Droop Gain Research Articles

Variable droop gain frequency supporting control with maximum rotor kinetic energy utilization for wind-storage system

To address the emerging frequency stability issue brought by the large replacement of synchronous generators with renewable generations, wind turbine generators are required to possess frequency-supporting capability. However, existing frequency-supporting control strategies lack the assessment of the frequency support capability of wind turbine generators, leading to degraded control performance under various situations. Aiming to solve this problem, this paper proposes a variable-droop-gain control for wind turbine generators with maximum rotor kinetic energy utilization. Firstly, an analytical relationship was established between droop gain, disturbance scale, and rotor speed. Subsequently, the released energy of the wind turbine generator is evaluated, which equals the difference in the rotor kinetic energy under the initial and the post-disturbance steady-state rotor speed. It was proved that the released kinetic energy cannot exceed a certain proportion of total rotor kinetic energy. Accordingly, a variable initial gain scheme is proposed, which determines the initial droop gain as per the disturbance scale for maximizing the kinetic energy release of wind turbines. Moreover, an additional real-time droop gain adjustment rule is added to prevent the over-deceleration of wind turbines. The simulation results show that the proposed scheme may provide the maximum KE release and effectively improve the system frequency nadir while ensuring the safe operation of wind turbine generators.

Enhanced Dynamic Droop Control for Microgrid Frequency and Voltage Stabilization Using Hybrid Energy Storage Systems: A SECANT Method Approach

Due to their variable and intermittent nature, the integration of renewable energy sources poses control challenges related to voltage and frequency stability in isolated microgrids. This paper proposes an enhanced dynamic droop control strategy optimized in active time along with a Hybrid Energy Storage System (HESS) comprising Battery Energy Storage System (BESS), supercapacitors (SUPCA), and Superconducting Magnetic Energy Storage (SMES) to improve microgrid stability. The Dynamic Droop Gains (DDG) are continuously tuned using the rapid-converging SECANT numerical method to enhance transient response and steady-state performance, this was achieved using MATLAB/Simulink. The HESS combines the complementary characteristics of BESS, SUPCA and SMES to balance steady power supply and temporary overload capacity. Detailed simulation studies on a microgrid test system verify that the proposed control strategy significantly enhances voltage/frequency regulation, power sharing accuracy, BESS lifespan and overall stability compared to conventional droop techniques. The SUPCA further improves the transient performance and power quality by mitigating fluctuations. The research demonstrates an innovative way to harness advanced control algorithms and emerging storage technologies for next-generation resilient and sustainable microgrids.

A Low-Complexity Artificial Neural Network-Based Optimal Droop Gain Design Strategy for DC Microgrids Onboard the More Electric Aircraft

A Low-Complexity Artificial Neural Network-Based Optimal Droop Gain Design Strategy for DC Microgrids Onboard the More Electric Aircraft

Design of Battery Energy Storage System Torsional Damper for a Microgrid with Wind Generators Using Artificial Neural Network

Ancillary frequency controllers such as droop controllers are beneficial for frequency regulation of a microgrid with high penetration of wind generators. However, the use of such ancillary frequency controllers may cause torsional oscillation in the doubly fed induction generator (DFIG). In this paper, a supplementary torsional damper in a battery energy storage system (BESS) is designed to improve the damping ratio for the DFIG torsional mode. Since the optimal damper gain depends on system variables such as the number of diesel generators, the number of wind generators, and BESS droop gain, an artificial neural network (ANN) is trained using these system variables as inputs and the desired BESS damper gain as the output. After the ANN has been trained with the training patterns, it can provide the desired BESS damper gain in an accurate and efficient manner. The effectiveness of the proposed ANN approach for BESS damper design is demonstrated by MATLAB/SIMULINK R2022b simulations.

