Background: Considering the role of aging in brain atrophy and cerebral vascular demand, we carried out this study to clarify the role of aging in duplex indices of vertebral arteries.

SUMMARYThis paper presents a novel linearized model of a power system installed HVAC parallel-connected with a HVDC system based voltage source converter (VSC HVDC) in order to determine the most effective input to apply supplementary controller and design of phase compensator. The power system linearized model cannot be appropriate during the severe disturbances such as three phase faults. In this paper, a nonlinear model of a power system is obtained and in addition, an adaptive neural damping controller based neural identifier is proposed to improve stability and overcome the drawbacks of conventional phase compensator. Simulation results carried out by SIMULINK/MATLAB show the proposed strategy is most effective in comparison with phase compensator.

No abstract

No abstract

I. INTRODUCTIONFixed series capacitors have long been used in power systems as a cost-effective option for enhancing power transfer capability of HV transmission systems. However, the series capacitor has caused shaft damage in multistage steam turbine system in couple of occasions through subsynchronous resonance (SSR). SSR is an electric power system condition where the electric network exchanges energy with a turbine generator at one or more of the natural frequencies of the combined system under the subsynchronous frequency of the system [1]. This leads to turbine-generator shaft failure and system instability.Rotor oscillations of generator at a torsion mode frequency, (fm) induce armature voltage components at frequencies (fern) given by:(1) When the subsynchronous component fern is close to electrical resonant frequency, the subsynchronous torques produced by subsynchronous voltage components can be Manuscript received June 8, 2010. Ali Ajami is with the electrical engineering department of Azarbaijan university of Tarbiat Moallem, Tabriz, Iran, phone: +98-914-3183362; fax: +98-412-4327548; e-mail: ajami@azaruniv.edu Naser Taheri is with the electrical engineering department of, Islamic Azad University, Quchan Branch, Iran, e-mail: N.Taheri.1362@gmail.com sustained. This interplay between electrical and mechanical systems is termed as torsional oscillations [2].Flexible AC Transmission Systems (FACTS) technology is an important tool for permitting existing transmission facilities to be loaded, at least under contingency situations, up to their thermal limits without degrading system security [3][4][5][6]. The most striking feature is the ability to directly control transmission line flows by structurally changing parameters of the grid and to implement high-gain type controllers, based on fast switching.The uses of FACTS devices like STATCOM are increasing in the network for enhancing power transfer, voltage stability and damping of power oscillations. As a promising technology, the control of STATCOM has been discussed in many literatures [7][8][9][10]. In this paper, the damping of torsional oscillations using STATCOM, connected at the middle bus is studied. A supplementary fuzzy control signal is added to control circuit to damping oscillations caused by series compensator capacitance. To improve STATCOM controller, hybrid Fuzzy/LQR controller is proposed. II. DYNAMIC MODELLING OF STATCOMThe system considered is an IEEE benchmark used to study subsynchronous resonance. The modeling aspects of the electromechanical system are given in detail in reference [11]. This system is shown in Fig.1. One convenient method for studying balanced three-phase system (especially in synchronous machine problems) is to convert the three phase voltages and currents into synchronous rotating frame by abc/dq transformation. Computer and Electrical Engineering, Vol. 3, No. 2, April, 2011 1793-8163 185 are decoupled so that the use of classic control method is possible, and even more physical meaning for each control...