Droop Gain Research Articles

Damping of Inter-Area Oscillations via Modulation of Aggregated Loads

Low frequency electromechanical oscillations can pose a threat to the stability of power systems if not properly addressed. This paper proposes a novel methodology to damp these inter-area oscillations using loads, the demand side of the system. In the proposed methodology, loads are assigned to an aggregated cluster whose demand is modulated for oscillation damping. The load cluster control action is obtained from an optimal output feedback control (OOFC) strategy. The paper presents an extension to the regular OOFC formulation by imposing a constraint on the sum of the rows in the optimal gain matrix. This constraint is useful when the feedback signals are generator speeds. In this case, the sum of the rows of the optimal gain matrix is the droop gain of each load actuator. Time-domain simulations of a large-scale power system are used to demonstrate the efficacy of the proposed control algorithm. Two different cases are considered: a power imbalance and a line fault. The simulation results show that the proposed controllers successfully damp inter-area oscillations under different operating conditions and with different clustering for the events considered. In addition, the simulations illustrate the benefit of the proposed extension to the OOFC that enable load to provide a combination of droop control and small signal stability augmentation.

Adaptive Coordination for Power and SoC Limiting Control of Energy Storage in an Islanded AC Microgrid With Impact Load

Isochronous controlled battery energy storage system (ESS) can maintain constant frequency in an islanded ac microgrid, but its power and state of charge (SoC) may go beyond limit due to intermittent renewable energy resources and impact load change. This paper introduces a control strategy integrated with bang-bang and backlash mechanism to coordinate distributed energy resources (DERs). When overloading will take place, ESS will change from isochronous control to droop control, with droop gain varying based on bang-bang control, by which the overloaded power will be spontaneously transferred from ESS to other DERs. Moreover, frequency will be automatically restored when impact load disappears, which eliminates the effort of secondary frequency control. When SoC of ESS comes to limit, the diesel generator will change from droop control to isochronous control, whereas ESS will adopt virtual inertia and grid-supporting droop control to help stabilize frequency. A backlash mechanism is also employed to latch the control mode near SoC limit. Comprehensive tests on real-time simulator demonstrated the effectiveness of the proposed control strategy.

Frequency support from photovoltaic power plants using offline maximum power point tracking and variable droop control

With higher penetration of converter-connected renewable energy sources (RES) into power systems, the successful operation of the system is challenged by significant reductions in system inertia. Presently, given the dominant share of the conventional synchronous power plant, RES power plants are not demanded to provide ancillary services. However, as RES connections increase, RES power plants will play a major role in power system operation, contributing to frequency control. This study demonstrates that photovoltaic power plants (PVPPs) can provide effectively different types of frequency support based on a power reserve and an offline maximum power point tracking (MPPT) technique. An innovative method to de-load the PVPP without significantly increasing the MPPT complexity is proposed. Results from different PVPP frequency support methods, under varying levels of photovoltaic penetration, are presented which demonstrate their capability to provide inertia support comparable to that of synchronous generators. A new variable droop control method, which releases maximum power during the inertial response and returns to fixed droop gain value after a specified time is also presented. The results from using the variable droop show that the frequency nadir and the rate-of-change-of-frequency can be significantly reduced and some power reserve still maintained after a frequency event.

A Distributed Control Method Based on a Voltage Sensitivity Matrix in DC Microgrids With Low-Speed Communication

A distributed control method is proposed to improve the power sharing performance of bidirectional distributed generators (BDGs) and the voltage regulation performance of a dc bus in a dc microgrid (MG). To analyze the structural characteristics of a dc MG, voltage sensitivity analysis based on power flow analysis is presented. Based on this, a distributed control method using a voltage sensitivity matrix is proposed. The proposed method uses information received through the communication line and performs the droop gain variation method and voltage shifting method without additional controllers. This approach achieves improved power sharing and voltage regulation performance without output transient states, even at slow communication speed. The proposed method is implemented through an experimental environment consisting of two BDGs, a load, and a non-dispatchable distributed generator in a four-bus ring type model. The experimental results demonstrate improved power sharing accuracy and voltage regulation performance.

