Droop Method Research Articles

Fractional order PI control combined with improved frequency droop method for power management in standalone LVDC microgrids

According to increasing growth of DC loads, DC microgrids (MGs) have an effective role in future power systems. Improvement of power management is one of the important issues in DC MGs. In this paper, in order to provide an accurate power management in standalone low-voltage DC (LVDC) MG, including photovoltaics (PVs) and battery energy storage systems (BESSs), an AC component is superimposed to the output voltage of DC-DC converters by inspecting the frequency droop control of the synchronous generators. In addition, two corrective terms based on the calculation of AC active and reactive powers are added to this AC component, which is named improved frequency droop method (IFDM). Different operating modes for PV system comprising maximum power point tracking (MPPT) and voltage control mode (VCM); and BESS consisting of charge, discharge, and idle are considered in the proposed power management scheme based on the state of charge (SoC) of local BESS. On the other hand, the conventional proportional-integral (PI) controllers in DC-DC converters' control structure present poor transient dynamic performance and low loading level. Hence, fractional order PI (FOPI) controllers are designed by optimization of integral time absolute error (ITAE) criteria. The effectiveness of the proposed FOPI controllers combined with IFDM scheme is evaluated by simulations conducted in MATLAB/SIMULINK software. The FOPI-IFDM approach provides accurate current sharing and better voltage regulation without dependence on communication link in comparison with PI-IFDM under load, line impedance, and solar irradiance variations.

Reactive power sharing in microgrid using virtual voltage

The traditional droop control strategy has been applied previously in microgrids (MGs) to share accurately the active power. However, in some cases the result obtained when sharing reactive power is not the best, because of the parameters related to the distances from distributed generators (DGs) to the loads and the power variations. Therefore, this paper proposes a reactive power control strategy for a low voltage MG, where the unequal impedance related to the distances between generators and loads requires adjustments to work with the conventional frequency and voltage droop methods. Thus, an additional coefficient is calculated from parameters of the network that relate the location of elements. The test is perfomed by simulations in the MATLAB-Simulink software, considering a three-node MG with three DGs and a load that can change power at different periods of time. The results show that it is possible to improve reactive power sharing between the DGs located in the MG according to the load changes simulated and to improve voltages with this method.

Small-signal modelling and stability analysis of island mode microgrid paralleled inverters

The autonomous operation mode of paralleled inverters in microgrids can be intentional or unintentional in order to ensure the continuity of supply. In this mode the voltage and frequency magnitudes are held by local controllers using droop control, this latter is generally considered to be the most adopted technic for the primary layer in a multilayer control structure due to their main feature of sharing the power equally between inverters, without needing communication infrastructure, the design of droop parameters is very crucial because a bad design can lead to the instability of the system. This paper presents a small-signal analysis for an MG composed of parallel-connected inverters in island mode and controlled using the droop method, aiming to analyze the stability by performing eigenvalues and sensitivity analysis which allows obtaining the behavior of the system, analyze the interaction between the different elements and study the influence of the droop parameters on this later which helps in the design procedure, small-signal model and Simulink block model was developed and simulated. Simulation results show a high correspondence and agreement between the model developed using Matlab Simulink-SimPowerSystem library and the developed small-signal model which confirms the validity of this later.

A flexible power management strategy for PV-battery based interconnected DC microgrid

Abstract In this paper, a coordinated power-sharing strategy for interconnected DC-microgrid (DC-MG) is proposed. The DC-MG consists of two subgrids with an interlinking bidirectional DC/DC converter (IBDDC). Each subgrid has a secondary-1 controller based on a state of charge (SoC) balancing based droop control strategy of the battery unit (BU). The proposed droop strategy regulates the DC bus voltage according to the SoC of BU. With the SoC balancing based droop method, BU with higher SoC supplies more power to the microgrid (MG) as compared to low SoC BU. The SoC information of batteries in all subgrids is communicated through low bandwidth communication (LBC). In case of failure of LBC, a secondary-2 controller is implemented for the battery controller to regulate the DC bus voltage considering the SoC of BU. Secondary-2 does not depend on the communication line. Considering the levels of DC bus voltages, a secondary power regulating controller is introduced for IBDDC. Further, a coordinated power control strategy is proposed for distributed generation to avoid overcharging of batteries. The whole system operates in a distributed way without a central controller. The proposed strategy has been verified in MATLAB/Simulink.

