Robust control for voltage and transient stability of power grids relying on wind power

Common practice in stabilization of power grids is to refer to different stability categories (transient stability, voltage stability, rotor angle stability) and to address these by designing dedicated controllers separately based on models linearized around nominal operation points. Furthermore, the controllers of a generating unit contained in the grid are usually synthesized without considering other grid nodes. This work, in contrast, proposes a scheme for unified synthesis of controllers which conjunctively address rotor angle stability and voltage stability for grids containing synchronous generators as well as wind energy conversion systems based on doubly-fed induction generators. First, a procedure is proposed to describe the generating units by linear-parameter-varying (LPV) systems, in which fluctuations imposed by the grid or the wind are mapped into time-varying model parameters. For appropriate ranges of these parameters, decentralized robust controllers can be synthesized by semidefinite-programming, such that the power grid is stabilized for the considered fluctuations and disturbances. The effectiveness of the approach is demonstrated for a multi-bus benchmark system, where the grid oscillations are well damped and the LPV-controller stabilizes the grid after permanent changes.     

A two-level emergency control scheme against power system voltage instability

A two-level adaptive control scheme against power system voltage instability is proposed, to deal with emergency conditions by acting on distribution transformers and/or by curtailing some loads. The lower level includes distributed controllers, each acting once the voltage at a monitored transmission bus settles below a threshold value. The upper level benefits from wide-area monitoring and adjusts in real-time the voltage thresholds of the local controllers. Emergency detection is based on the sign of sensitivities. The proposed scheme is robust with respect to communication failures. Its performance is illustrated through detailed simulations of a small but realistic 74-bus test system.     

Periodic event-triggering in distributed receding horizon control of nonlinear systems

How to efficiently use limited system resources in distributed receding horizon control (DRHC) is an important issue. This paper studies the DRHC problem for a class of dynamically decoupled nonlinear systems under the framework of event-triggering, to efficiently make use of the computation and communication resources. To that end, a distributed periodic event-triggered strategy is designed and a detailed DRHC algorithm is presented. The conditions for ensuring feasibility of the designed algorithm and stability of the closed-loop system are developed, respectively. We show that the closed-loop system is input-to-state stable if the energy bound of the disturbances, the triggering condition and the cooperation matrices fulfill the proposed conditions.     

Global stabilisation via switching output-feedback for power integrator systems with unknown control direction

This paper considers the global output-feedback stabilisation for power integrator systems with unknown control direction. The presence of uncontrollable and unobservable linearisation renders the strategy based on Nussbaum function rather difficult (even impossible) to compensate the unknown control direction. In this paper, to dominate the inherent nonlinearities and unknowns, a powerful switching strategy is proposed by combining an output-feedback scheme and a switching mechanism. First, a special case with known control direction is considered to provide a basic structure and selection rules of design parameters for the expected controller, and then, the essential case with unknown control direction is investigated to build a suitable switching logic to online tune the design parameters. The proposed switching-type output-feedback controller guarantees that all the system signals are globally bounded and ultimately converge to zero. A numerical example is given to illustrate the effectiveness of the proposed methods.     

Modeling and control of discrete event systems using finite state machines with variables and their applications in power grids

Control theories for discrete event systems modeled as finite state machines have been well developed to address various fundamental control issues. However, finite state machine model has long suffered from the problem of state explosion that renders it unsuitable for some practical applications. In an attempt to mitigate the state explosion problem, we propose an efficient representation that appends finite sets of variables to finite state machines in modeling discrete event systems. We also present the control synthesis techniques for such finite state machines with variables (FSMwV). We first present our notion and means of control under this representation. We next present our algorithms for both offline and online synthesis of safety control policies. We then apply these results to the control of electric power grids.     

Nonlinear control design for excitation controller and power system stabilizer

The paper focuses on the design of excitation controller and power system stabilizer (PSS) control schemes to enhance voltage regulation and transient stability. To design the excitation controller, the nonlinear models of the synchronous generator and electrical system are suitably arranged to give an input-output nonlinear model, which is subsequently made linear by a compensating law. The PSS control scheme is designed according to a new approach, based on the inverse system theory, which guarantees satisfactory swing damping by reducing the mutual influence between transient stability and voltage regulation. The results obtained by numerical simulations in different operating conditions confirm the effectiveness of the proposed design, also in the presence of model parameter inaccuracy.     

