This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON POWER SYSTEMS 1 Coordinated Switching Controllers for Transient Stability of Multi-Machine Power Systems Y. Liu, Q. H. Wu, Fellow, IEEE, and X. X. Zhou, Fellow, IEEE Abstract—This paper proposes two switching controllers working in a coordinated manner to enhance the transient stability of multi-machine power systems. One is switching excitation con- troller (SEC) and the other is switching governor (SG). The SEC switches between a bang-bang excitation controller (BEC) and a conventional excitation controller (CEC) and the SG switches between a bang-bang governor (BG) and a conventional governor (CG), via a state-dependent switching strategy. The BEC and the BG are designed as bang-bang constant funnel controllers (BCFC), which are able to provide fast switching of excitation voltage and valve position between their upper and lower limits. A detailed steam turbine model including boiler pressure effect is considered in the controller design process. Simulation studies are undertaken in the IEEE 16-generator 68-bus power system to evaluate the control performance of the switching controllers, which include the cases that three-phase-to-ground fault and transmission line outage occur in the system, respectively. Coor- dination between the excitation loop and speed governing loop is studied. Two system resilience indexes are introduced, and the short-term resilience is investigated for the power systems with and without switching controllers installed, respectively. Index Terms—Bang-bang excitation controller, bang-bang gov- ernor, switching controller, short-term resilience. I. INTRODUCTION T RANSIENT stability has always been regarded as a key issue in the operation of power systems. The excitation control of synchronous generators is a traditional but effective way to enhance the transient stability of power systems [1]. The existing research concerning the excitation control can be sum- marized as follows. With respect to the linear excitation control methods, the PID excitation controller combined with an AVR compensator was proposed in [2]. With the power system stabilizer (PSS) im- plemented, the PID excitation controller was capable of im- proving both the transient and the small-signal stability of power Manuscript received May 12, 2015; revised August 21, 2015 and October 16, 2015; accepted October 22, 2015. This work was supported in part by the State Key Program of National Natural Science of China (NO. 51437006), and in part by the Guangdong Innovative Research Team Program (NO. 201001N0104744201), China. Paper no. TPWRS-00666-2015. Y. Liu is with the School of Electric Power Engineering, South China Uni- versity of Technology, Guangzhou 510640, China . Q. H. Wu is with the School of Electric Power Engineering, South China Uni- versity of Technology, Guangzhou 510640, China and also with the Department of Electrical Engineering and Electronics, The University of Liverpool, Liver- pool L69 3GJ, U.K. (e-mail: wuqh@scut.edu.cn). X. X. Zhou is with China Electric Power Research Institute, State Grid Cor- poration of China, Qinghe, Beijing 100192, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRS.2015.2495159 systems [3]. To ensure the robustness of the PSS in terms of damping the oscillation of power systems, the PSS tuning tech- niques considering multiple operation points were investigated in [4], [5]. Moreover, extensive effort was dedicated to the study of linear optimal and suboptimal excitation control methods [6]. Then the linear output feedback excitation controller was inves- tigated in [7], which was adopted to stabilize the torsional os- cillation of synchronous generators. As for nonlinear excitation control methods, the exact linearization feedback excitation control of synchronous gen- erators was studied in [8], and a partial linearization feedback excitation control scheme was investigated in [9]. Moreover, the fuzzy set theory, Lyapunov theory and recursive design method were applied to the design of the excitation controller in [10]–[12], respectively. Nevertheless, the aforementioned nonlinear excitation control methods all required accurate system parameters and thus lacked robustness to parameter uncertainties of power systems. Meanwhile, to the best of our knowledge, there was no excitation control scheme that required no system information. Besides the excitation control, the speed control of syn- chronous generators was believed to have influence on the transient stability of power systems as well [13]. However, the speed control loop and the excitation control loop of a synchronous generator were generally considered to be inde- pendent with each other and they were decoupled in different timescales [14]. But this is no longer the situation of modern power systems, in which the faster switching of valve position is allowed with the aid of accumulators and hydraulic speed governing systems. Hence, there exists tight mutual interaction between the excitation loop and speed governing loop of the synchronous generator [15]. The coordination between the excitation loop and speed control loop was studied in [16], which was verified to be able to widen the stability margin and achieve better transient stability of power systems. Apart from the aforementioned continuous control methods, bang-bang control was applied to improve the transient sta- bility of small-scale power systems as well [17], [18]. Based upon the minimum principle, the bang-bang control law was obtained through solving the canonical equation of the Hamil- tonian of the power system. The bang-bang controller utilized the largest damping energy to damp out the oscillation of the power system in the shortest time, thereby achieving a time-op- timal damping performance. Although it has shown its poten- tial in the transient stability control of the power system, the bang-bang control law involved the calculation of the deriva- tives of the Hamiltonian of the system. Thus the requirement of accurate system parameters and the complexity of the Hamil- 0885-8950 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.