Design of Optimal Droop Control for Multi-Terminal High-Voltage Direct Current Systems During Line Outages

This article investigates the required modifications to droop control design for optimum operation of Multi-Terminal High-Voltage Direct Current (MTHVDC) systems during line and converter outages. A typical 6-bus MTHVDC system is used in this study. Different scenarios are considered during the design process such as different outage types, violation of power ratings of system components, and variability of wind power injected into the system. A variable droop control gains technique is proposed to ensure minimum power loss and maximum annual energy collection. The study is extended to investigate the effect of employing constant droop control gains calculated during rated conditions on the system performance. Moreover, the results of the constant and the proposed variable droop gain schemes are compared. Simulation results show the effectiveness of the proposed techniques for designing optimum droop gains during abnormal conditions. Additionally, general guidelines are presented for the implementation of the proposed techniques.

Approximation to Frequency Control Capability of a DFIG-Based Wind Farm Using a Simple Linear Gain Droop Control

In the literature, many droop control loops have been proposed to enhance the frequency control capability of the wind farm (WF) when a large frequency disturbance occurs. However, some loops are not feasible to be implemented in the dynamic equivalent model (DEM) of the WF. The kinetic energy (KE)-based droop control loop is an example. The droop gain of this loop is expressed as a quadratic function of the wind turbine (WT) rotor speed. However, it is not feasible to implement a nonlinear function in DEM of WF. Therefore, in this study, a new linear-gain droop control loop is proposed for the doubly fed induction generator-based WF. In the proposed control loop, the droop gain is a linear function of the WT rotor speeds. By selecting the proper coefficients of the linear function, the proposed linear droop gain can achieve a good approximation to the quadratic droop gain. The performance of the proposed droop control loop is demonstrated based on three initial conditions. To verify the responses of system frequency and WF power output, four indices are developed. The simulation results demonstrate that the proposed linear-gain droop control loop is capable of approximating closely to the KE-based droop control loop.

A Critical Assessment of Oscillatory Modes in Multi-Microgrids Comprising of Synchronous and Inverter-Based Distributed Generation

This paper provides a critical assessment of oscillatory modes for a droop-controlled multi-microgrid (MMG) consisting of inverter-based and synchronous-based distributed generation (DG). Local and inter-microgrid oscillatory modes similar to conventional interconnected power systems are observed. However, an additional new and more “dominant” oscillatory mode caused by interactions between close-by coherent synchronous-based and inverter-based DGs in the MMG is identified. Sensitivity analysis is employed to assess the impact of various MMG configurations and parameters such as: interconnecting line length, droop gain, exciter gain, and governor gain on MMG oscillatory modes. Parametric studies are conducted to highlight the impact of oscillatory modes on MMG transient and dynamic performances. Consequently, power system stabilizers (PSSs) are developed for both inverter-based and synchronous-based DGs to improve the transient and dynamic performances of the MMG. Residues are calculated and therefore utilized to determine the effect of PSS placement on the MMG oscillatory modes. Finally, small-signal stability analysis results and the performances of the PSSs are validated through time domain simulations on MATLAB/Simulink.

Primary frequency control of a microgrid with integrated dynamic sectional droop and fuzzy based pitch angle control

In a microgrid (MG), employing conventional fixed droop gain control of wind turbine for mimicking conventional generators may threaten grid stability. In this paper, an integrated control strategy of inertia emulation and dynamic sectional droop control for generating active power reference of the wind power system is presented to provide primary frequency control (PFC) services. The dynamic sectional droop is implemented with the fuzzy logic controller (FLC) regulated pitch angle control mechanism and compared with the conventional proportional-integral (PI) pitch angle control method in the isolated MG. Two different gain values are selected for developing the proposed sectional droop control which is based on the sensitivity of frequency deviation in comparison to the single fixed gain in conventional droop control. The maximum power margin of wind turbine is considered as 25% and 15% for a wind speed higher than (or equal to) the rated speed and lower than the rated speed, respectively. The effectiveness of the proposed method is investigated through Matlab/Simulink based real-time simulation studies. Simulation results demonstrate that the proposed sectional droop control provides superior performance than conventional fixed-gain method and achieves optimum frequency regulation at the rated wind speed and even for a wind speed lower than the rated wind speed.

