This study addresses the robust stability analysis for an islanded microgrid with droop-controlled inverter-based distributed generators (DGs). Owing to large load changes, microgrid structure ...reconfiguration, and higher-power demands, the low-frequency (LF) dominant modes of a microgrid stir toward unstable zone and make the system more oscillatory or even unstable. In this study, a robust two-degree-of-freedom (2DOF) decentralised droop controller, which is the combination of the conventional droop with a robust transient droop function, is utilised for each inverter-based DG unit. Unlike conventional tuning of 2DOF droop controllers, a new design procedure is proposed to robustly determine the transient droop gains to effectively damp the LF oscillatory modes of the islanded microgrid irrespective of disturbances, equilibrium point variations, and uncertain parameters of a microgrid. To mitigate the LF power oscillations at different microgrid conditions, inspired by Kharitonov's stability theorem, a robust D-stability analysis is performed to determine the specific ranges of the transient droop gains to provide a robustness margin for the disturbances, equilibrium point variations, and uncertain parameters of the islanded microgrid. Finally, digital time-domain simulation studies are performed in MATLAB/SimPowerSystems software environment to verify the effectiveness of the proposed method.
The universal droop control (UDC) can be applied to power inverters having an impedance angle between -pi/2 rad and pi/2 rad to achieve voltage and frequency regulation and accurate proportional ...power sharing without the need of knowing the type or value of impedance. However, there is an increasing need for voltage and frequency regulation within predetermined ranges even under some abnormal conditions, such as overloading, sensor faults, and large set-point changes, etc. In this paper, a bounded nonlinear dynamics is introduced into the UDC to ensure that the voltage and frequency can always stay within predetermined ranges under normal and some abnormal conditions, up to hardware limits when current protection needs to be triggered. As a result, the proposed controller can extend the operational range of power inverters in terms of voltage and frequency regulation to cover different scenarios. More importantly, the ranges for both voltage and frequency can be chosen independently from each other. Since the original structure of the UDC is kept in the proposed controller, the properties of the UDC, such as without the need of knowing the type or value of impedance, are well maintained. Furthermore, the closed-loop stability of the system is established via the Lyapunov method. Extensive simulation and experimental results are presented to validate the effectiveness of the proposed controller.
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.
In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control. ...Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time. Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements. With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode. Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control. The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process. Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results.
In the modern-day distribution network, converter-based distributed generators (DGs) are increasingly employed to supply power from renewables. One of the prominent issues in the use of decentralized ...control for this purpose is the inaccurate real power sharing amongst these DGs due to the voltage drops associated with the feeder/line impedances. The use of virtual voltage drop in the controller of these DGs is a popular approach to mitigate the impedance mismatch effects. This impedance shaping technique demonstrates improved power sharing, however, the selection of an appropriate value for it is a big challenge. This paper elaborates upon a droop-based VI shaping technique for converter-based DG through a non-linear variation for islanded networks which are predominantly resistive. Performance evaluation of this technique for a wide variety of loads including balanced (constant impedance), constant power, unbalanced, non-linear, and induction motor loads is done through detailed simulations. The proposed method is also tested for a mesh network. DG plug-and-play functionality performance and load variation on a modified 13-bus network is verified using the proposed control scheme. The range for control parameters for stability is verified through modeling and eigen value analysis. Experimental validation on a laboratory setup for the same has also been presented.
•A non-linear relation between the output current and virtual impedance values is proposed to improve the proportional power-sharing of the DGs.•Improved proportional power-sharing is achieved by keeping the PCC voltage under acceptable limits and the non-linear parameter is also adjusted automatically.•The scheme idea is designed for low-voltage islanded systems and isolated systems where the DGs operate in inverse drop control.
Renewable energy-based energy conversion technologies have become more relevant due to environmental considerations even though they are intermittent in nature. As a result, the concept of microgrid ...and microgrid control techniques have been evolved as major areas of power system research. Among different inverter control methods, the droop-based control method is more popular in microgrid systems due to its simplicity and non-requirement of expensive communication systems. The transient performance, power-sharing accuracy and decoupling between real and reactive power are improved by modifying the natural droop control method. In this study, the selected microgrid system consists of two inverters operating in parallel, two interconnecting lines and three loads. A state-space model of the microgrid is created based on the small-signal stability and the transient response is improved by introducing virtual impedance and dynamic droop gains. The different controller parameters are optimised using particle swarm optimisation ensuring stability. Eigenvalue analysis is done to analyse stability. The analysis of the response of the system for various disturbances validates the effectiveness of the proposed controller. The strategy developed ensures improved power-sharing capability with high values of natural droop gains without compromising stability by using optimised dynamic droop gains.
