Published in IET Generation, Transmission & Distribution Received on 29th August 2011 Revised on 10th January 2012 doi: 10.1049/iet-gtd.2011.0625 ISSN 1751-8687 Damping of subsynchronous oscillations in power system using static synchronous series compensator M. Farahani Department of Electrical Engineering, Bu-Ali Sina University, Hamedan, Iran E-mail: m.farahani@basu.ac.ir Abstract: In this study, a static synchronous series compensator (SSSC) is used to damp the subsynchronous oscillation in a power system compensated by the series capacitor. In order to achieve an effective damping, a supplementary subsynchronous damping controller (SSDC) is added to the SSSC. The only input signal for the SSDC is the rotor speed deviation to generate the modulation index for controlling the injected voltage of the voltage-sourced converter (VSC). Also, the chaotic optimisation algorithm is employed to tune the parameter of SSDC. The design objective is to suppress the subsynchronous resonance (SSR) caused by the series capacitor. By using the SSDC, the SSSC connected at the transmission line is able to damp the SSR. The first system of IEEE second benchmark model is used to evaluate the effective of SSDC on the torsional oscillations. The several simulations are used to demonstrate the ability of SSDC in damping the SSR. 1 Introduction The use of series capacitor is a conventional method for reducing high reactance of long transmission lines. This method has some advantages such as increase in transient stability, improvement of load carrying of transmission lines and by controlling this reactance, they allow better control over load sharing between parallel transmission lines. However, despite these benefits, these series capacitors can increase the risk of interaction between electrical power systems and turbine – generators’ rotor torsional system. This problem is known as subsynchronous resonance (SSR) or subsynchronous oscillations. ‘Subsynchronous oscillation is an electric power system condition where the electric network exchanges significant energy with a turbine – generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system following a disturbance from equilibrium’ [1]. The researchers have used many techniques to overcome this problem and proposed many controllers in the literatures. In general, most of these techniques can be divided into two main groups. The first one contains the controllers based on the excitation system of generator [2–4]. The second one consists of the flexible AC transmission systems (FACTS) devices. The FACTS devices provide a powerful mechanism in order to control the reactive power and voltage in power systems. Besides these abilities, the different types of these devices can be used in improving the stability of system. A lot of articles have been published about the use of these devices in damping the SSR [5–13]. In this paper, a static synchronous series compensator (SSSC) along with the supplementary subsynchronous damping controller (SSDC) connected at the transmission line is used to damp the SSR. The parameters of SSDC are tuned by the chaotic optimisation algorithm to achieve an effective damping. The SSSC is used as a voltage source in series with a fixed capacitor. So, this combination can prevent the subsynchronous oscillations that may be caused by conventional fixed capacitor. This factor, along with the simple control method, makes the proposed configuration highly effective in damping the SSR. 2 System under study In this study, the first system of IEEE second benchmark model shown in Fig. 1 is used to evaluate and analyse the risk of SSR [14]. In this model, a 600-MVA synchronous generator via two 500-kV transmission lines is connected to a large grid which is approximated by an infinite bus. The turbine–generator system as shown in Fig. 2 is modelled by four masses. As seen in Fig. 1, the SSSC injects a voltage V s in series with the transmission line where it is connected. Voltage- sourced converter (VSC) using insulated gate bipolar transistor (IGBT)-based pulse width modulation (PWM) inverters is used in this study. However, as details of the inverter and harmonics are not represented in the SSR studies, the same model can be used to represent a gate turn off (GTO)-based model. The overall performance of SSSC is completely explained in [10–11]. 3 Proposed approach 3.1 Structure of control for the SSSC An SSSC has an inherent damping capability and that only under certain circumstances it may be not sufficient [11], IET Gener. Transm. Distrib., 2012, Vol. 6, Iss. 6, pp. 539–544 539 doi: 10.1049/iet-gtd.2011.0625 & The Institution of Engineering and Technology 2012 www.ietdl.org