Robust Wide-Area Damping Controllers for compensating Inter-Area Oscillations in Power System

Abstract

Inter-area low-frequency oscillation is one of the major problems in an interconnected power system. The low frequency oscillations are observed to when a group of generators or a single generator on one side of the tie line oscillate against a group of generators or a single generator on the other side of the tie line. The power transfer capability through tie lies is reduced due to inter-area poorly damped oscillation. Sometimes these inter area oscillations may pose a serious threat to power system stability. Local signal based conventional power system stabilizer(PSS) is used to stabilize the low-frequency oscillation. However, due to the low observability of the local controller, these are not sufficient to provide adequate damping to the inter-area oscillation. A wide-area damping controller is efficient to provide adequate
damping to inter-area oscillation. Use of wide-area signal is more effective than the local area signal in damping out the inter-area oscillations. Wide Area Measurement System(WAMS) is convenient to transmit the wide area signals by using a communication channel to a remote location. The operating condition of the power system changes very often hence a wide-area damping controller(WADC) should be designed such that perform in all operating condition. The focus of this research is to design centralized robust wide area damping controller for inter-area oscillations in power system. Proportional-Integral(PI) controller is one of the most popular controllers due to its simple structure to implement. Hence, a loop shaping based PI controller is designed to improve the damping of the inter-mode of oscillations. The gains of the PI controller are optimally determined by solving a convex optimization problem by employing the primal-dual interior point method. Although the wide-area signal can be used to obtain accurate dynamic behavior of power system, but time delay appears in the wide-area signal through transmission from a remote area to a controller site. Thus, there lies a great deal of challenge to design a suitable damping controller. A Unified Smith Predictor (USP) based loop shaping H¥ controller is then designed to compensate for the time delay effect by using wide-area signal. To achieve robust stabilization, the normalized coprime factor problem is converted into a generalized H¥ optimization problem satisfying additional pole placement constraints. Communication failure of the wide-area signal is another disastrous phenomenon that occurs in a communication channel. Hence, a Dual Input Single Output (DISO) H¥ controller is proposed to achieve the control resiliency by employing two highest observability ranking wide-area signals with respect to the critical damping inter-area mode. The proposed controller provides sufficient damping to the power system ensuring its stability even when one of the wide-area signal is lost. The involvement of time-delay in the wide-area signal transmission may deteriorate the controller performance. Hence, USP approach is adopted to design the H¥ controller with additional pole placement constraints to compensate a range of communication delay. Although the higher-order controller exhibits good damping performance, but its design remains always a challenge while implement is in the large power system. A speed-based fixed low-order Wide-Area Damping Controller (WADC) is proposed by using the non-convex and non-smooth optimization technique. The controller is synthesized by minimizing the objective function defined on the basis of the spectral abscissa, complex
stability radius, and H¥ norm minimization. The time delay effect is considered in the synchronized and non-synchronized feedback loop. The fixed low-order synchronized and non-synchronized WADC is proposed considering the delay in synchronized as well as in the non-synchronized feedback signals. The performances and effectiveness of all the above designed controllers namely loop shaping PI controller, USP based loop shaping H¥ controller, USP based DISO H¥ controller and fixed order H¥ controller have been verified by considering two case studies namely the Kundurs two-area system and IEEE-39 bus system considering different operating points. It is observed from small-signal analysis and time-domain simulation in MATLAB/Simulink that the proposed wide-area damping controllers provide adequate damping to inter-area oscillations and compensate for the adverse effect of the time delay. In addition, it is also observed that the proposed fixed low-order (2nd) wide-area damping controller provides slightly better damping performance than a higher order controller.