Droop Control Method Research Articles

Investigations of Decoupled Trigonometric Saturated and Fuzzy Logic Techniques for the Automatic Frequency Control of Islanded Microgrid

This paper involves the investigation of new techniques for the automatic load frequency control of islanded Microgrids. Microgrids are being established as a part of smart grid environment. In modern power systems, smart grid represents the solution for many of traditional power system problems such as frequency fluctuations. Frequency fluctuations have negative consequences in terms of electrical equipment life, production cost and production losses relative to costumers and electricity producers. So, the frequency of power system must be kept in acceptable range. In order to operate a power system with fixed frequency, it is necessary to always maintain a balance between the generation and the consumption of active power. The frequency droop control methods are widely used to control active power and frequency of the parallel synchronous generators in the traditional power grid. At present, this method has also been applied to the control of parallel inverters to share the load demand in proportion to their ratings. In order to improve the frequency control of traditional droop control technique applied in islanded microgrid, It is proposed to investigate the Decoupled Trigonometric Saturated (DTS) and fuzzy droop control techniques on islanded microgrids which uses meshed parallel inverter systems. To verify the performance of the proposed ALFC based on Decupled trigonometric saturated controller, fuzzy PI controller the MATLAB/SIMULINK environment is used.

Investigations of Decoupled Trigonometric Saturated and Fuzzy Logic Techniques for the Automatic Frequency Control of Islanded Microgrid

This paper involves the investigation of new techniques for the automatic load frequency control of islanded Microgrids. Microgrids are being established as a part of smart grid environment. In modern power systems, smart grid represents the solution for many of traditional power system problems such as frequency fluctuations. Frequency fluctuations have negative consequences in terms of electrical equipment life, production cost and production losses relative to costumers and electricity producers. So, the frequency of power system must be kept in acceptable range. In order to operate a power system with fixed frequency, it is necessary to always maintain a balance between the generation and the consumption of active power. The frequency droop control methods are widely used to control active power and frequency of the parallel synchronous generators in the traditional power grid. At present, this method has also been applied to the control of parallel inverters to share the load demand in proportion to their ratings. In order to improve the frequency control of traditional droop control technique applied in islanded microgrid, It is proposed to investigate the Decoupled Trigonometric Saturated (DTS) and fuzzy droop control techniques on islanded microgrids which uses meshed parallel inverter systems. To verify the performance of the proposed ALFC based on Decupled trigonometric saturated controller, fuzzy PI controller the MATLAB/SIMULINK environment is used.

Voltage-Based Droop Control of Electric Vehicles in Distribution Grids under Different Charging Power Levels

If left uncontrolled, electric vehicle charging poses severe challenges to distribution grid operation. Resulting issues are expected to be mitigated by charging control. In particular, voltage-based charging control, by relying only on the local measurements of voltage at the point of connection, provides an autonomous communication-free solution. The controller, attached to the charging equipment, compares the measured voltage to a reference voltage and adapts the charging power using a droop control characteristic. We present a systematic study of the voltage-based droop control method for electric vehicles to establish the usability of the method for all the currently available residential electric vehicle charging possibilities considering a wide range of electric vehicle penetrations. Voltage limits are evaluated according to the international standard EN50160, using long-term load flow simulations based on a real distribution grid topology and real load profiles. The results achieved show that the voltage-based droop controller is able to mitigate the under voltage problems completely in distribution grids in cases either deploying low charging power levels or exhibiting low penetration rates. For high charging rates and high penetrations, the control mechanism improves the overall voltage profile, but it does not remedy the under voltage problems completely. The evaluation also shows the controller’s ability to reduce the peak power at the transformer and indicates the impact it has on users due to the reduction in the average charging rates. The outcomes of the paper provide the distribution grid operators an insight on the voltage-based droop control mechanism for the future grid planning and investments.