Reduced-Order Modeling and Comparative Dynamic Analysis of DC Voltage Control in DC Microgrids Under Different Droop Methods

Droop control has been well adopted for multi-source multi-load DC microgrids (MGs) due to its inherent modularity and reliability by only using local measurements, especially with the most commonly-used voltage-current droop ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</i> ) mode. While another implementation named current-voltage droop ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i> ) mode also can be adopted. With the same value of droop gain in these two modes, a DC MG will have the same steady-state feature, but its dynamic performance of DC voltage control may be different. Our primary motivation is to investigate this phenomenon from the perspective of equivalent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> circuits. This paper proposes a generic reduced-order modeling method suitable for exploring the dynamic stability of DC voltage control with these two modes. By ignoring fast inner current control dynamic, each droop based DC voltage control unit can be modeled as a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> parallel circuit in these two modes, which is convenient for modular modeling and extension. With these models, the essential cause of system dynamic stability difference and the physical meaning of key control parameters in these two modes can be revealed in an intuitive way. In addition, if the inner current control with slow dynamic cannot be ignored in some specific scenarios, a modified <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> model can be still obtained to analyze its influence. Moreover, based on reduced-order models, analytical solutions of dynamic performance indexes have been obtained, through which the impact of control parameters on dynamic performance of DC bus voltage can be characterized. Finally, the effectiveness of the proposed reduced-order modeling method has been verified by detailed simulation and experiment results.

Load frequency control and dynamic response improvement using energy storage and modeling of uncertainty in renewable distributed generators

Nowadays to improve the dynamic performance of the power system and control the frequency stability, load frequency control (LFC) based on droop strategy must be provided. PID controller is also used in addition to the droop method to maintain the stable load frequency. In the new power systems such as microgrids (MGs) with different low and high-inertial distributed generators (DGs), using only LFC method is not sufficient to provide the stable load frequency and balanced power sharing. Thus, modified LFC approach is needed. Energy storage element is a precious solution presented to combat the non-desirable transient conditions on load frequency and power sharing. Among different storage elements, superconducting magnetic energy storage (SMES) is selected in this paper because of fast dynamic response and desirable inertial characteristic. Using accurate modeling of SMES, the reserved power in off-peak times can be exploited in on-peak times to inject the required power in sudden load changes to provide stable frequency and balanced power sharing. Moreover, the uncertainty of renewable DGs such as photovoltaic (PV) and wind turbine (WT) can be assumed as disturbance input to the MG. Applying SMES in the MG, accurate modeling of SMES dynamic behavior in control block and modeling of DGs uncertainties with suitable probability functions in the proposed control structure leads to more stable LFC approach shown in simulation results of this paper. Inertial property improvement by SMES is also analyzed while comparing with other LFC methods, proposed modified LFC approach presents more stable load frequency. In addition, the frequency fluctuation in the proposed control structure is reduced 15–20% in average compared to other methods.

A Two-Layer Control Scheme Based on P – V Droop Characteristic for Accurate Power Sharing and Voltage Regulation in DC Microgrids

The increasing penetration of dc distributed energy resources (DERs) and dc loads has motivated the utilization of dc microgrids (MGs). In this paper, a new two-level control scheme is proposed for accurate power sharing and appropriate voltage regulation in dc MGs during islanded operation mode. In the primary control level, a P-· V droop method is proposed to eliminate the dependency of power sharing among DERs on line resistances. Since the P-· V droop deviates the voltage derivative to a nonzero value, a voltage derivative restoration mechanism is adopted in the secondary control level to provide voltage stability. The secondary control level also compensates the voltage deviations by cascading an outer voltage control loop with the inner voltage derivative restoration loop. The secondary control signal is broadcasted to each DER via a unidirectional low-bandwidth communication link. Small signal stability of the proposed scheme is analyzed by studying the locus of the system eigenvalues. To verify the efficacy of the proposed method and compare it with the conventional scheme, real-time simulations in OPAL-RT lab setup are provided.

