Controller For Stability Enhancement Research Articles

An Adaptive BESS Controller for Stability Enhancement of Islanded Low Voltage Microgrids

An Adaptive BESS Controller for Stability Enhancement of Islanded Low Voltage Microgrids

Stability enhancement control strategy for grid-connected wind power system based on composite feedforward damping

The stability of grid-connected wind power system (GCWPS) is prone to deteriorate due to the impedance interaction between wind turbines and the weak grid. For purpose of finding out the cause of power oscillation and effectively improving the stability of GCWPS under weak grid, firstly of all, a frequency coupling impedance model (FCIM) for GCWPS is established. Through analyzing the impedance characteristic of GCWPS, a significant coupling effect between GCWPS impedance and grid impedance in the middle-low frequency region under weak grid is observed, which is easy to cause power oscillation. For purpose of restraining the power oscillation of GCWPS under weak grid, this paper proposes an oscillation suppression strategy based on composite feedforward damping. Virtual damping is incorporated into the feedforward loop to rectify the impedance characteristic curve of GCWPS. Then, a FCIM of GCWPS that adopts the proposed oscillation suppression strategy is established, and the stability analysis demonstrates that the proposed strategy effectively enhances the GCWPS stability under weak grid. Additionally, compared with conventional control strategy, GCWPS exhibits enhanced adaptability to the increased output power and phase-locked loop (PLL) bandwidth under weak grid. Finally, experiments are undertaken to evidence the feasibility of the proposed oscillation suppression strategy.

Transient stability enhancement control strategy based on grid dispatching for multi-paralleled grid forming inverters during LVRT

In this article, the transient synchronization process of multi-paralleled grid forming (GFM) inverters is studied detailly. Firstly, based on the multi-paralleled inverters control structure and complementary cluster center of inertia relative motion (CCCOI-RM), the single machine infinite bus (SMIB) system is proposed. Subsequently, the power angle characteristic (PAC) equation of multi-paralleled inverters for different dispatching strategies with simplified damping coefficient is built. Meanwhile, a grid dispatching strategy considering communication delay is proposed, which avoiding the cascading failures of grid faults. Furthermore, according to the grid dispatching strategy, a cluster control strategy optimization method including critical cluster identification (CCI) method and transient stability prediction analysis method is designed, which is proposed to identify unstable cluster in multi-paralleled inverters. In addition, the synchronization stability criteria for dispatching strategies of multi-paralleled inverters are obtained based on the PAC equation, in which the transient instability phenomenon and mechanism for different fault stages is explained by extended equal area criterion (EEAC) method. Finally, the simulations results validate the effectiveness of the theoretical analysis and proposed method.

Assessment and enhancement control for small‐signal voltage stability of VSC‐HVDC systems supplying passive industrial loads

AbstractThis paper presents a comprehensive assessment for the small‐signal voltage stability of voltage source converter (VSC)‐HVDC systems supplying industrial loads represented by the induction motors (IMs) and provides a stability enhancement control for the VSCs. First, the steady‐state voltage stability is analysed to seek the factors that affect the existence of the stable equilibrium point. Then, the small‐signal voltage stability analysis is performed based on the developed linear model. Eigenvalue analysis is carried out to capture the critical factors that induce instability phenomena correlative to the system voltage. It demonstrates that decreasing either the proportional gain K of the VSC's PI controller or the electrical distance between the VSC and IM is detrimental to voltage stability. Next, the enhancement control strategy is elaborated. It adopts virtual reactance and virtual admittance control for the VSC to improve the voltage control capability. Finally, the effectiveness and robustness of the proposed enhancement control are validated via time‐domain simulations in a multi‐IM system supplied by VSC‐HVDC.

