基于SVG的风电场接入局域电网的电压稳定性分析

研究了用静止无功发生器(SVG)改善基于双馈感应发电机组的风电场的暂态电压稳定性。在DIgSILENT/PowerFactory中建立了双馈感应发电机组及SVG控制模型,通过包含风电场的电力系统仿真,验证了SVG对风电场暂态电压稳定性的作用。仿真结果表明,SVG能够有效地帮助风电场在电网发生故障后迅速恢复电压,提高风电场的故障穿越能力,确保风电机组连续运行及电网安全稳定。     

Robust grid‐oriented control of high voltage DC links embedded in an AC transmission system

Several studies have shown that the way to design controllers for the high‐voltage direct current (HVDC) links impacts the transient behavior of the electric system in which the latter are inserted. This can be exploited to improve the performances of the stability of the power system. In this paper, a robust multivariable control design for the converters of an HVDC link is proposed. It is based on the coordination of the control actions of the HVDC converters and the use of a control model. The latter takes into consideration, in addition to the dynamics that mostly impact the stability of the neighbor zone of the HVDC link, several cases of faulted situations modeled as uncertainties. An H_∞ controller allowed us to achieve robustness against such uncertainties. The new controller is tested in comparison with the standard vector control and an optimal linear quadratic controller using the EUROSTAG simulation software (Tractebel Engineering, Brussels, Belgium and Réseau de Transport d'Electricité (RTE) ‐ France) on both academic and realistic large‐scale power systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Stability analysis of the drive‐train of a wind turbine with quadratic torque control

This paper investigates the stability of a variable‐speed wind turbine operating under low to medium wind speed. The turbine is controlled to capture as much wind energy as possible. We concentrate on the mechanical level of the turbine system, more precisely on the drive‐train with the standard quadratic generator torque controller. We consider both the one‐mass and the two‐mass models for the drive‐train, with the inputs being the deviation of the active torque from an arbitrary positive nominal value and the tracking error of the generator torque. We show that the turbine system is input‐to‐state stable for the one‐mass model and integral input‐to‐state stable for the two‐mass model. Copyright © 2008 John Wiley & Sons, Ltd.     

Stable reactive power balancing strategies of grid‐connected photovoltaic inverter network

In this paper, a distributed reactive power control based on balancing strategies is proposed for a grid‐connected photovoltaic (PV) inverter network. Grid‐connected PV inverters can transfer active power at the maximum power point and generate a certain amount reactive power as well. Because of the limited apparent power transfer capability of a single PV inverter, multiple PV inverters usually work together. The communication modules of PV inverters formulate a PV inverter network that allows reactive power to be cooperatively supplied by all the PV inverters. Hence, reactive power distributions emerge in the grid‐connected PV inverter network. Uniform reactive power distributions and optimal reactive power distributions are considered here. Reactive power balancing strategies are presented for both desired distributions. Invariant sets are defined to denote the desired reactive power distributions. Then, stability analysis is conducted for the invariant sets by using Lyapunov stability theory. In order to validate the proposed reactive power balancing strategies, a case study is performed on a large‐scale grid‐connected PV system considering different conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

A sliding mode control and artificial neural network based MPPT for a direct grid‐connected photovoltaic source

A robust sliding mode controller for a grid‐connected photovoltaic source is proposed in this paper. The objective of the presented control scheme is to force both the output voltage of the photovoltaic PV source and the power factor at the inverter output to follow a certain trajectory reference. The main idea is to apply the robust sliding mode controller directly to the nonlinear state model of the system composed of the PV source and the inverter with its input and output filters. In order to operate the PV system at the maximum power point and to satisfy the environmental factors, such as solar irradiance and temperature, we included a rigorous maximum power point tracker based on an artificial neural network. Simulation results are presented to illustrate the performance of the proposed control scheme. In addition, we show that the grid current satisfies the harmonic limits of the IEEE standard for interconnecting distributed energy sources with electric power systems.     

Reduced‐order design of high‐order sliding mode control system

To design an rth (r>2) order sliding mode control system, a sliding variable and its derivatives of up to (r ? 1) are in general required for the control implementation. This paper proposes a reduced‐order design algorithm using only the sliding variable and its derivatives of up to (r ? 2) as the extension of the second‐order asymptotic sliding mode control. For a linear time‐invariant continuous‐time system with disturbances, it is found that a high‐order sliding mode can be reached locally and asymptotically by a reduced‐order sliding mode control law if the sum of the system poles is less than the sum of the system zeros. The robust stability of the reduced‐order high‐order sliding mode control system, including the convergence to the high‐order sliding mode and the convergence to the origin is proved by two Lyapunov functions. Simulation results show the effectiveness of the proposed control algorithm. Copyright © 2010 John Wiley & Sons, Ltd.     