Primary frequency support from local control of large-scale heat pumps

Increasing penetration of volatile renewable sources in the electric power system increases the need of new sources of balancing capacity. As a power-to-x technology, heat pumps (HPs) couple the electricity and heating systems, while offering a potential source of regulating capacity. This paper presents new findings on primary frequency support from large-scale HPs through local droop and synthetic inertia control methods with the objective of improving the system frequency control performance following a disturbance. The complexity and operation of large-scale HPs are different from the common model representation of HPs as thermostatically controllable load, which makes the existing studies and HP frequency support methods inaccurate. This work applies a state space model representation of the large-scale HPs, which enables a performance evaluation of existing local frequency control methods. Simulation results show the improved grid frequency control performance following a disturbance supported by large-scale HPs. Simultaneously, the heat and power response of large-scale HPs with frequency control support is compared to that of small-scale HPs.

Imbalance‐based primary frequency control for converter‐fed microgrid

Frequency droop control is a simple and effective frequency control method. However, it is not appropriate as a primary frequency control for microgrids with energy storage systems and voltage source converters (VSCs) owing to their slow response speed. In addition, it is difficult to determine the grid balance only with frequency deviation. In this paper, a new converter-fed microgrid primary frequency control method based on the estimated active power imbalance for fast response and less variability in the transmission system is proposed. The active power imbalance is estimated via frequency deviation, frequency drop speed and the modified swing equation. The estimated imbalance indicates the status of the power imbalance more quickly and accurately than the frequency deviation. The proposed method does not require communication devices, responds quickly to frequency drops, and prevents unnecessary operation of the grid-connected VSC because it uses the estimated imbalance instead of the frequency or angle for droop control. Simulation results show that the proposed method is significantly faster than frequency droop control methods, improves the rate of change of frequency and frequency nadir and reduces variability in the transmission system.

Distributed secondary consensus fault tolerant control method for voltage and frequency restoration and power sharing control in multi-agent microgrid

This paper proposes a distributed secondary consensus fault-tolerant control (FTC) method for the multi-agent microgrid (MG). The proposed controller is applied to compensate for the errors in the system frequency and voltage profiles in the presence of MG faults. The proposed controller also enables the system to ensure an accurate sharing of the active and reactive power among the connected distributed generations (DGs) in MG. The consensus-based controller depends on the information transferring between MG neighbor agents through a graph communication network. The proposed secondary controller improves the primary controllers' performance, including droop control and inner voltage and current controllers. The proposed controller has been verified using a hypothetical multi-agent MG system in MATLAB/Simulink environment. A comparative analysis between the results obtained from applying the proposed controller and the primary droop control method (DCM). The proposed controller's performance is evaluated using the controller response time, the maximum frequency deviation, and the steady-state restoration time. The results show the proposed secondary FTC method's effectiveness in mitigating the faults' effect in multi-agent MG.

A Finite-Time Robust Distributed Cooperative Secondary Control Protocol for Droop-Based Islanded AC Microgrids

In this research work, a resilient finite-time consensus-based distributed secondary control protocol is presented for droop-based distributed generating (DG) units of an islanded AC microgrid (MG). Through a multi-agent control structure, the DG units of the microgrid adjust their active power outputs so that they reach an agreed-upon value in a finite time. Concurrently, all the DG units are forced to operate with their frequencies regulated to the reference MG frequency in a finite time, despite time-varying load perturbations. Each DG unit is provided with a hierarchical control architecture, where the primary control is achieved using the droop control method, while the secondary control is established through the proposed distributed control protocol. The communication between DG units takes place over a sparse communication network. The proposed control protocol is robust to both small and sufficiently large communication latencies and it supports the plug-and-play feature of DG units. Different time-domain-based numerical simulations are carried out on a small as well as large microgrid testbenches in Matlab/Simulink and demonstrate the correctness and effectiveness of the proposed distributed control protocol. A comparative study is also presented with the existing distributed control protocol, and it is found that the proposed strategy is superior in its performance.