Transient stability enhancement control through VSC-HVDC system and fast frequency response resources

In addition to the traditional long-distance and bulk power transmission application, the HVDC system can bring additional benefits to the system by providing ancillary service. The high-voltage direct-current transmission (HVDC) system can regulate its power flow in a very short time and can be used to provide emergency support to rescue the disturbed system from very bad conditions. However, because traditional generators cannot meet the great power demand of the HVDC system in a short time, the fast response of the HVDC system may cause severe power-unbalance events in the power supporting system. In this paper, a combination frequency response control (CFRC) is proposed for the fast frequency response (FFR) and HVDC system coordinating under the contingency. With the proposed combination frequency response control, the most use of the frequency response capacity of the HVDC system could be made. Meanwhile, the coordination between the HVDC system and fast frequency response could prevent the frequency of the power-source-provided system from dropping to an unacceptable level. The simulation study is conducted on a highly reduced power system model of the three North American interconnections. The simulation verification shows the effectiveness and feasibility of the proposed combination frequency response control and indicates that the frequency stability for both side systems of the HVDC system could be significantly improved with the proposed combination frequency response control. In the future, the communication delay between energy storage and HVDC will be considered in the strategy to further improve the accuracy of fast frequency response.

Leader-follower formation of light-weight UAVs with novel active disturbance rejection control

Considering that the formation composed of light-weight UAVs is highly susceptible to the interference, which may from the changes in the external environment and the uncertainty of system, a formation control method based on the inner and outer loops is proposed. In the inner loop, the single UAV stable flight can be achieved via controlling the state variables of UAV angles. In the outer loop, we can achieve multi-UAVs cooperative flight through the leader-follower strategy. In this paper, we mainly focus on the stability control of each member UAV, which is the basic composition of formation control. For this purpose, an optimized active disturbance rejection control (ADRC) is proposed which combined an improved prescribed-time extended state observer (PTESO) and weight optimization module based on broad learning. In this way, the single UAV can dynamically adjust the control weights according to different wind degrees, so that the influence of environmental changes on the control system is reduced. Then, the effectiveness and superiority of the proposed formation control methods are verified by simulations. The stability enhancement control method proposed in this paper provides a new and effective theoretical support for the actual control of the light-weight UAV formation, which has a well engineering application prospect.

Battery and supercapacitor assisted controller for stability enhancement of DC microgrid

AbstractDespite the widespread popularity of DC microgrid (DCMG), one of the key issues faced by it is the instability arising from supply‐demand imbalance, which is conventionally tackled via energy storage units such as batteries. But the life cycle of the battery, which may reduce when subjected to transients, can be prolonged by integrating fast acting supercapacitor (SC) with higher power density. Such a combination of energy storage units enables the DCMG to handle steady state and transient conditions effectively. However, the reduction in the operating voltage of SC may bring in ringing in the DC bus voltage, which will affect DCMG stability. Hence, this paper focuses on analysing the impact of SC voltage deviation on DCMG stability and to propose a controller design procedure with conventional PI controller and power management strategy to mitigate the aforementioned consequences. The simulation in MATLAB and real‐time validation in OP4510 real‐time simulator has been conducted. The results of both revealed that the proposed scheme has succeeded in eliminating ringing of DC bus voltage, irrespective of the SC operating voltage variations, thereby maintaining DCMG stability.

Transient stability enhancement control strategy for droop-controlled voltage source converter

The droop-controlled voltage source converter (VSC) can support the voltage at the point of common coupling (PCC) and realize frequency adjustment by simulating the swing equation of the synchronous generator (SG) and has a higher stability margin in the extremely weak grid. Under large-signal disturbances such as short-circuit faults in the power grid, the active power output of the VSC decreases, and the continuous increase of the power angle causes the VSC to have a power angle instability similar to that of the SG. In addition, in order to ensure the safety of the equipment, the VSC control system contains a current-limiting strategy, which will cause a large change in the transient external characteristics of the VSC under large-signal disturbances. This paper firstly studies the transient synchronization mechanism between VSC and the power grid under grid faults and expounds on the reasons for the transient instability of VSC with the current limiting strategy. Based on the above analysis, a transient enhancement strategy is proposed to reduce the imbalance of active power and reduce the risk of VSC transient instability. It is worth mentioning that the proposed transient control strategy will not affect the steady-state output characteristics of the VSC. Finally, simulation and experiment verify the effectiveness of the proposed control strategy.