A Novel Communication-Based Average Voltage Regulation Scheme for a Droop Controlled DC Microgrid

In a dc microgrid, enhanced voltage regulation (VR) and load sharing (LS) are necessary for the economic and reliable operation of the system. Traditionally, droop control is used for LS in microgrids. However, because of the nature of the droop control, good VR and accurate LS cannot be achieved simultaneously. To overcome this drawback, a low-bandwidth, communication-based average VR technique along with an algorithm for equal voltage correction factor is proposed in this paper. Here, precise LS is achieved via droop control using a high value of droop gain, while good VR is obtained through low-speed control area network (CAN) communication, which provides fault tolerance and expandability feature to the system. Only the local dc bus voltage information is exchanged over CAN by the grid connected converters, which ensure low communication traffic. Thus, sufficient bandwidth is available on the CAN bus that can be used for control and monitoring purpose—a highly desirable proposition for a smart grid application. Effect of communication delays on system stability has been modeled and analyzed. Simulation and experimental results are presented to validate the feasibility of the proposed scheme. Plug-n-play and fail-safe features of the proposed technique are highlighted.

Small‐signal stability analysis and particle swarm optimisation self‐tuning frequency control for an islanding system with DFIG wind farm

Linearised model for frequency control of an islanding system comprising an equivalent synchronous generator and an equivalent doubly fed induction generator (DFIG) is derived. Small signal stability analysis is performed to determine the stable regions for DFIG supplementary droop controller under various wind velocities. In order to achieve better dynamic frequency response under different system parameters, the DFIG droop gain is adapted in real-time using a self-tuning controller designed based on particle swarm optimisation technique. In order to validate the results from small signal analysis and to demonstrate the effectiveness of the proposed self-tuning frequency controller, digital simulations using MATLAB/SIMULINK are performed on a local power system in central Taiwan. It is concluded from the simulation results that the proposed self-tuning frequency controller offer better dynamic frequency response than the fixed-gain droop controller.

Large-scale provision of frequency control via V2G: The Bornholm power system case

This paper assesses the impact of electric vehicles (EVs) providing primary frequency regulation via vehicle-to-grid (V2G) technology. The aim of the work is to define a set of recommendations in order to guarantee a stable large-scale deployment of EV fleets as primary reserve providers. A realistic fleet model is proposed, which emulates the aggregated response of a number of EVs characterized by V2G hardware response times derived in laboratory tests. The effects of primary frequency control via EV fleets replacing conventional generating units are assessed with a sensitivity study in a single-bus power system with growing fleet sizes and response times. Two recommendations are derived to guarantee safe and stable operation: Recommendation 1 requires the share of EVs providing primary reserve to be smaller than the reserve from conventional units; Recommendation 2 requires response times below a given limit value, calculated as a function of the following power system parameters: the system inertia, the total primary reserve over the rotating generation capacity, and the employed droop gain. The full 60 kV power system of the Danish island of Bornholm is then employed to evaluate the validity of the proposed requirements on a real system with complex dynamics, non-linearities and voltage dependencies.

Dynamic Droop Control for Wind Turbines Participating in Primary Frequency Regulation in Microgrids

Wind power will provide a significant portion of electricity generation in the near future. This significant role requires wind power generators to contribute to the system frequency regulation. The droop method is one of the most popular methods to be implemented in these generators to mimic the governors of conventional generators and contribute to both transient and steady-state frequency regulation. However, the unpredictability of the variable wind speed complicates this implementation. In the present wind-based droop methods, the maximum allowable droop gain is a function of the wind speed. This dependency means either that the entire available capacity of the wind generator will not be used or that instability will threaten the implemented droop wind generators. This paper proposes the efficiency droop, a new droop-based method, which can be tuned regardless of the wind speed. Small-signal analyses are used to study the method in depth and compare its influences on both the transient and steady-state frequency performance to the influence of the present methods while adopting minimum approximation. Detailed time domain analyses are used to verify the analytical results.