Solution to Fault of Multi-Terminal DC Transmission Systems Based on High Temperature Superconducting DC Cables

In this paper, we developed the small-signal state-space (SS) model of hybrid multi-terminal high-voltage direct-current (HVDC) systems and fault localization method in a failure situation. The multi-terminal HVDC (MTDC) system is composed of two wind farm side voltage-source converters (VSCs) and two grid side line-commutated converters (LCCs). To utilize relative advantages of the conventional line-commutated converter (LCC) and the voltage source converter (VSC) technologies, hybrid multi-terminal high-voltage direct-current (MTDC) technologies have been highlighted in recent years. For the models, grid side LCCs adopt distinct two control methods: master–slave control mode and voltage droop control mode. By utilizing root-locus analysis of the SS models for the hybrid MTDC system, we compare stability and responses of the target system according to control method. Furthermore, the proposed SS models are utilized in time-domain simulation to illustrate difference between master–slave control method and voltage droop control method. However, basic modeling method for hybrid MTDC system considering superconducting DC cables has not been proposed. In addition, when a failure occurs in MTDC system, conventional fault localization method cannot detect the fault location because the MTDC system is a complex form including a branch point. For coping with a failure situation, we propose a fault localization method for MTDC system including branch points. We model the MTDC system based on the actual experimental results and simulate a variety of failure scenarios. We propose the fault localization topology on a branch cable system using reflectometry method. Through the simulation results, we verify the performance of fault localization. In conclusion, guidelines to select control method in implementing hybrid MTDC systems for integrating offshore wind farms and to cope with failure method are provided in this paper.

Robust Power Sharing and Voltage Stabilization Control Structure via Sliding-Mode Technique in Islanded Micro-Grid

With a focus on the problems of active power sharing and voltage deviation of parallel-connected inverters in an islanded micro-grid (MG), in this study, the power-voltage droop controller and the inner voltage regulator are redesigned based on a total sliding-mode control (TSMC) technique. As for the power-voltage droop control loop, a droop control relation error between the active power and the point-of-common-coupling (PCC) voltage amplitude is defined. Then, the TSMC scheme is adopted to reach the new droop control relation, where the active power sharing and voltage amplitude recovery can be achieved simultaneously. Owing to the faster dynamic response and strong robustness provided by the TSMC framework, high-precision active power sharing during transient state also can be ensured without the influence of line impedances. The power allocation error can be improved by more than 81.2% and 50% than the conventional and proportional-integral (PI)-based droop control methods, respectively, and the voltage deviation rate can be reduced to 0.16%. Moreover, a small-signal model of the TSMC-based droop-controlled system is established, and the influences of control parameters on the system stability and the dynamic response are also investigated. The effectiveness of the proposed control method is verified by numerical simulations and experimental results.

Research on Dual Mode Unified Control Strategy of Grid Connected Inverter Based on Droop Control

There is a big difference between the two modes of grid connected inverter in micro-grid operation. In order to avoid large voltage and current impact, advanced control strategy is needed to realize smooth conversion during operation mode switching. A droop control method based on voltage amplitude change rate is proposed, which is suitable for dual-mode unified control of inverter. While ensuring the stability of the system, the reactive power of the inverter can be accurately shared during islanding operation. This method can realize smooth switching between grid connected inverter and islanding mode. The feasibility of the method is verified by simulation and experiment.

A Decentralized Control Strategy Based on V-I Droop for Enhancing Dynamics of Autonomous Hybrid AC/DC Microgrids

This article presents a GPS-based decentralized control strategy for hybrid ac/dc microgrids. The proposed method uses V-I droop characteristics for the distributed energy resources (DERs) in each of ac and dc subgrids. In particular, the output voltage of each DER is adjusted as a piecewise linear function of the per-unit (pu) output current. In addition, a droop control method is proposed for the interlink converter (IC) to attain global power sharing among the dc and ac DERs. The IC controller changes the deviation of the ac bus voltage as a function of the deviation of the dc bus voltage. This way, the power flow between ac and dc subgrids is adjusted such that the ac and dc DERs experience the same pu voltage deviation and hence deliver the same pu current and active power. In contrast with the existing control approaches, the proposed method does not require frequency or power calculations but directly uses voltage and current measurements in the control loops. So the delay associated with the feedback signals is minimized, which avails accurate load sharing during transients. Experimental results show the proposed method eliminates the transient overshoots of DERs' output powers by offering a fast and overdamped dynamic response.

