Journal of Applied Engineering (JOAE), 2 (5), May-2014 (Volume-II, Issue-V) ISSN: 2348-4802 SSSC - Static Synchronous Series Compensator: Theory, Modeling Controlling and Voltage Level Improvement Of Power System Mohsin I Sujela 1 ,Amin S Nilesh Patel2, Anas I Shikari3 1,3 PG Student, 2 Assistant Professor Depart ment of Electrical Engineering, Parul Institute of Technology, Limda Abstract: The main aim of this work is to damp out power system oscillations, which has been recognized as one of the major concerns in power system operation. In a Static Synchronous Series Co mpensator (SSSC), a controllable A C voltage is generated by a voltage-source converter. There are two control channels for controlling the magnitude and phase of the voltage. This work describes the damping of power oscillations by static synchronous series compensator (SSSC) based damping controllers. The advantage of this approach is that it can handle the nonlinearit ies, at the same time it is faster than other conventional controllers and it improve the reactive power of the system. Simu lation studies will be carried out in Matlab/Simu link environ ment to evaluate the effectiveness of the proposed Static synchronous series compensator (SSSC) of mu lti -area power system. Results will show that the proposed SSSC based damping controllers improve the dampin g performance of the in the event of a major d isturbance. Keywords: AC transinission, FACTS, power fl ow controller, power converter, inverter, thyristor, GTO, etc . This work will presents an analysis of Static Synchronous Series Co mpensator (SSSC). Vo ltage Source Converter (VSC) based FACTS is the most recent approach in FACTS technology. The SSSC is the series compensation devices that open up new opportunities to control the power on transmission systems in order to enhance their utilization, increase power transfer capability and to improve voltage profile. The power system is a highly nonlinear system that operates in a constantly changing environment; loads, generator outputs, topology, and key operating parameters change continually [8,9] . When subjected to a transient disturbance,the stability of the system depends on the nature of the disturbance as well as the initial operating condition. The disturbance may be s mall or large. Small d isturbances in the form of load changes occur continually, and the system adjusts to the changing conditions. The system must be able to operate satisfactorily under these conditions and successfully meet the load demand. It is important I. INTRODUCTION Some of the major issues that are involved in bulk power transmission are enhancing the level of power transfer capability of existing transmission lines and flexib le control over power flow through these lines. To achieve the above goals, the current trend is to use solid state devices for faster control and reliable operation. Power electronic devices, wh ich are used for power flo w control, are categorized under the generic name of Flexib le A C Transmission Systems (FACTS). There are three major facets of FACTS. They are shunt compensation, series compensation and phase angle regulation. Of these three, the series compensation is used in this project Among all FA CTS devices, static synchronous series compensators (SSSC) plays much more important role in reactive power co mpensation and voltage support because of its attractive steady state performance and operating characteristics.[6,7] 82 Journal of Applied Engineering (JOAE), 2 (5), May-2014 (Volume-II, Issue-V) to damp these oscillations as quickly as possible because they cause mechanical wear in power plants and many power quality problems. To improve the voltage stability and the damping of oscillations in power systems, supplementary control laws can be applied to existing devices. These supplementary actions are referred to as voltage stability and power oscillation damp ing (POD) control. In this work, voltagestability and POD control will be applied to Static synchronous series compensator (SSSC).The SSSC using a voltage source converter to inject a controllable voltage is quadrature with the line current of a power network is able to rapidly provide both capacitive and inductive impedance compensation independent of the power line current [12-14]. These features make the SSSC an attractive FACTS device for power flow control, power oscillation damping and improving transient stability. This work will describe the damping of power oscillations by static synchronous series compensator (SSSC) based damping controllers and it also improve the reactive power of the system. Simu lation studies will be carried out in Matlab/Simulink environment to evaluate the effectiveness of the proposed Static synchronous series compensator (SSSC) In this work the damp ing performance of a two-Area power system will be co mpared for the cases of with and without SSSC based damping controllers in the event of a 3-phase short circuit faults .Simu lat ion results show that in damp ing power system oscillations, the SSSC with POD controller is more effective than the SSSC without POD controll II. THEORY Fig. 