Decentralized Control for Multi-Terminal Cascaded Medium-Voltage Converters Considering Multiple Crossovers

Decentralized control with multiple droop characteristics can significantly improve the accuracy of power flow in medium-voltage direct-current (MVdc) networks. However, multiple crossovers caused by different control characteristics can lead to the drifts of power and voltage and instability issues. When this type of control is implemented in the cascaded three-level neutral-point-clamped (C3L-NPC) converters, on one hand, the mechanism of such the power and voltage drifts was not investigated. On the other hand, power control accuracy, dc voltage balancing across submodules (SMs) and multiple crossovers should all be considered, which requires suitable control methods. To address the challenges, firstly, the mechanism behind the power and dc voltage drifts is analyzed. Secondly, a control scheme is presented to improve the power control accuracy and dc voltage balancing and concurrently, to avoid the multiple crossovers. This is achieved by suitable droop gain design and adding a secondary power compensator. The presented control scheme is verified in MATLAB/Simulink simulation and experimentally validated in a three-terminal MVdc testbed. Results show that the accuracy of steady-state power flow is improved by 15% due to the elimination of multiple crossovers, while the power accuracy at dynamics improved by 13% with the secondary power compensator.

Design of Fuzzy Logic based Adaptive Active Power Controller to Enhance Power sharing among DGs in an Autonomous Microgrid

This paper presents the design of an adaptive active power controller to enhance the power-sharing capabilities of distributed generators (DGs) in an autonomous microgrid. Each DG in an autonomous microgrid consists of a droop-controlled inverter to control active and reactive power by regulating frequency and voltage correspondingly. The high droop gain can be used in the power controller to encourage faster power sharing among the DGs. However, high droop gain can cause undamped growing oscillations during load fluctuations or generation losses. During such events, attaining faster power-sharing between DGs using high droop gains is difficult. So, the problem with an autonomous microgrid is the conflict between faster power sharing and stability. Stability needs to be compromised to attain quicker power sharing and vice versa. Hence, to achieve power-sharing swiftly with high droop gain and to diminish the growing oscillations caused, this paper proposes a fuzzy logic-based adaptive active power controller (FLAPC). The proposed FLAPC is adaptive and easy to implement. It offers faster power sharing for different values of droop gains and step change in load. The FLAPC is developed in MATLAB 2018a/Simulink environment, and time domain simulations are performed to see the efficacy of the proposed controller. The results of time domain simulations are compared with a droop controller without any additional controller, conventional lead-lag power system stabilizer, and proposed controller for step change in load at different droop gains. The results show that the proposed controller enhances the power-sharing performance and also ameliorates the system’s stability by reducing the settling time and overshoot in active power responses of DGs.

Improving small-signal stability of inverter-based microgrids using fractional-order control

Augmenting the range of the active droop gain in inverter-based microgrids (IBMGs) is required for faster power sharing and improved control at the secondary level. However, IBMG stability is degraded when active droop gain is increased, which reveals an obvious tradeoff. Therefore, fractional-order control (FOC) is used in this paper to increase the inverter-based (IB) MGs’ stability margin by increasing the stable range of the active droop gain. So, a thorough small-signal model of IBMGs is developed to assess the MG stability and tune the control parameters for the case of using FOCs. Also, a fractional-order low-pass filter (FO-LPF) is proposed for the outer control loop which increases the damping factor and reduces the power oscillations at the same active droop gain. Moreover, a further boost in stability margin is achieved by combining the proposed FO-LPF in the power control loop with FO-PI controllers in the inner control loops. The stability results of the developed FO small-signal model are validated on a MATLAB/SIMULINK MG stability benchmark system. Also, a comparative case study has shown the superiority of the proposed approach over recent ones in the case of a reconfigured and meshed network. Moreover, the discretization scheme of FOCs is outlined and tested under a load disturbance event.

Adaptive Droop Gain Control for Optimal Kinetic Energy Extraction From Wind Turbines to Support System Frequency

Adaptive Droop Gain Control for Optimal Kinetic Energy Extraction From Wind Turbines to Support System Frequency

Disturbance size estimation in Great Britain power system including combined cycle gas turbine power stations

With the substantial popularity of combined cycle gas turbine (CCGT) power plants in the nowadays power systems, special care must be taken to regulate frequency due to unique frequency response characteristic of the full-loaded CCGT units. This unique feature is documented in the literature; however, its effect on determining frequency response of the power systems was not addressed in detail. This study proposes a new analytical method to achieve a more accurate estimated size of a loss-of-generation disturbance. This method considers demand-side power deviations and transmission lines power loss as well as unique frequency response of the CCGT units following the event. Firstly, it is exposed that there is an approximately linear relationship between power and frequency deviations of these plants in a real-world power system despite the complexity of the CCGT model. This relationship may be represented by a negative droop gain. Next, the derived CCGT’s linear characteristic is formulated in the disturbance size estimation process. Finally, the effectiveness of the proposed modifications is demonstrated through extensive simulations on a 36-zone Great Britain equivalent test system.