Soft-computing combining of distributed power control algorithms

In traditional distributed power control (DPC) algorithms, every user in the system is treated in the same way, i.e., the same power control algorithm is applied to every user in the system. In this paper, we divide the users into different groups depending on their channel conditions and use different DPC accordingly. Our motivation comes from the fact that different DPC algorithms have its own advantages and drawbacks, and our aim in this paper is to “combine” the advantages of different DPC algorithms, and we use soft computing techniques for that. In the simulations results, we choose Foschini and Miljanic Algorithm in [3], which has relatively fast convergence but is not robust against time-varying link gain changes and CIR estimation errors, and fixed step algorithm of Kim [3], which is robust but its convergence is slow. By “combining” these two algorithms using soft computing techniques, the resulting algorithm has fast convergence and is robust. Acknowledgments This work was supported in part by GETA (Finnish Academy Graduate School on Electronics, Telecommunications and Automation), Finland.     

Coordination and control for energy distribution in distributed grid networks: Theory and application to power dispatch problem

This paper presents a new framework considering decentralized energy coordination and generation, and flow control for supply–demand balance in distributed grid networks. Consensus schemes using only local information are employed to produce energy coordination, generation, and flow control signals. For the supply–demand balance, it is required to determine the amount of energy needed at each distributed resource. Also, due to the different generation capacities of each energy resource, coordination of energy flows among distributed energy resources is essentially required. Thus, this paper proposes a new framework which gives decentralized energy coordination scheme, generation, and flow control method considering these constraints based on distributed consensus algorithms. The proposed framework in this paper can be nicely utilized in energy dispatch or energy flow scheduling. Furthermore, it can be applied to various engineering problems including water irrigation systems, traffic networks, and building automation systems since it deals with attributed distribution and resource allocation in large scale distributed systems. Through illustrative examples, the effectiveness of the proposed approaches is illustrated. A possible application to power dispatch problem in the IEEE-14bus is also addressed for more detailed and realistic evaluation.     

Distributed model predictive control of constrained nonlinear systems with communication delays

Distributed model predictive control (MPC), having been proven to be efficient for large-scale control systems, is essentially enabled by communication network connections among involved subsystems (agents). This paper studies the distributed MPC problem for a class of continuous-time decoupled nonlinear systems subject to communication delays. By using a robustness constraint and designing a waiting mechanism, a delay-involved distributed MPC scheme is proposed. Furthermore, the iterative feasibility and stability properties are analyzed. It is shown that, if the communication delays are bounded by an upper bound, and the cooperation weights and the sampling period are designed appropriately, the overall system state converges to the equilibrium point. The theoretical results are verified by a simulation study.     

Current control based power management strategy for distributed power generation system

The integration of renewable energy sources (RES) based distributed generation (DG) systems into electric grid has many challenges such as synchronization, control, power management (PM) and power quality problems. This paper proposes a references current generator (RCG) based PM strategy to control three phase inverter and manage power flow among the DG energy sources, electric grid and load demand under balanced and unbalanced grid conditions. The amplitudes of active and reactive power oscillations are also eliminated and controlled through only one flexible control parameter (FCP) under grid faults and harmonic distortions. Compared with previous similar studies, one of the important contributions is capable to inject maximum active power and minimum reactive power into electric grid and load at inverter power capacity under grid faults Another contribution is to extract the positive and negative sequence (PNS) voltage and current components with the improved fast and robust dual adaptive filters based phase locked loop (DAF-PLL). Fractional order proportional integral (FOPI) is selected as an attractive solution for AC current regulation to exhibit fast transient response and to achieve zero steady-state errors as compared with conventional current regulation controllers in synchronous or stationary frames.     

Designing a resource broker for heterogeneous grids

Grids provide uniform access to aggregations of heterogeneous resources and services such as computers, networks and storage owned by multiple organizations. However, such a dynamic environment poses many challenges for application composition and deployment. In this paper, we present the design of the Gridbus Grid resource broker that allows users to create applications and specify different objectives through different interfaces without having to deal with the complexity of Grid infrastructure. We present the unique requirements that motivated our design and discuss how these provide flexibility in extending the functionality of the broker to support different low‐level middlewares and user interfaces. We evaluate the broker with different job profiles and Grid middleware and conclude with the lessons learnt from our development experience. Copyright © 2007 John Wiley & Sons, Ltd.     

