Nonlinear large disturbance attenuation controller design for the power systems with STATCOM
Introduction
With the development of power industry, one of the tremendous achievements is the proposal of the flexible AC transmission system (FACTS) and its applications. The static synchronous compensator (STATCOM), as a member of the core devices and technology of FACTS, is used to handle the reactive power compensation, maintain the connection point voltage, enhance the stability of system voltage, and improve the steady and transient state performance of the power system. However, the nonlinear model of STATCOM system is quite complicated, which is sensitive to the influence of external disturbances, parameters changes, interconnected terms between subsystems, and residual nonlinearities. Moreover, the uncertain factors and the burst larger interference possess the unpredictability. The conventional STATCOM controller is designed mostly based on the local linearization of model and based on PI control method etc, which only ensures the system stability within a small region around the operating point. The performance of such controller will decrease significantly in the case that a large disturbance occurred and/or the operation point is far away from the normal value. Meanwhile, the adaptability of controller is also very important [1], [2]. How to protect STATCOM of the power system from the external disturbance and residual nonlinear influences for the purpose of normal and stable operation of the system? It is an important research work with the scientific significance and academic values.
In recently years, advanced control theory and design method, aiming at application research on transient stability of systems [3], have been introduced to the traditional STATCOM control devices, which effectively improve the recovery of system stability [4], [5]. In [6], a single input fuzzy logic controller is designed to determine the control signal of static synchronous compensator to improve the stability of power systems. The influence of uncertainties, however, is not considered in the proposed fuzzy rule. In [7], the robust coordinated controller is designed for both excitation system and STATCOM by constructing the Hamiltonian energy function [8]. The simulation result shows that stability of overall system can be enhanced. For the STATCOM control device, the energy function is constructed and combined with standard power flow control method to achieve the purpose of controlling the flexible AC transmission system in [9]. However, this method is not perfect, and energy function cannot easily be obtained, and the controller design is very complicated. In [10], [11], a robust controller is designed for the single machine infinite bus system with a STATCOM to improve the transient stability. The combination of the state feedback exact linearization with the linear control theory is adopted to avoid the difficulty of solving Hamilton–Jacoby inequality. Lin [12] investigated the external disturbance attenuation of a system, and used the switched method to design a class of L-2 gain disturbance attenuation controllers for delay systems with time-varying exogenous disturbances. The fuzzy variable structure control method is introduced to STATCOM control of power systems [13]. In [14], the unified power flow controller (UPFC) with strong functions is designed by using Lyapunov theory, and it is applied to the flexible AC transmission system (FACTS). In [15], an optimal output feedback controller of STATCOM is designed to solve the stability problem of power systems. However, the methods mentioned above just consider the single uncertainty. Accordingly, it is a valuable study in the further to make the system insensitive to the large disturbances and uncertainties, in the condition that both the various uncertainties in the model and the influence degree of disturbance to the system are taken into account.
In the present paper, the minimax robust design method is investigated for the STATCOM control system. First, the uncertainty of the system is assumed to be expressed in the form of a parameter perturbation. The stability of the system with the parameter uncertainty is then ensured by designing an adaptive backstepping controller. The present study focuses on the dynamic response of the system during mechanical power disturbances and transmission line faults. The minimax method is used to estimate effectively the effect of sudden disturbances to the system. This method effectively reduces the conservativeness of the disturbance treatment, and ensures that the STATCOM system is insensitive to large disturbances. Finally, considering the single-machine infinite-bus system as an example, simulation results also show that the proposed method is more superior to some conventional nonlinear controller design methods.
Section snippets
Robust model of STATCOM systems
Consider the following single-machine infinite-bus system (SMIB) with STATCOM described in Fig. 1a. Its equivalent circuit is shown in Fig. 1b.
Let the generator be noted by the constant voltage source behind the transient reactance. By replacing the STATCOM dynamic process by the one order inertial link equivalently, the mathematical dynamic model of STATCOM control system is expressed as follows.
Design of nonlinear adaptive controller
The design problem of the nonlinear large disturbance attenuation controller for the single-machine infinite-bus system with STATCOM can be summarized as follows. For any given considering the case of the system with the large external disturbance and uncertain perturbation, then the state feedback controller can be designed, and system (2)-(5) such that the following dissipative inequality holds for any final time :at the same time it is
Simulation analyses
In this section, a simulation analysis of the large disturbance attenuation controller is given by considering a single-machine infinite bus system with STATCOM as an application example. The physical parameters are selected as follows:
, , , , and the weight coefficients of the adjusting output are chosen as: , , , , , , = 0.9 pu. Adaptive gain coefficients are: , and the level coefficient of
Conclusions
The current paper aims to investigate the large disturbance attenuation problem of the nonlinear STATCOM. The minimax method is used to design the nonlinear large disturbance attenuation controller, and the stability control for the STATCOM system is realized. The clever process of the minimax method enables the design process to consider fully the effect of external disturbances, and also reduces conservativeness in dealing with disturbances. The dynamic response of a system is effectively
Acknowledgements
The first author’s work was supported by the Fundamental Research Funds for the Central Universities, under grant 110404032, and the National Natural Science Foundation of China, under grant 61233002 and 71271047, and Specialized Research Fund for the Doctoral Program of Higher Education, under grant 20020145007.
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