Practical stability analysis of sliding‐mode control with explicit computation of sampling time

Sliding‐mode control is a wide‐spread approach to a number of practical problems. The classical stability analyses of sliding‐mode control had to face the difficulty of defining a trajectory of a system whose dynamics are discontinuous in the state variable. Different generalizations of the system trajectory were suggested, such as Filippov solutions. Another generalization is based on the sample‐and‐hold framework, where the system dynamics are in continuous time and the control is changed only at certain discrete times. The current work addresses the sample‐and‐hold stability analysis of sliding‐mode control with special attention to an explicit computation of the required controller sampling time. A computational example is provided.     

Control‐oriented modeling and robust nonlinear triple‐step controller design for an air‐feed system for polymer electrolyte membrane fuel cells

The efficient operation of polymer electrolyte membrane fuel cells (PEMFCs) significantly relies on the reliable control of air‐feed system. The core control objective in air‐feed system is to track a pre‐defined reference of the oxygen excess ratio to avoid oxygen starvation and stack damage. In this paper, we focus on the modeling of the air‐feed system in a PEMFC and the robust nonlinear controller design for the oxygen excess ratio tracking control. To facilitate the subsequent nonlinear controller design, a specific affine‐like, second‐order, control‐oriented model of oxygen excess ratio dynamic behavior is developed, and the modeling uncertainty is estimated and compensated by using an extended state observer (ESO). The control‐oriented model is verified via a high‐fidelity plant model. A nonlinear controller for oxygen excess ratio tracking control is proposed based on the triple‐step technique which is robust against the system disturbances. The tuning rule of the controller parameters is discussed in the scheme of the linear system. Finally, simulations are conducted to demonstrate the effectiveness and advantages of the proposed controller under variant operating conditions compared with baseline controllers.     

Robust FOPID controller design for fractional‐order delay systems using positive stability region analysis

In this paper, a robust fractional‐order PID (FOPID) controller design method for fractional‐order delay systems is proposed based on positive stability region (PSR) analysis. Firstly, the PSR is presented to improve the existing stability region (SR) in D‐decomposition method. Then, the optimal fractional orders λ and μ of FOPID controller are achieved at the biggest three‐dimensional PSR, which means the best robustness. Given the optimal λ and μ, the other FOPID controller parameters k_p, k_i, k_d can be solved under the control specifications, including gain crossover frequency, phase margin, and an extended flat phase constraint. In addition, the steps of the proposed robust FOPID controller design process are listed at length, and an example is given to illustrate the corresponding steps. At last, the control performances of the obtained robust FOPID controller are compared with some other controllers (PID and FOPI). The simulation results illustrate the superior robustness as well as the transient performance of the proposed control algorithm.     

Stability analysis of interconnected nonlinear fractional‐order systems via a single‐state variable control

This paper is devoted to investigating the stability of interconnected nonlinear fractional‐order systems via a single‐state variable control. First of all, based on stability theory, the Grönwall‐Bellman lemma and the Mittag‐Leffler function, the relevant stability results are derived. The obtained results are general and can further extend the application range. Meanwhile, an improved single‐state variable control method is introduced. The control scheme only needs to control some state variable of the system or some subsystem(s) to realize and any additional restrictions are not added. Finally, the effectiveness of the obtained results is demonstrated by several typical examples. Besides, by comparison, simulation results show that the proposed control method can indeed decrease the design and control cost and improve flexibility of control.     

An improved model predictive control of low voltage ride through in a permanent magnet synchronous generator in wind turbine systems

This research investigates a wind energy conversion system based on a permanent magnet synchronous generator (PMSG). In addition, a model predictive control (MPC) is proposed for the PMSG in normal and fault conditions. The most efficient mode of the control algorithm is found for maximum power point tracking in normal conditions and fast dynamic response in fault conditions following the selection of the optimum voltage vector. This method prevents a sudden increase in the DC‐link voltage by storing the active power in the generator rotor inertia. Moreover, during the low voltage, the grid code adoption of the reactive current is injected into the grid side. The performance of the proposed control scheme is evaluated for a wind power generator using MATLAB software. The simulation results demonstrate that the proposed method can safeguard the DC‐link during the fault.     

Robustness analysis of systems' safety through a new notion of input‐to‐state safety

In this paper, we propose a new robustness notion that is applicable for certifying systems' safety with respect to external disturbance signals. The proposed input‐to‐state safety notion allows us to certify systems' safety in the presence of the disturbances, which is analogous to the notion of input‐to‐state stability for analyzing systems' stability.     

Circumnavigation of a speedy unpredictable target by a group of speed‐ and acceleration‐limited robots

An unpredictably maneuvering speedy target travels in a plane, which also hosts a team of fully actuated robots whose velocities and accelerations are upper‐bounded in magnitude. The robots should approach the target and then follow it at a prespecified distance. They also should achieve an even self‐distribution around the target and a given angular velocity of rotation about the target. Every robot has access to the relative position of the target and other robots (in the latter case, within a finite “visibility” range) and to the angular speed of its own pure rotation; access to the coordinates of its own linear velocity in its own local frame is also employed in some cases. The robots are not equipped with communication facilities and cannot distinguish among one another; assignment of different roles to various robots is infeasible. Necessary conditions for the mission feasibility are first obtained. A distributed control law is then presented, and its global convergence and collision avoidance property are rigorously justified under slight enhancement of the just mentioned necessary conditions. The performance of the control law is illustrated by computer simulations.     