An Islanded Microgrid Droop Control using Henry Gas Solubility Optimization

Coordination of various distributed generation (DG) units is required to meet the growing demand for electricity. Several control strategies have been developed to operate parallel-connected inverters for microgrid load sharing. Among these techniques, due to the lack of essential communication links between parallel-connected inverters to coordinate the DG units within a microgrid, the droop control method has been generally accepted in the scientific community. This paper discusses the microgrid droop controller during islanding using the Henry Gas Solubility Optimization (HGSO). The most important goals of droop control in the islanded mode of operation are the frequency and voltage control of microgrid and proper power sharing between distributed generations. The droop controller has been designed using HGSO to optimally choose PI gains and droop control coefficients in order to obtain a better microgrid output response during islanding. Simulation results indicate that the droop controller using HGSO improves the efficiency of micro-grid power by ensuring that variance in microgrid frequency and voltage regulation and effective power sharing occurs whenever micro-grid island mode or when variation in load occurs.

Power sharing through interlinking converters in adaptive droop controlled multiple microgrid system

Multiple microgrids (MMGs) may be interconnected through voltage source converters (VSC) for controlled power sharing among themselves. VSCs in large interconnected micro- grids may use non-communication based droop control to share total power demand in the ratio of their droop coefficients. However, a constant droop ratio restricts the power delivery capability of the VSCs although surplus power may be available in their respective sources. In this paper, along with a unified power management strategy for intra and inter microgrid power sharing in a hybrid AC-DC MMG system, a novel adaptive droop control algorithm is presented that dynamically changes droop coefficients of the VSCs to ensure maximum utilization of available resources in each microgrid under different operating modes. Stability analysis of the proposed control method is performed and comparison of the system performance with fixed and adaptive droop ratio is illustrated by simulating the MMG in MATLAB SIMULINK using real solar irradiance field data. It is validated from simulation results that the adaptive droop control method increases utilization of renewable energy sources within microgrids and reduces energy import from utility grid.

A Comprehensive Review on Control Techniques for Power Management of Isolated DC Microgrid System Operation

The present research investigations in power management fraternity are focused towards the realization of smart microgrid (MG) technologies. Due to intrinsic advantages of Direct Current (DC) system in terms of compatibility with power generation sources, modern loads and storage devices, DC MG has becoming popular over Alternate Current (AC) system. A secondary voltage and current control schemes of DC MG system are mainly based on the distributed consensus control of Multi-agent system (MAS) to balance generation and the demand. The basic concern of the cooperative control of MAS is consensus, which is to design a suitable control law such that the output of all agents can achieve synchronization. The distributed consensus algorithm requires much less computational power and information exchange in between the neighbor’s agent. Meanwhile the other controllers such as model predictive control (MPC) requires accurate dynamic models with high computational cost and it suffers from lack of flexibility. The hierarchical consensus control technique is classified into three levels according to their features, namely primary, secondary, and tertiary. MAS is a popular distributed platform to efficiently manage the secondary control level for synchronization and communication among the power converters in autonomous MGs. In this article, various primary control techniques for local voltage control, voltage restoration in the secondary control level and tertiary control for energy management techniques are discussed. With this, the key emphasis to reduce the voltage deviation and disturbances in a heterogeneous DC MG network solutions are discussed. Furthermore, to analyze the system response and the charging and discharging characteristics of the battery unit, the developed second order heterogeneous consensus controller is compared with the traditional homogeneous consensus control and droop control methods. Finally a detailed discussion on simulation case study using heterogeneous consensus control method over the traditional methods are provided using MATLAB/Simulink platform.