A novel load flow method for islanded microgrids with optimum droop coefficients

The integration of renewable energy resources into the power system has led to growing interest in the field of microgrids. These microgrids can operate in grid-connected or islanded modes.Load flow analysis is an important step for the operation and planning of the power system. The traditional load flow techniques, which use the concept of a reference bus, are not valid in microgrids, particularly when operating in islanded mode. Thus, in this study, a modified Gauss-Seidel method has been implemented in which to find the optimum values for droop coefficients, different optimisation techniques have been used. The value of droop gain impacts the load distribution among various DGs, as the fast action of droop is indicated by a higher value of droop gain. But the inappropriate value of droop gain affects the stability of the system, so typically the values are kept low due to steady state stability concerns. The optimum droop gain helps to keep the voltage and frequency within the permissible limits. The voltage profile has been improved with optimal gains for the droop coefficients, and about 4% loss reduction has been reported, proving the effectiveness of the proposed method. The method has been tested on 6-bus, 38-bus, and 40-bus islanded microgrid systems.

DC‐side stability analysis of grid‐tied converter with different control modes based on electrical torque analysis

AbstractThe DC‐side stability of the grid‐tied converter under different control modes is fully investigated using electrical torque analysis. The small‐signal model of a single converter connected to an ideal DC bus under various control modes is formulated. Accordingly, the damping and synchronising coefficient contributed by the DC network and controllers of grid‐tied converter are separately accessed using the electrical torque analysis method and the stabilising conditions of the grid‐tied converter operating under different control modes are further derived. The system stability mainly corresponds with DC network dynamics under constant active power control mode. On the contrary, the grid‐tied converter under constant DC‐link voltage control mode has no stability problem. Generally, elevating the DC‐link capacitance or decreasing the droop gain can greatly improve the stability margin reserve of the VSC‐HVDC links. In addition, the control gains of the classical PQ controller are proven to have limited impacts on DC‐side system stability. Finally, the results of numerical simulation prove the validity of the proposed stability analysis method and the stable boundary for the grid‐tied converter with different control modes.

Modified Droop Control for Microgrid Power-Sharing Stability Improvement

Isolated microgrid (IMG) power systems face the significant challenge of achieving fast power sharing and stable performance. This paper presents an innovative solution to this challenge through the introduction of a new droop control technique. The conventional droop controller technique used in inverter-based IMG systems is unable to provide satisfactory performance easily, as selecting a high droop controller gain to achieve fast power sharing can reduce the system’s stability. This paper addresses this dilemma by proposing a modified droop control for inverter-based IMGs that effectively dampens low-frequency oscillations, even at higher droop gain values that would typically lead to instability. The design is described step-by-step, and the proposed controller’s effectiveness is validated through time domain simulation analysis. The results demonstrate the significant improvement in stability and fast power sharing achieved with the proposed controller. This innovative technique presents a promising solution for achieving fast power sharing and stable performance in IMG power systems.

Solar PV and BESS Plant-Level Voltage Control and Interactions: Experiments and Analysis

In real world operation, low-frequency oscillations associated with plant-level voltage control of inverter-based resources (IBRs) have been a concern of grid operators. This research aims to demonstrate and analyze the effect of plant-level voltage control on system stability and potential interactions among IBRs. The experiment results of a 13.2-kV system interconnected with two IBRs—a 1-MW battery energy storage system (BESS) and a 430-kW solar photovoltaic (PV) plant—are presented. Both 0.5-Hz and 3-Hz oscillations become visible at certain conditions. The phenomenon is further explained by use of a feedback system consisting of the plant-level droop control and the effect of reactive power or var injection on voltage. The main components of the block diagram are obtained via frequency scan measurements. The analysis offers an understanding of the main contributors and other influencing factors of the 3-Hz and 0.5-Hz modes. It also reveals the role of the volt-var droop gain, the low-pass filter time constant, and the system impedance in stability.