Droop Method Development for Microgrids Control Considering Higher Order Sliding Mode Control Approach and Feeder Impedance Variation

Due to the growing power demands in microgrids (MGs), the necessity for parallel production achieved from distributed generations (DGs) to supply the load required by customers has been increased. Since the DGs have to procure the demand in parallel mode, they are faced with several technical and economic challenges, such as preventing DGs overloading and not losing network stability considering feeder impedance variation. This paper presents a method that upgrades the droop controller based on sliding mode approach, so that DGs are able to prepare a suitable reactive power sharing without error even in more complex MGs. In the proposed strategy, the third-order sliding mode controller significantly reduces the V-Q error and increases the accuracy in adjusting the voltage at the DG output terminals. Various case studies conducted out in this paper validate the truthfulness of the proposed method, considering the stability analysis using Lyapunov function. Finally, by comparing the control parameters of the proposed technique with existing methods, the superiority, simplicity and effectiveness of the 3rd order sliding mode control (SMC) method are determined.

Comparison between alternative droop control strategy, modified droop method and control algorithm technique for parallel-connected converters

Most of the active current sharing methods are based on a communication network. The communication link is also used with the improved droop control methods to achieve a precise load current sharing and regulate the voltage at the common DC bus. Conversely, the conventional droop method that is considered a decentralized method becomes more attractive for controlling parallel-connected converters in DC microgrids. The conventional droop methods' main drawbacks are associated with the unequal load current sharing and voltage deviation at the common DC bus. In this paper, the modified droop method as a conventional droop method is augmented with a virtual droop and adaptive voltage control gains to improve the load current sharing and the voltage regulation, respectively. In contrast with other improved droop approaches, the control approach proposed in the paper does not require a communication link to exchange information between parallel modules. Instead, it uses the converters' theoretical load regulation characteristics to estimate the voltage set point for each converter locally. The proposed virtual resistive gain manipulates the modified droop method to regulate each module's droop gain, which ensures equal current sharing. The proposed method also eliminates the tradeoff between current sharing difference and voltage regulation by implementing the adaptive voltage control, which compares the estimated voltage at the point of common coupling with the rated bus value and adjusts the droop gains based on the compared values to ensure a constant voltage at various load conditions. The load current sharing and voltage restoration improvements of the proposed method versus the modified droop method and the control algorithm technique are observed in this paper. The proposed method's effectiveness is demonstrated by MATLAB/Simulink simulation and validated by an experimental prototype.

Neurogenic induction of human dental pulp derived stem cells by hanging drop technique, basic fibroblast growth factor, and SHH factors

The progressive destruction of nerve cells in nervous system will induce neurodegenerative diseases. Recently, cell-based therapies have attracted the attention of researchers in the treatment of these abnormal conditions. Thus, the aim of this study was to provide a simple and efficient way to differentiate human dental pulp stem cells into neural cell-like to achieve a homogeneous population of these cells for transplantation in neurodegenerative diseases. In this basic research, human dental pulp stem cells were isolated and characterized by immunocytochemistry and flow cytometry techniques. In the following, the cells were cultured using hanging drop as three-dimensional (3D) and tissue culture plate as 2D techniques. Subsequently, cultured cells were differentiated into neuron cell-like in the presence of FGF and Sonic hedgehog (SHH) factors. Finally, the percentage of cells expressing Neu N and β tubulin III markers was determined using immunocytochemistry technique. Finally, all data were analyzed using the SPSS software. Flow cytometry and immunocytochemistry results indicated that human dental pulp-derived stem cells were CD90, CD106-positive, but were negative for CD34, CD45 markers (P ≤ 0.001). In addition, the mean percentage of β tubulin positive cells in different groups did not differ significantly from each other (P ≥ 0.05). Nevertheless, the mean percentage of Neu N-positive cells was significantly higher in differentiated cells with embryoid bodies' source, especially in the presence of SHH than other groups (P ≤ 0.05). It is concluded that due to the wide range of SHH functions and the facilitation of intercellular connections in the hanging droop method, it is recommended that the use of hanging drop method and SHH factor can be effective in increasing the efficiency of cell differentiation.

A Decentralized Reliability-Enhanced Power Sharing Strategy for PV-Based Microgrids

Microgrid (MG) technologies facilitate reliable, efficient, and economic operation of distributed resources such as photovoltaic and battery storage systems. The well-known droop method controls different sources in an MG to properly share power supply. However, utilizing the droop method poses two major challenges. First, while the droop method can prevent converter overloading, it cannot protect them from overstressing, thus deteriorating system reliability. Second, operating a 100% renewable-based MG requires a supervisory unit to monitor and control energy flow for load-generation balance. However, the supervisory unit relies on communication systems which impacts overall system reliability by being exposed to single-point failures and cyberattacks. This article proposes a decentralized power sharing approach that restricts thermal damage of converter components to avoid over-stressing converters. The main goal is to improve overall system performance and reliability by appropriately sharing active and reactive power among different sources without using communication systems. The simulations and numerical analysis show that the proposed decentralized strategy will properly control the power and energy flow among different sources. Moreover, it prevents over-stressing converters, consequently enhancing the overall reliability of the MG. An experiment is also presented to demonstrate the effectiveness of the proposed decentralized control approach.