A novel droop control method to achieve maximum power output of photovoltaic for parallel inverter system

In general, the output power of PV inverter should be coordinated with the load. In the initial stage of the design, when the PV inverter operating in islanded mode, the load power is supposed to be no more than the maximum PV output power with the purpose to stabilize the system. However, it may waste PV cells' energy. Consequently, more and more solar panels are linked to other energy sources in order to create a parallel inverter system, which makes it feasible to fit the power output of each source. The power distribution of parallel inverter is achieved by the use of droop control in microgrid system. For such a parallel inverter system consisted of PV inverters and non-regeneration energy sources inverters without energy storage devices in islanded mode, if the shared load power is no more than the available maximum PV inverter output power, there is a power waste for PV inverter; Moreover, due to the intermittency of PV sources, the system may become unstable if the shared load power is more than the available maximum output power of PV inverter. However, the maximum utilization of the PV output energy cannot be achieved automatically with the traditional methods to add virtual impedance or adjust the droop coefficients. Thus, in order to avoid power waste and potential instability caused by insufficient PV power by traditional droop control, this paper recommends an improved droop control scheme to maximize the power output of PV units. Required by the load, the remaining power is composed of the other inverters, which could effectively improve the utilization rating of renewable energy sources and system stability. At the same time, according to the system stability analysis based on the small signal modeling, it has been designed about the droop coefficients of the improved droop control loop. In the end, the experimental results have showed that the suggested scheme has a varied validity and robustness.

Study and modelling of droop-controlled islanded mesh microgrids

The active and reactive power sharing of distributed generation sources (DGs) connected to isolated microgrids with a single point of common coupling (mono-PCC) to which the loads are also connected has already been the subject of several studies. A high penetration rate of DGs based on renewable energies has as a logical consequence the development and implementation of mesh and more complex multi- PCC microgrids. In this paper, a developed droop control method for synchronization and power sharing between different DGs connected to a mesh islanded multi-PCC microgrid with many distributed generation sources (DGs) and different type of loads (including active load (CPL)) randomly connected to different PCCs is applied. Then, a state model of the entire mesh microgrid is developed integrating the generators with their controllers, power lines, droop algorithms and dynamic loads. This model is then used to study the asymptotic stability and robustness properties of the system. The simulation results confirm the effectiveness of the applied strategies for the synchronization of the different DGs to the microgrid while ensuring an efficient active and reactive power sharing. also, they confirm the validity of the developed state space model of the system.

Droop control method based on fuzzy adaptive virtual resistance for DC microgrids

DC microgrid is a high-efficiency and high-reliability solution for integrating energy sources, energy storage units and loads. Droop control method based on the virtual resistance is the most popular control method for DC micro grids (MGs). However, this method has two limitations: 1) unequal load sharing in case of unequal line resistances of converters; 2) DC bus voltage deviation with increasing the load. In this paper, in order to solve these limitations, a new control method based on fuzzy controller is proposed to determine instantaneous virtual resistance considering current sharing error and DC bus voltage deviation. In the proposed method, a low bandwidth communication network is used for current sharing of converters and DC bus voltage regulation. The performance of the proposed method has been investigated for difference line resistances and load changes. The simulation results show that the proposed control method leads to increasing load sharing accuracy and DC bus voltage restoration improvement.

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.

Droop control method based on fuzzy adaptive virtual resistance for DC microgrids

DC microgrid is a high-efficiency and high-reliability solution for integrating energy sources, energy storage units and loads. Droop control method based on the virtual resistance is the most popular control method for DC micro grids (MGs). However, this method has two limitations: 1) unequal load sharing in case of unequal line resistances of converters; 2) DC bus voltage deviation with increasing the load. In this paper, in order to solve these limitations, a new control method based on fuzzy controller is proposed to determine instantaneous virtual resistance considering current sharing error and DC bus voltage deviation. In the proposed method, a low bandwidth communication network is used for current sharing of converters and DC bus voltage regulation. The performance of the proposed method has been investigated for difference line resistances and load changes. The simulation results show that the proposed control method leads to increasing load sharing accuracy and DC bus voltage restoration improvement.

Analysis of the Droop Control Method in DC Microgrids with Regard to the Types of Load

The droop control approach divides load demand among voltage source converters in microgrids in a decentralized manner. Different factors, such as the microgrid line impedances, have an impact on the power sharing accuracy of the droop approach. This research evaluates the droop controller's power-sharing accuracy for the first time with respect to three different load types: constant impedance (CI), constant current (CC), and constant power (CP). Additionally, the impact of DC microgrid configuration on the accuracy of power sharing is investigated in this work. To support the conclusions, the simulation results in the Matlab/Simulink software environment are shown.