Improved Droop Control with Washout Filter

In this paper, a droop washout filter controller (DWC), composed of a conventional droop controller and a washout filter controller, is proposed. The droop controller is used to ensure the “plug-and-play” capability, and the droop gain is set small. The washout filter is introduced to compensate the active power dynamic performance (APDP). Compared to the droop controller, the DWC can achieve accurate active power sharing and smaller frequency difference without losing the APDP. Additionally, a novel modeling technology is proposed, using which a small-signal model for an island microgrid (MG) is constructed as a singular system. The system’s stability is analyzed and the DWC is verified using real-time (RT-LAB) simulation with hardware in the loop (HIL).

Observer-Based DC Voltage Droop and Current Feed-Forward Control of a DC Microgrid

An observer-based dc voltage droop and current feed-forward control for a dc microgrid has been proposed in this paper. With the proposed control scheme, dynamic response of dc voltage control can be improved effectively through using an observer. Moreover, the feedback current for dc voltage droop and the feed-forward current are both obtained from the observer without additional current measurement. Furthermore, system stability analysis shows that, the propose method has enhanced robustness and stability when compared to the traditional droop method under variations of system parameters, such as loads, cable impedances, and droop control gains. In a droop controlled dc microgrid, there exists a trade-off between the load demand sharing accuracy (which needs a high droop gain) and stability (which limits the droop gain). With the above mentioned advantages, the proposed method can effectively address this trade-off in such a system. The effectiveness of the proposed method is verified by experiments in a laboratory dc microgrid with two boost dc–dc converters based distributed generations and a buck type constant power dc load.

Quality-Index Based Distributed Secondary Controller for a Low-Voltage DC Microgrid

Sources in a dc microgrid are required to have a proportional current sharing and an improved voltage regulation. Conventional droop-based control techniques achieve the proportional current sharing, but the voltage regulation of an individual bus may be large due to a high value of droop gains. In the literature, secondary controllers are discussed to improve either the voltage regulation of one bus or the average voltage regulation of the entire system. Most of these techniques rely on the communicated values of both the terminal voltage and the output current of individual sources. This paper puts forth a new approach to address the aforementioned issues. A figure of merit called quality index is introduced, which constitutes a weighted average of terms representing the voltage regulation and the power sharing at each source bus/node. An online algorithm to find the optimal droop coefficient based on the quality index is suggested. It does not require the knowledge of the system parameters. Furthermore, it reduces congestion in the communication network, as only output current information is exchanged among sources. Equations for the current sharing error and the voltage regulation in a microgrid system are derived based on droop gain values of the sources. Operation of the proposed algorithm is verified using a computer simulation. The method is validated on a scaled-down dc microgrid prototype.

Control Approach for the Multi-Terminal HVDC System for the Accurate Power Sharing

This paper presents a new control strategy for voltage source converter based Multi-Terminal High Voltage Direct Current (MTDC) systems. The proposed control approach ensures accurate power sharing between the droop-controlled converter stations. By communicating the power-sharing index between neighboring converters, the proposed approach achieves exact droop control operation independent of the DC system topology and line parameters. The pilot voltage droop based controller, which is an alternative communication-based approach for achieving precise power sharing, was used as a base case for comparison. Modal analysis is carried out to reveal the sensitivity of the system's eigenvalues to the changes in control parameters (e.g., power droop gain, proportional integral gains of the proposed controller) and the latencies in the communication. It is demonstrated that the proposed strategy remains in the stable operation even when excessive latencies are encountered in the communication. Nonlinear simulations are conducted in the MATLAB/Simulink environment in four- and five-terminal MTDC grids, validating the capability of the proposed controller to achieve the desirable performance under various operational conditions.