DC‐link capacitor current observation based stability enhancement control for back‐to‐back converter

This article presents a stability enhancement control scheme applied in the three-phase back-to-back (BTB) converters with L filter. The stability enhancement control scheme is concocted using the feedback of DC-link capacitor current without additional sensors. Both grid-side converters are equipped with this control scheme, which can also be called the coordinated compensation control scheme. In this article, DC terminal impedance models of two converters before and after optimization and stability criterion are introduced. Furthermore, means of impedance analysis, including Bode plot and Nyquist curve, are discussed to categorize the causes of BTB converter instability caused by the increased power. The coordinated scheme is compared with the conventional scheme for revealing the theoretical basis. The proposed method is capable of optimizing the disturbance suppression capability of DC-link voltage, which would potentially reduce capacitor volume and physical size of the DC-link. The impedance-based stability analysis indicates that the controllability and stability of the BTB converter is enhanced with the proposed DC-link capacitor current coordinated control. Furthermore, the proposed control scheme is implemented in the MATLAB/Simulink environment. Moreover, for further validation, the feasibility of the coordinated control scheme is verified with the experimental results.

Comprehensive transient stability enhancement control of a VSG considering power angle stability and fault current limitation

Comprehensive transient stability enhancement control of a VSG considering power angle stability and fault current limitation

A review of STATCOM control for stability enhancement of power systems with wind/ PV penetration: Existing research and future scope

In recent years, the development of renewable energy sources (RESs) and their integration with the conventional power network have increased significantly. Due to the power grid transformation brought about by the large penetration of power electronics converter-based RESs such as wind and solar photovoltaic (PV), the operation and control of the interconnected power system has become a challenging task with respect to sustaining its stability and reliability. Power systems are confronted by several new stability issues because the dynamic behavior of converter-interfaced renewable sources differs from conventional generation. In this respect, static synchronous compensator (STATCOM), a shunt connected flexible AC transmission system (FACTS), is recognized as a fundamental solution for maintaining power system stability. This paper presents a thorough and state-of-the-art review of STATCOM control in wind- and/or PV-interfaced power systems for enhancing system performance by addressing key stability issues related to rotor angle stability, voltage stability, and resonance stability. A comprehensive analysis of various control techniques of STATCOM based on conventional, adaptive, nonlinear, predictive, robust, and coordinated control and soft computing techniques is provided. Furthermore, the function of the grid-side converter of the wind/PV plant as a STATCOM is also evaluated. Finally, the shortcomings of existing research are highlighted, current control challenges are presented, and several topics for future research are suggested. This paper provides researchers the opportunity to consider the current state of research and develop new control schemes for STATCOM to further improve the stability of modern power grids.

Transient Stability Enhancement Control Strategy Based on Improved PLL for Grid Connected VSC during Severe Grid Fault

In this article, the transient synchronization process of the grid-connected voltage source converters (VSC) is studied detailly. Firstly, the phase-locked loop (PLL)-synchronized VSC is modeled according to the rotor motion equation of synchronous generator (SG). Furthermore, the VSC's damping ratio is derived, and the effects of the VSC's control parameters and the operation status on the equivalent power angle (EPA) is investigated. Moreover, the influence of the PLL's parameters and the voltage dips degree on the VSC's transient frequency behavior are analyzed. Analysis result reveals that the VSC's damping ratio decreases once the grid voltage drops, so that the EPA and frequency of the VSC may appear large overshoot and oscillation, which may trigger the frequency protection and deteriorate the VSC's transient stability. In order to ensure that the VSC can smoothly operate to new equilibrium points, an improved PLL is proposed, which can adaptively adjust the VSC's damping ratio for different voltage sags. The proposed method can not only reduce the overshoot of the EPA but also restrain the VSC's frequency dips degree. Finally, the simulations and experimental tests validate the effectiveness of the theoretical analysis.