Colloidal semiconductor quantum dots (QDs) are a highly promising materials platform for implementing solution-processable light-emitting diodes (LEDs). They combine high photostability of ...traditional inorganic semiconductors with chemical flexibility of molecular systems, which makes them well-suited for large-area applications such as television screens, solid-state lighting, and outdoor signage. Additional beneficial features include size-controlled emission wavelengths, narrow bandwidths, and nearly perfect emission efficiencies. State-of-the-art QD-LEDs exhibit high internal quantum efficiencies approaching unity. However, these peak values are observed only at low current densities (J) and correspondingly low brightnesses, whereas at higher J, the efficiency usually exhibits a quick roll-off. This efficiency droop limits achievable brightness levels and decreases device longevity due to excessive heat generation. Here, we demonstrate QD-LEDs operating with high internal efficiencies (up to 70%) virtually droop-free up to unprecedented brightness of >100,000 cd m–2 (at ∼500 mA cm–2). This exceptional performance is derived from specially engineered QDs that feature a compositionally graded interlayer and a final barrier layer. This QD design allows for improved balance between electron and hole injections combined with considerably suppressed Auger recombination, which helps mitigate efficiency losses due to charge imbalance at high currents. These results indicate a significant potential of newly developed QDs as enablers of future ultrabright, highly efficient devices for both indoor and outdoor applications.
Droop control is a well-known strategy for the parallel operation of inverters. However, the droop control strategy changes its form for inverters with different types of output impedance, and so ...far, it is impossible to operate inverters with inductive and capacitive output impedances in parallel. In this paper, it is shown that there exists a universal droop control principle for inverters with output impedance having a phase angle between -(π/2) rad and (π/2) rad. It takes the form of the droop control for inverters with resistive output impedance (R-inverters). Hence, the robust droop controller recently proposed in the literature for R-inverters actually provides one way to implement such a universal droop controller that can be applied to all practical inverters without the need of knowing the impedance angle. The small-signal stability of an inverter equipped with the universal droop controller is analyzed, and it is shown to be stable when the phase angle of the output impedance changes from -(π/2) rad to (π/2) rad. Both real-time simulation results and experimental results from a test rig consisting of an R-inverter, an L-inverter, and a C-inverter operated in parallel are presented to validate the proposed strategy.
•An adaptive droop control scheme is designed to improve the frequency stability.•The proposed scheme could solve the issue of regaining the rotor speed to the initial value without resulting in a ...large second frequency drop.•The proposed scheme is investigated under various wind speed conditions and wind power penetration levels.•Performance comparison among constant gain, variable gain, and adaptive gain schemes.
This paper presents an adaptive droop control scheme with smooth rotor speed recovery capability for doubly-fed induction generators (DFIGs, type III wind turbine generators) to improve the frequency nadir, minimize the second drop in the frequency, and regulate the time to meet the MPPT curve. To this end, an adaptive droop gain that decays with time is proposed, the initial value of which determined according to the wind speed. The effectiveness of the proposed adaptive droop control scheme is verified using a modified IEEE 14-bus system under various wind speeds, wind power penetration levels, and variable wind speed conditions. Simulation results in EMTP-RV indicate that the proposed adaptive droop control scheme is beneficial to improving the frequency nadir, preventing DFIGs from stalling, and minimizing the second drop in the frequency under various conditions.
In the distributed generation environment, parallel operated inverters play a key role in interfacing renewable energy sources with the grid or forming a grid. This can be achieved by operating the ...inverters in parallel with an effective control strategy. Hence, qualitative output in terms of voltage and power in a parallel inverter demands a control strategy for abating the certain performance parameters. The technical constraints like load sharing capability, stability, total harmonic distortion, steady state and transient response determines the performance indices of the control strategy in connection with micro grid. The paper presents a comprehensive review of control strategies of parallel inverter based on the communication and control loops. The classification and transfiguration of control strategy from centralized approach to distributed approach and from distributed approach to droop approach were reviewed in the paper. The paper also focuses on the ancillary services that improve the performance indices of the system with multi loop approach of droop control in connection to micro grid. This paper provides an insight of the researcher's perception in emphasizing the power quality issues to achieve regulated and qualitative output with different control strategies. The paper also cites the progress of the research on parallel inverter control by inoculating the issue and the resolution in the literature. This could help the future researchers in addressing the power quality issues associated with parallel inverter control in connection to micro grid. Based on the reviewed state of art, the paper finally reports the suitability and future scope of the control strategy as its concluding remark.
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