Distributed Control for DC Microgrid Based on Optimized Droop Parameters

ABSTRACTThe droop control approach is widely used in case of parallel operation of DC sources. The conventional droop control method is realized by linearly reducing the load voltage as the load current increases. This droop control is unable to achieve effective equal load sharing as well as the bus voltage regulation. The load sharing error is enhanced when cable line parameters of parallel-connected DC sources are not equal. The droop gain and nominal voltage reference of source converters are the key parameters to achieve the bus voltage deviation within the permissible limits and effective load sharing. In this paper, the droop parameters of the proposed distributed control scheme are optimized with the help of the Particle Swarm Optimization technique for the aforementioned control objectives of the DC microgrid for unequal cable line resistances. The performance of the proposed control scheme based on optimal droop parameters is verified through MATLAB/Simulink environment and validated by a hardware-in-the-loop real-time simulator based on the dSPACE 1202 platform.

A virtual energy storage system for voltage control of distribution networks

—Increasing amount of Distributed Generation (DG) connected to distribution networks may lead to the voltage and thermal limits violation. This paper proposes a Virtual Energy Storage System (VESS) to provide voltage control in distribution networks in order to accommodate more DG. A VESS control scheme coordinating the demand response and the energy storage system was developed. The demand response control measures the voltage of the connected bus and changes the power consumption of the demand to eliminate voltage violations. The response of energy storage systems was used to compensate for the uncertainty of demand response. The voltage control of energy storage system is a droop control with droop gain values determined by voltage sensitivity factors. The control strategy of the VESS was applied to a medium-voltage network and results show that the control of VESS not only facilitates the accommodation of higher DG capacity in the distribution network without voltage violations or network reinforcements but also prolongs the lifetime of transformer on-load tap changer.

Ideal current‐based distributed control to compensate line impedance in DC microgrid

To achieve proportional power sharing among similar sources in DC microgrid, droop controllers are used. However, due to interconnecting cable impedances, power shared by the sources deviate from their desired values. To alleviate this problem, techniques utilising shift in droop characteristics and/or modification in droop gain are suggested in literature. Methods which use both of them offer better performance due to higher degree of freedom in control. However, these methods utilise distributed proportional-integral control, which leads to winding-up phenomenon. To address this problem, a distributed secondary controller using full communication is proposed. The current of each source is shared with other sources using low-bandwidth communication. Using this information, each source calculates the ideal value of current to be supplied, which is then used to update its droop gain. To improve the average system voltage, shifting of droop characteristics is incorporated. Detailed small signal model of the proposed controller is developed. Effect of communication delay on the performance of the proposed controller is analysed using the characteristic equation of the system. To evaluate the performance of the proposed scheme, detailed simulation studies are carried out. A scaled down laboratory prototype is developed to determine the viability of the proposed controller.

Coordinated Design of Droop Control in MTDC Grid Based on Model Predictive Control

A new multi-terminal DC (MTDC) grid controller based on model predictive control (MPC) for coordinating all droop controllers is proposed. This grid controller is designed to minimize the deviations of the DC voltages of the MTDC grid and avoid control mode change, so that to preserve the ability of droop voltage source converters (VSC) in controlling DC voltages. This is achieved by coordinately adjusting the gains of all droop controllers when changes in the MTDC grid power flow occur. A systematic discrete-time linear model of MTDC grid for this grid controller is derived in detail. In addition, the limitations on over shoot of DC voltages, converter capacity, and droop gain rate of change are taken into account in the MPC. Note that the constant droop gain will lead to a constant ratio $\Delta P/\Delta {U_{dc}}$ . Minimize the $\Delta {U_{dc}}$ means $\Delta P$ is minimized. So if necessary, the active power of a particular droop VSC can be fixed at a constant level by setting the lower and upper constraint on the corresponding droop gain to be equal. Tests are conducted on the system with six-terminal converters. The performance of the proposed grid controller is compared with those of fixed droop scheme. Simulation results have demonstrated the superiority and flexibility of the proposed grid controller under various operating conditions.