Impedance-Based Modeling and Common Bus Stability Enhancement Control Algorithm in DC Microgrid

In this paper, impedance modeling of a DC microgrid system consisting of a source and load converter, including an input filter, is performed. Impedance-based modeling has been used to derive mathematical models of the output impedance of the source converter and the input impedance of the load converter. The correlation between the converter interaction and system stability is analyzed based on the mathematical model. An impedance-based stability analysis is used to determine the system stability by analyzing the interactions among the converters in the DC microgrid system. Middlebrook’s stability criterion, which uses the impedance transfer function, is applied to determine system stability. Moreover, in this paper, a stability enhancement control algorithm is proposed to resolve the system instabilities resulting from interaction among the converters and the distortion caused by the harmonics emanating from the AC input. The proposed stability enhancement control algorithm consists of a feed-forward type virtual impedance (VI) and a proportional-resonant (PR) controller. The validity of the proposed method is demonstrated by the results of the response characteristics in the frequency domain, and the effectiveness of the proposed control algorithm is verified via simulations and prototype experimental models.

Sliding-mode perturbation observer-based sliding-mode control design for stability enhancement of multi-machine power systems

This paper presents the design of a sliding-mode perturbation observer-based sliding-mode control for stability enhancement of multi-machine power systems. The combinatorial effect of nonlinearities, parameter uncertainties, unmodelled dynamics and time-varying external disturbances is aggregated into a perturbation, which is rapidly estimated by a sliding-mode state and perturbation observer and then fully compensated by a sliding-mode controller in real time. The attractiveness of the sliding surface is analysed theoretically in the context of the Lyapunov criterion. The proposed control does not require an accurate system model and only one state measurement is needed. In addition, an over-conservative control effort can be effectively avoided via perturbation compensation. Simulation results for a three-machine power system and the New England power system verify the effectiveness of the proposed approach.

A System Transient Stability Enhancement Control Method Using Doubly Fed Induction Generator Wind Turbine with Considering Its Power Constraints

With the rapid development of wind power, the effects of doubly fed induction generators (DFIGs) on the transient stability of power system have attracted more attention. However, the effects are still not clear due to the lack of deep and theoretical analysis. A control method for enhancing system rotor angle stability enhancement is proposed with considering the power constraints of DFIG. The rotor angle oscillation of the synchronous generator (SG) can be reduced and overspeed and the overcurrent of DFIG can be avoided. In this paper, the effects of the DFIG on the rotor angle characteristics of SG are analyzed. The change law of system rotor angle varying with transient power of the DFIG is obtained. Then, the power equation in the emergency pitch control process is explored. The reactive power constraints of the stator and grid-side converter are deduced. Finally, the system rotor angle stability enhancement control method is proposed based on the power constraints of DFIG. The effectiveness of proposed method is proven by simulations.

Robust tuning of excitation controller for stability enhancement using multi-objective metaheuristic Firefly algorithm

Abstract The proposed approach focuses on investigating the optimum values of Power System Stabilizer (PSS) parameters by the implementation of Firefly algorithm (FFA) based optimization technique. It minimizes the low frequency oscillations such that both maximum overshoot and settling time are reduced simultaneously, since the reduction of both these parameters will considerably improve the stability of the power system. In this paper, eigenvalue and overshoot based multi objective function is used to enhance damping of electromechanical oscillations in the system. Firstly, the conventional lead-lag structure of PSS, which has its design based on phase compensation technique, was applied to the systems under study. Then, Firefly optimization technique is implemented on three different standard test systems and a comparative analysis is carried out with the classical techniques (under the disturbances). Moreover, the performance of FFA tuned PSS is also compared with PSS tuned using Genetic algorithm (GA). Based on the simulations, it is seen that Firefly optimization technique based PSS converges faster as compared to conventional PSS and GAPSS. Thus, the implementation and evaluation of firefly algorithm has emerged as an evolving platform and can be considered as a very impressive catalytic method to tune the PSS parameters.