Power sharing for transmission systems with 100% inverter‐based generating resources

This study proposes controls and power sharing design and architecture for a 100% inverter-based transmission system. Such an operation scenario has already occurred for short periods in portions of several systems in the United States, Europe, and Australia, and is likely to be more frequent in the future. The proposed algorithm enables the inverter-based resources (IBR) to participate in power sharing based on an angle droop method that explicitly takes into account the IBR ratings and preferred set points. This strategy results in an essentially constant-frequency operation of the power system without relying on secondary controllers or communication for frequency restoration. The performance of the proposed architecture under different operating conditions is evaluated via extensive simulation case studies in PSCAD/EMTDC software.

Coordinated Power Sharing in Islanding Microgrids for Parallel Distributed Generations

Optimal power sharing between parallel inverters and the demand load in microgrids is challenging and particularly critical for power grids in islanding operation. This paper introduces a novel control approach for managing parallel distributed power sources in the presence of variable load in islanding regime. The proposed scheme is based on the modified sliding mode control (MSMC) which is combined with the optimal Riccati control method to achieve convergence at the slip level with higher accuracy. The mathematical principles of the network equations are derived and its stability is obtained using the Lyapunov function. The MSMC simulation results are discussed in relation to the conventional droop method, while the laboratory evaluation was carried out to characterize its dynamic and static response. The results show that the proposed scheme control is able to manage the distributed power generation for static and dynamic load scenarios, and as such, guarantying microgrid frequency stability.

Decentralised coordinated energy management for hybrid AC/DC microgrid by using fuzzy control strategy

The islanded hybrid AC/DC microgrid consisting of battery energy storage (BES) systems, photovoltaic (PV) generators, and bidirectional power converter (BPC) possesses the advantages of flexibility and extendibility. To ensure the safety and stability of operation, the power of BES and BPC needs to be restrained within their designed limits regardless of the power fluctuation in the hybrid microgrid. Besides, the over-charge and over-discharge of BES should be prohibited. In this study, a decentralised generation-storage-subgrid coordination control for power management is proposed to assure the power limitation and state of charge (SOC) protection. In the control strategy of BES, a modified droop method is adopted to deliver the storage's both SOC and output power signals without communication links. Meanwhile, a fuzzy logic controller is adopted in BPC, which can prevent the overuse of BESs in both subgrids and limit the maximum output power of BPC. In addition, a modified P&O strategy is applied in PV systems as a supplement of overuse protection. Finally, the effectiveness of the proposed control strategy is verified on RT-Lab experimental platform.

Enhanced Frequency Droop Method for Decentralized Power Sharing Control in dc Microgrids

This article proposes two novel approaches to improve the superimposed frequency droop scheme for the control of dc microgrids (MGs). Conventional voltage-based control strategies suffer from issues such as undesirable voltage regulations, poor power sharing among the sources, and negative effects of line resistances on the equivalent droop characteristics. To overcome these challenges, a superimposed frequency droop scheme has been introduced. However, this method suffers from three major issues: 1) instability in terms of load variation, which is due to the location of system dominant poles; 2) limitation in system loading due to the limitation in the transferred reactive power; and 3) poor voltage quality caused by injection of the ac voltage. In this article, two methods are presented to stabilize the system and enhance its loading condition, consequently improving its viability for control of the dc MG. Furthermore, the system voltage quality is improved by limiting the amplitude of the injected ac voltage. The effectiveness of the proposed schemes is shown by different simulations and is further validated by experiments.