Distributed receding horizon control of large-scale nonlinear systems: Handling communication delays and disturbances

This paper studies the robust distributed receding horizon control (DRHC) problem for large-scale continuous-time nonlinear systems subject to communication delays and external disturbances. A dual-mode robust DRHC strategy is designed to deal with the communication delays and the external disturbances simultaneously. The feasibility of the proposed DRHC and the stability of the closed-loop system are analyzed, and the sufficient conditions for ensuring the feasibility and stability are developed, respectively. We show that: (1) the feasibility is affected by the bounds of external disturbances, the sampling period and the bound of communication delays; (2) the stability is related to the bounds of external disturbances, the sampling period, the bound of communication delays and the minimum eigenvalues of the cooperation matrices; (3) the closed-loop system is stabilized into a robust invariant set under the proposed conditions. A simulation study is conducted to verify the theoretical results.     

智能电网中电力线通信网络负载均衡的机会路由协议

针对智能电网（SG）中电力线通信（PLC）网络中负载均衡的问题,提出了一个自适应机会路由协议——负载均衡的机会路由协议（LBORP）。在LBORP中,所有收到数据包的候选转发节点都有机会参与到数据包的转发中,不再局限于一条路由路径,避免了流量仅从一条链路经过导致的负载不均衡现象;而且候选转发节点的转发优先级不仅考虑到转发节点到目的节点的距离,还考虑到了PLC链路的不稳定性以及流量的变化。除此之外,在LBORP中采用一种隐式确认方案,进一步减少协议的端到端时延。在仿真实验中,与基于有序树的PLC路由协议（PLC-TR）和PLC机会路由协议（PLC-OR）相比,LBORP在时延上分别降低了19.7%和45.8%,在丢包率上分别降低了23.4%和32.5%。实验结果表明,LBORP能够实现网络的负载均衡,提升网络的可靠性并减小端到端时延。     

Global control with application to bifurcating power systems

A multilevel nonlinear control scheme that is capable of system control over wide ranges of operating conditions is proposed. The first control level confines the system operation to some reference segments in its state space. Each reference segment is further subdivided using the bifurcation analysis for establishing the bounds of the state space of the system with different control requirements. The principles of global control lead to a hierarchical control structure which combines regional (linear or nonlinear) controllers smoothly. The scheme is illustrated for control of a power system known to have diverse dynamical behavior. The control will accomplish desired steady-state (fixed point or small oscillation) operation of the power system for a wide range of loading levels.     

Adaptive passivity-based control for maximum power extraction of stand-alone windmill systems

This paper addresses the high performance regulation of stand-alone windmill systems consisting of a wind turbine coupled to a generator and a battery charging system, which is a challenging problem for at least two reasons. First, the dynamics of the overall system are described by a highly coupled set of nonlinear differential equations. Since the range of operating points of the system is very wide, classical linear controllers yield below par performances. Second, in many applications it is desirable to extract from windmill systems their maximum power. This operating point is a nonlinear function of the wind speed, which is hard to measure. In this paper, a nonlinear passivity-based controller that ensures asymptotic convergence to the maximum power extraction point, which is rendered adaptive combining it with a wind speed estimator previously proposed by the authors, is proposed. Detailed computer simulations are presented to validate the approach.     

Limitation control procedures, required for power plants and power systems: Possibility for reducing future blackouts

For maintaining the reliability of interconnected power systems also in cases of extreme power transit the protective system devices should not become active whenever possible.For this reason, it is necessary to provide decentral automatic limitation control procedures, and this not only on the side of power plants, where it has been common practice for many years to keep at least the main manipulated and controlled variables within their foreseen operation areas, but also within the grid to contain the transmission flows of heavily overloaded system components within their maximum allowable transmission capacity. In this respect, manual short-term bottleneck management on the system operation side turns out to be not sufficient enough, in the case of unexpected events caused by increased power transits.