Conservation Voltage Reduction Strategy for Autonomous Microgrid with Improved Voltage-CurrentDroop-Based Inverter Control Framework

In a conventional AC distribution system, the conservation voltage reduction (CVR) strategy is widely employed to lower down the voltage of specific load to lower the power consumption. The wide applicability of demand side management (DSM) using CVR in a stand-alone microgrid through VSI-based energy sources is a thrust area, that is, not examined yet and needs to be explored. The fast dynamics and flexible control are the characteristics of the voltage-current droop method which further increases the inertia of the voltage source inverter. For utilizing these advantages of this droop method, it is required to determine the accurate droop gain to properly coordinate the distribution of power among DGs. In this paper, the voltage-current droop method is utilized to carry out the function of DSM, and a modified droop computation method for voltage-current droop is formulated to determine the impedance from initial of the DG point to the downstream end for multifeeder network. As most of the droop control techniques emulate the conventional power grid such as Q-V droop control which reduces voltage with the increase of reactive power, the research prospect is very high in devising the new droop computation method for voltage-current droop for accurate control of power. In addition to it, the work is extended to apply the benefits of voltage-current droop to execute DSM strategy in standalone MG. Moreover, the capability of the proposed estimation of droop parameter is implemented on a standalone 5-bus single-feeder multi-DGs network, and furthermore, the scheme is applied to IEEE-9 bus multifeeder multi-DGs network to show the applicability of the proposed scheme. The simulation results produced from MATLAB/Simulink are compared with the decentralized power-based droop method and conventional voltage-current droop technique to analyse the performance of the devised scheme.

Voltage-sag mitigation by stability-constrained partitioning scheme

Microgrids have the ability to function in islanded or grid-connected modes of operation. The increased penetration of inverter-based Distributed Energy Resources (DERs) in the system promotes the concept of partitioning the system into self-governing and self-adequate microgrids. Most of the partitioning techniques determine virtual boundaries and did not consider the survivability of the constructed microgrids. In this paper, a stability-constrained partitioning scheme is proposed based on small-signal stability to ensure microgrids' survivability when physically partitioned. Moreover, a sensitivity analysis of active power droop gain is utilized to define a novel index for the microgrid’s marginal stability. The application targeted in this paper is the mitigation of voltage-sag events caused by low impedance faults. The system will be partitioned into clusters of survivable microgrids during faults to isolate the faulted zone that caused the voltage-sag event. By isolating the voltage-sag origin from the rest of the system, voltage-sag mitigation is accomplished. Also, a microgrid re-connection method is proposed. This method allows multiple droop-controlled DERs to adjust the frequency, phase, and magnitude of their output voltages to facilitate seamless re-connection of microgrids. Simulation results are given that show a seamless re-connection of two different microgrids when the proposed method is utilized. The effectiveness of the proposed mitigation algorithm is validated using a modified IEEE 33-bus distribution system simulated on MATLAB/SIMULINK platform.

Island microgrid power control system based on adaptive virtual impedance

When the microgrid is in the islanding operation mode, affected by the line impedance difference between the distributed power sources (DGs), the traditional droop control strategy will lead to the fact that the reactive power of the system cannot be reasonably distributed according to the droop gain, which makes the power grid prone to failure. To solve this problem, an improved sag control strategy based on adaptive virtual impedance is proposed in this paper. The central controller calculates the given power according to the capacity of each inverter and the total load capacity of the line, and then sends it to each inverter, and the inverter determines the value range of the virtual impedance according to the given reactive power. A voltage compensation link is added to the control strategy to compensate for the voltage drop difference caused by the line impedance difference, thereby ensuring the stability of the output power and realizing the power control of the microgrid. The MATLAB/Simulink simulation platform was built to establish the microgrid simulation model in island mode. The simulation results show that when the inverters with the same capacity are connected in parallel, the output active power is the most stable, and the reactive power maintains an equalized state; When inverters of different capacities are connected in parallel, the output active power takes the shortest time to stabilize, and the reactive power basically matches the rated capacity ratio. Therefore, it is fully demonstrated that this method has a good control effect on the power control and operation of the islanded microgrid.