Droop‐free hierarchical control strategy for inverter‐based AC microgrids

Hierarchical schemes are widely used for the designing of the inverter-based AC microgrids control strategies. To ensure reliable operation, hierarchical control must consider together all the functionalities that allow the regulation of key variables and guarantee a safe transition between operation modes. Conventionally, in the literature are proposed three-layer schemes which present relevant drawbacks: they include limited functionalities and they use droop method for the primary layer which, despite its decentralised nature, suffers from issues that have motivated the development of alternative strategies. Considering this, the contribution of this study is two-fold. First, a droop-free hierarchical control strategy that satisfies a proper operation of AC microgrids is proposed. Control objectives such as power-sharing, frequency regulation, optimal power dispatch and voltage regulation are considered. Second, a closed-loop small-signal model, which facilitates the control parameters design and fills a gap in the literature is presented. Differences between the proposal and previous controls are discussed. Selected tests are carried out in a laboratory microgrid under different conditions, including normal operation and the response to failures in the central controller and to communication impairments. The experimental results show a good performance of the proposal even in adverse conditions.

Fault Current Control and Protection in a Standalone DC Microgrid Using Adaptive Droop and Current Derivative

This article presents a novel fault detection, characterization, and fault current control algorithm for a standalone solar-photovoltaic (PV) based dc microgrids. The protection scheme is based on the current derivative algorithm. The overcurrent and current directional/differential comparison based protection schemes are incorporated for the dc microgrid fault characterization. For a low impedance fault, the fault current is controlled based on the current/voltage thresholds and current direction. Generally, the droop method is used to control the power-sharing between the converters by controlling the reference voltage. In this article, an adaptive droop scheme is also proposed to control the fault current by calculating a virtual resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> droop</sub> , and to control the converter output reference voltage. For a high impedance fault, differential comparison method is used to characterize the fault. These algorithms effectively control the converter pulsewidth and reduce the flow of source current from a particular converter, which helps to increase the fault clearing time. Additionally, a trip signal is sent to the corresponding dc circuit breaker (DCCB), to isolate the faulted converter, feeder or a dc bus. The dc microgrid protection design procedure is detailed, and the performance of the proposed method is verified by simulation analysis.

Power Control in an Islanded Microgrid Using Virtual Impedance

Abstract The common droop method is one of the most effective methods for controlling power-sharing in islanded microgrid. Due to the dependence of the active and reactive power on the voltage and frequency in resistor microgrid, the use of this method in controlling the power-sharing in these types of a microgrid to the undesirable performance of active and reactive power-sharing. In this paper, by applying virtual impedance in the local controller of distributed generation resources, this paper has modified the common droop method and improved power-sharing. The proposed virtual impedance has an adjustable resistor section and a fixed reactance section. Simulation of a typical islanded microgrid in PSCAD / EMTDC software shows that the proposed method is capable of improving the active and reactive power sharing in resistive microgrid controlled by the droop method.

Mode-triggered droop method for the decentralized energy management of an islanded hybrid PV/hydrogen/battery DC microgrid

In a hydrogen-based DC microgrid (MG), the integration of hydrogen subsystem increases the system complexity and flexibility. Effective power split, bus voltage stability and reliable operation become significant control issues of hydrogen-based DC MG. This paper proposes a simple and effective decentralized energy management strategy based on a mode-triggered droop scheme for an islanded PV/hydrogen/battery DC MG. In a decentralized manner, the proposed decentralized energy management strategy is implemented through two control steps: mode divisions and droop control. For practical implementation purposes of the proposed energy management strategy, the mode divisions scheme autonomously divides this DC MG into eight operating modes based on the system local information; the adaptive droop control method controls the distributed generation units based on their droop relationships under different operating modes. In addition, an islanded DC MG hardware-in-the-loop (HIL) RT-LAB simulation platform and a lab-scaled experimental platform are built to validate the effectiveness of the proposed decentralized strategy. The simulation and experiment results show that the proposed energy management strategy has an efficient power distribution ability without communication link under various operating modes.

An Intelligent Type-2 Fuzzy Stabilization of Multi-DC Nano Power Grids

This paper proposes a robust and intelligent droop control method to stabilize a cluster of DC nanogrids (DCNGs) with constant power loads (CPLs). Conventional droop methods suffer from voltage regulation in parallel converters due to the CPLs destructive effects, which cause instability in the DCNGs. To address the shortcomings of the conventional methodologies, this paper proposes a new droop-based single input interval type-2 fuzzy logic controller (SI-IT2-FLC), which enhances the transient response and improve the steady-state performance. The suggested method aims to boost the stability of the paralleled DCNGs in a cluster under the effect of the CPLs, which is inevitable in the practical applications. The efficacy of the suggested scheme is validated in different operating conditions. Moreover, real-time model-in-the-loop (MiL) results show the performance improvements in the transient and steady-state obtained by the novel suggested control approach.