Distributed Optimal Control for Stability Enhancement of Microgrids With Multiple Distributed Generators

The distributed secondary control in a microgrid can be used for complementing the function of the primary droop-based control. However, its dynamic performance may be undesirable and furthermore it may introduce new less-damped modes to the system leading to oscillatory responses. Unfortunately, mechanism analysis of the undesirable dynamic performance and the possible oscillations, and more importantly, stabilization of the microgrid with the distributed secondary control have not been reported. To fill this gap, this paper first develops a unified small-signal dynamic model of the microgrid. Based on the developed model, a small-signal stability analysis is utilized to perform the aforementioned mechanism analysis. A distributed optimal controller is thereafter proposed to enhance the system stability and improve the system dynamic performance. The distributed optimal controller can coordinate multiple distributed generation units in the microgrid and exhibits robust performance under a wide range of operating conditions. Finally, theoretical analysis and time-domain simulation results on a benchmark microgrid system are provided to verify the effectiveness of the proposed methods.

Design and Implementation of Delay-Dependent Wide-Area Damping Control for Stability Enhancement of Power Systems

In this paper, the hardware and software design and implementation are proposed for the wide-area damping control (WADC) strategies. First, the hardware-in-the-loop (HIL) real-time (RT) simulation system based on the RT-Lab is established as the closed-loop test platform for the WADC applied in multi-machine interconnected power system. The hardware, which contains the microcontroller, the peripheral analog to digital converter, and the digital to analog converter, are developed based on the AVR 32-bit development board. Three WADC algorithms, i.e., the phase-compensation, the delay-dependent state-feedback, and the dynamic output-feedback method, are introduced. Then, the software design of the above algorithms is proposed and the bilinear transform method is employed to get the discrete-time model of the controllers. The flowchart of each algorithm is designed for the implementation in the environment of hardware. The designed WADC is embedded in a real-time power system. Finally, case studies are carried out based on the HIL real-time simulation system. The proposed control concept and WADC design methods are validated by considering the big/small disturbance and the signal time-varying delay. The effectiveness of the proposed HIL closed-loop test system is also confirmed.

Transient stability enhancement control of power systems with time‐varying constraints

Transient control performance of power systems is extremely important, especially during the recovery stage from a system temporary fault. A large power angle error or generator relative speed could result in unrecoverable failure or system damage. In this study, a class of novel excitation controllers is proposed for power systems to place user-defined time-varying constraints on the system errors. First, the direct feedback linearisation technique is employed to transfer the original non-linear system to a linear one. An error transformation method is introduced to generate a new non-linear system, whose stability ensures that the time-varying constraints on the original system are satisfied. Thus, a backstepping-based excitation control methodology is designed for ideal case by assuming all system parameters are known. Thereafter, the uncertainties of system dynamics are also considered and compensated by designing an adaptive excitation controller. By utilising the standard Lyapunov synthesis, the closed-loop system is proved to be asymptotically stable, and all the other signals are bounded for the two control schemes. Furthermore, the system transient performance can be shaped arbitrarily with predefined trajectories, in the sense that the system errors never cross certain boundaries during transient phase. Eventually, extensive simulation studies demonstrate the feasibility of the theoretical results.

Delay-dependent wide-area damping control for stability enhancement of HVDC/AC interconnected power systems

In this paper, a delay-dependent wide-area damping control strategy associated with a high-voltage direct current (HVDC) supplementary controller is presented to enhance the overall stability of parallel HVDC/AC interconnected systems. First, the HVDC/AC interconnected systems are linearized into a nominal delay-dependent linear model using varying-delay characteristics. Next, the robust H∞ design approach is used to solve the optimal state-feedback-type controller parameters. All of the controller design problems are formulated in the framework of linear matrix inequalities (LMIs), which is convenient for converting the controller design problem into a standard nonlinear optimization problem represented as an optimization objective with a set of LMIs constraints. An iterative algorithm is presented to solve the optimization problem. The effects of the varying-delay of the wide-area signal on the damping performance are considered during the controller design. Additionally, the state observer is used to convert the state-feedback control into an output-feedback control that is convenient for practical applications. Two case studies are conducted on the Kundur׳s 4-machine 2-area system and the New England/New York interconnected system to validate the presented control concept and the designed HVDC wide-area damping controller.