the line impedance between inverters and the point of common coupling (PCC) in microgrid are not always equal. This im balance in line impedance often results in a reactive power mismatch among inverters. This problem has been solved by introducing a virtual impedance loop in the conventional droop control scheme. However, the reactive power sharing performance of this method is still deteriorate d when the line impedances change during operation. To overcome such a problem , a new control scheme that is based on a virtual impedance loop and an impedance estimation scheme is proposed. To monitor the changes in line impedances , t he impedance estimator is implemented by using the output voltages and currents of inverters as well as the voltages at the PCC. To compensate for the reactive power mismatch due to the line impedance changes , t h e estimated line impedance is fed to the virtual impedance loop in which it adjusts the virtual impedance value . Comparative s imulation results with the conventional ones verify the effectiveness of the proposed adaptive virtual impedance based droop control scheme .

Research on Droop Control of Single Phase Parallel Inverter

Parallel operation of Multi-inverter can greatly improve the system's flexibility , power system's Capacity, redundancy and reliability, current through the power switch is divided, so the current stress for power switch can also be greatly reduced, the performance is improved with costs effective and power density. The conventional droop-control strategy which is adopted generally for parallel inverter regulates the phase and the amplitude of the output voltage according to output active power and reactive power，respectively.And active power and reactive power are related to both output amplitude and output phase for double closed-loop feedback voltage—source inverter．The control method of the droop control with virtual impedance is applied in this paper. The PQ droop control strategy for parallel single phase inverter is illustrated. PQ droop control scheme can effectively stabilize the droop control system to automatically exit, and also can achieve the expected load-sharing flow effect. Droop control without mutual connection, the inverter can be conveniently connected and removed. Finally， simulation results by obtained MATLAB / Simulink software is given to verify the effectiveness of theoretical analysis .

Robust droop control of DC distribution networks

DC droop control strategy is usually used to improve load sharing in increasingly popular DC distribution networks. However, the conventional droop control strategy suffers from considerable terminal voltage drop and influences by line impedance. In this study, the authors propose a new robust droop control strategy to overcome these drawbacks. They first develop a mathematical model of the proposed robust controller and use the load terminal voltage as a feedback signal. They further treat the line impedance as part of the equivalent output impedance of individual power converters and minimise the inaccuracy of load sharing by regulating robust coefficients. They analyse the influence of the robust coefficients on the system stability. They have verified the robust control strategy with both power system computer-aided design/electromagnetic transients including DC simulation and control hardware in the loop semi-experimental method. It is shown that the robust concept improves the load sharing accuracy and suppressing the load voltage fluctuation.

Review of Active and Reactive Power Sharing Strategies in Hierarchical Controlled Microgrids

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.