A novel stability index-based virtual synchronous machine droop scheme for performance improvement of hybrid AC/DC microgrid under volatile loading conditions

The large-scale electric vehicle penetration into the grid and the volatile nature of the electric vehicle loading adversely affect grid stability. This paper presents a novel index-based droop scheme for improving the performance of the hybrid AC/DC microgrid under volatile loading conditions. The novelty of the proposed control scheme is that the adaptiveness in selecting active droop gain based on the volatile stability index generates an accurate power reference rather than a constant power reference as in conventional droop gain methods. The state–space modelling of the proposed controller and index-based approach is used to determine the theoretical stability margin. The simulation experiment results agree with the theoretical stability margin limit. The efficacy of the control scheme is validated using real-time experiments on a hardware-in-the-loop real-time simulator, OPAL-RT. The results show that the proposed control scheme improves the performance of the hybrid AC/DC microgrid by maintaining the frequency/voltage fluctuations in the AC and DC sub-grids within the acceptable operating limits of ± 2% and ± 10% as per IEEE 1547 standards. As a result, the percentage improvement in voltage profile is 2.68% at the AC sub-grid and 5.55% at the DC sub-grid. • A novel stability index to reflect volatile loading conditions of HMG. • HMG stability enhancement using HVI-based VSM-V droop controller. • Mathematical modelling and analysis of VSM-V droop scheme. • Real-time experimental verification using OPAL-RT.

Synchrophasor Data Based Q-V Droop Control of Wind Farm Integrated Power Systems

With increasing scalability of grid-integrated wind farms (WFs), reactive power ancillary service through Q-V droop control of doubly-fed inducton generators (DFIGs) is one of the prime interests of power system operators. However, variable and uncertain availability of wind-resource at the wind turbines creates challenge against allocating reactive power reserve at the WFs for maintaining voltage stability of the grid. This issue can be resolved through online coordination among the WFs. This paper proposes a synchrophasor data based Q-V droop (SQVD) control technique for the WFs integrated to the grid through different point of common couplings (PCCs) in the power system transmission network. The data from the phasor measurement units (PMUs) at the WF-connected PCCs are used to obtain synchronized Q-V droop gain for each WF. Further, distributed coordination among DFIGs within each WF is performed to allocate optimum reactive power share at the converters. The total reserve of each WF is employed by their synchronized droop, in improving voltage stability at both the local PCC and neighboring PCCs. Performance of SQVD is verified at 418-bus practical Indian power system, in different dynamic situations of the grid, and found to be more effective as compared to the constant Q-V droop and variable Q-V droop control methods.

Voltage control and power sharing in DC Microgrids based on voltage-shifting and droop slope-adjusting strategy

• Designed a dynamic droop controller based on average of instantaneous power, droop control and voltage. • Proposed a new distributed control algorithm for DC microgrids. • Accurate power sharing and fast voltage recovery. To overcome the disadvantages of droop control, this paper proposes a novel method of hierarchical distributed secondary control for DC microgrids. With the load changes, distributed energy resources can achieve power sharing and restore the DC bus voltage. Based on the rated voltage and instantaneous power of the DC grid (DG), a hierarchical distributed control method including shift voltage and droop slope adjustment has been developed. By increasing the shifted voltage and adjusting the droop gain, it can effectively compensate the voltage drop caused by the droop controller. In this way, the DC bus voltage can always be adjusted automatically, regardless the load changes. In addition, this paper introduces the design process of the controller in detail and proves the stability of the system. Finally, simulations and experiments have verified the effectiveness and superior performance of the proposed control method.

Efficiency Focused Energy Management Strategy Based on Optimal Droop Gain Design for More Electric Aircraft

Due to the substantial increase in the number of electrically driven systems onboard more electric aircraft (MEA), the onboard electric power systems (EPSs) are becoming more and more complex. Therefore, there is a need to develop a control strategy to manage the overall EPS energy flow and ensure the operation of safety-critical systems (which are electrical loads) under different operating scenarios and to consider EPS losses minimization, exploiting the thermal capability of generators, different load priorities, and available batteries with their charging and discharging schedules. This article presents an energy management (EM) strategy that considers the aforementioned objectives. The optimal droop gain approach is employed as a power-sharing method to minimize the total EPS losses in MEA. A finite state machine (FSM) has been used to implement the control strategy to realize the EPS reconfiguration operation. The proposed EM strategy is implemented and simulated using MATLAB/Simulink and hardware-in-the-loop (HIL) under different operational scenarios, such as normal operations, failure of one of the power generation channels, and failure of all power generation channels. The proposed EM method has shown its capability to efficiently manage the EPS under different operating conditions to reduce the overall system losses.