Analysis and design of chattering‐free discrete‐time sliding mode control

In this paper, the disturbance observer–based chattering‐free discrete‐time sliding mode control (DSMC) approach is proposed for systems with external disturbances. The proposed disturbance observer, which makes full use of the state and input information at the current and last steps, improves the estimation accuracy and achieves accurate compensation for disturbances. Then, with the help of disturbance observer, a new reaching law, which contains not only a nonsmooth term with a dynamically adjusted gain parameter but also a second order difference of the disturbance, is proposed to reduce the range of the quasi‐sliding mode band and eliminate chattering. The proposed DSMC approach realizes the active disturbance rejection and strong robustness. Finally, a simulation example is presented to verify the effectiveness of the proposed method.     

Robust non‐fragile approach to resilient design of PID‐based blade pitch control for wind energy conversion system

The design of a blade pitch controller (BPC) for wind energy conversion system (WECS) applications is load‐dependent and has to be adjusted for each operating condition. Thus, BPC robustness is important for coping with the endless variations in operating conditions. The boundaries of a robust stability region are determined in regards to the controller parameters plane using their relevant set of polynomial inequalities via Referential Integrity between Routh‐Hurwitz criterion and Root‐Locus (RI‐RH/RL) approach. Constrained and unconstrained stability regions respectively are defined through a novel hybrid control technique based on the combination of both RI‐RH/RL and Kharitonov (Kh) theorem. The hybrid RI‐Kh method is used for globally analyzing all vertex plants to ensure the proposed controller robustness, non‐fragility, and resilience by selecting its parameters at the center of the robust stability region. The optimal BPC‐PID parameters estimated using different optimization techniques are always located within the specified stability region. Thus, the capability of the RI‐Kh approach in determining the most robust, non‐fragile and resilient controller is verified. Through simulation results, the effectiveness of the proposed approach and its applicability to WECS' global stabilization are validated.     

Stability analysis and H∞ control design for a class of nonlinear time‐delay systems

This paper investigates the stability and H_∞ control problem for a class of nonlinear time‐delay systems with a nonsingular Jacobian matrix, and provides a number of new results regarding stability analysis and control design. Firstly, an equivalent form is obtained for this class of systems by means of coordinate transformation and/or orthogonal decomposition of vector fields. Then, based on the equivalent form and free‐weighting matrix method, several sufficient conditions, in terms of nonlinear matrix inequalities, are derived for the stability analysis of the time‐delay systems by constructing suitable Lyapunov functionals. Finally, we use the equivalent form and the obtained stability results to investigate the H_∞ control problem, and present a control design procedure for this class of time‐delay systems. A study of illustrative examples shows that the results obtained in this paper have less conservatism, and work very well in the stability analysis and control design of some nonlinear time‐delay systems. Copyright © 2010 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society     

Two relay controller for real time trajectory generation and its application to inverted orbital stabilization of inertia wheel pendulum via quasi‐continuous HOSM

A quasi‐continuous high‐order sliding mode (QC‐HOSM) control is developed to solve the tracking control problem for an inertia wheel pendulum. A first step towards the solution of the tracking control problem in underactuated systems is to find the set of reference trajectories. A reference model based on the two relay controller idea is then developed for generating a set of desired periodic trajectories for the pendulum centered at its upright position. The two relay controller produces oscillations at the scalar output of the reference underactuated system where the desired amplitude and frequency are reached by choosing its gains. The HOSM will be capable of making the pendulum move, tracking the prescribed reference signals determined by the trajectory generator. Performance issues of the controller constructed are illustrated in an experimental study. Copyright © 2011 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society     

Stability Analysis and Controller Design of Discrete Interval Type‐2 Fuzzy Systems

This paper is concerned with trajectory stabilization of a computer simulated model car with uncertain velocity via type‐2 fuzzy control systems. First, stability conditions of discrete interval type‐2 fuzzy control systems are given in accordance with the definition of stability in the sense of Lyapunov. Then, we approximate a computer simulated model car, whose dynamics are nonlinear and velocity is uncertain. A type‐2 Takagi–Sugeno TS fuzzy controller is designed to handle system uncertainty. The control rules, which guarantee stability of the system, are derived from the approximated model. The simulation results show that the type‐2 fuzzy control rules can effectively stabilize the car model.     

Stability analysis and output‐feedback stabilization of discrete‐time systems with a time‐varying state delay and nonlinear perturbation

This paper aims to derive stability conditions and an output‐feedback stabilization method for discrete‐time systems with a time‐varying state delay and nonlinear perturbation. With a new way of handling the Lyapunov stability criterion, linear matrix inequality conditions are obtained for estimating bounds on delay to ensure the asymptotic stability. Based on the conditions, a synthesis procedure is developed for finding stabilizing output‐feedback gains, which are formulated as direct design variables. Three numerical examples are employed to demonstrate the effectiveness and advantages of the proposed method. Copyright © 2010 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society