1 shows a single line diagram of a simp le transmission line with an inductive reactance, XL connecting a sending-end voltage source, vs and a receiving-end voltage source, vr respectively To design and optimize the SSSC- based damping controller, a mu lti area power system with SSSC, shown in Fig is considered. It is similar to the power systems used in references. The system consists of two generators divided in two subsystems and are connected via an intertie. Following a disturbance, the two subsystems swing against each other resulting in instability. To improve the stability the line is sectionalized and a SSSC is in between the bus-1and 2. In the Figure, G1and G2 represent the generators; T/F1 and T/F2 represent the transformers in the bus-line1 and bus-line 2 respectively. The Static Synchronous Series Compensator (SSSC) is a series device of the Flexible A C Transmission Systems (FACTS) family using power electronics to control power flow and improve power oscillation damp ing on power grids [1]. The SSSC in jects a voltage Vs in series with the transmission line where it is connected III A STATIC SYNCHRONOUS SERIES COMPENSATOR shows an SSSC connected in series with a simp le transmission line between BUS 1 and BUS 2. The transmission line has an inductive reactance, XL, and a voltage source, vs at the sending-end and an inductive reactance, Xr, and a voltage source, Vr , at the receivingend, respectively. The SSSC consists of a 24- pulse harmonic neutralized voltage source inverter, VSZ2, a magnetic circuit, MC2, a coupling transformer, T2, a mechanical switch, MS2, t wo electronic switches, ES2 and ES22, current and voltage sensors, and a controller. The power circuit wh ich consists of a voltage source inverter and a magnetic circu it and the controller are described below. 83 Journal of Applied Engineering (JOAE), 2 (5), May-2014 (Volume-II, Issue-V) are cancelled by connecting filters at the AC side of the VSC. Th is type of VSC uses a fixed DC voltage Vdc. Vo ltage V_conv is varied by changing the modulation index of the PWM modulator. The SSSC (Phasor Type) block models an IGBT -based SSSC (fixed DC voltage). However, as details of the inverter and harmonics are not represented, it can be also used to model a GTO-based SSSC in transient stability studies. B-The control system consists of: A phase-locked loop (PLL) wh ich synchronizes on the positive-sequence component of the current I. The output of the PLL (angle Θ=ωt) is used to compute the direct-axis and quadrature-axis components of the AC three-phase voltages and currents (labeled as Vd, Vq or Id, Iq on the diagram).Measurement systems measuring the q components of AC positive-sequence of voltages V1 and V2 (V1q and V2q) as well as the DC voltage Vdc.AC and DC voltage regulators which co mpute the two components of the converter voltage (Vd_conv and Vq_conv) required to obtain the desired DC voltage (Vdcref) and the in jected voltage (Vqref). The Vq voltage regulator is assisted by a feed forward type regulator which predicts the V_conv voltage fro m the Id current measurement. The SSSC b lock is a phasor model which does not include detailed representations of the power electronics. You must use it with the phasor simu lation method, activated with the Powergui b lock. It can be used in three-phase power systems together with synchronous generators, motors, dynamic loads and other FACTS and Renewable Energy systems to perform transient stability studies and observe impact of the SSSC on electro mechanical oscillations and transmission capacity at fundamental frequency. IV. SINGLE-LINE DIA GRAM OF A SSSC A ND ITS CONTROL SYSTEM BLOCK DIA GRAM As the SSSC does not use any active power source, the injected voltage must stay in quadrature with line current. By varying the magnitude Vq of the in jected voltage in quadrature with current, the SSSC performs the function of a variable reactance compensator, either capacitive or inductive. The variation of injected voltage is performed by means of a Voltage-Sourced Converter (VSC) connected on the secondary side of a coupling transformer. The VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage V_conv fro m a DC voltage source. A capacitor connected on the DC side of the VSC acts as a DC voltage source. A small active power is drawn fro m the line to keep the capacitor charged and to provide transformer and VSC losses, so that the injected voltage Vs is practically 90 degrees out of phase with current I. In the control system b lock d iagram Vd_conv and Vq_conv designate the components of converter voltage V_conv which are respectively in phase and in quadrature with current. A-Two VSC technologies can be used for the VSC: VSC using GTO-based squarewave inverters and special interconnection transformers. Typically four three-level inverters are used to build a 48step voltage waveform.Special interconnection transformers are used to neutralize harmon ics contained in the square waves generated by individual inverters. In this type of VSC, the fundamental co mponent of voltage V_conv is proportional to the voltage Vdc. Therefore Vdc has to varied for controlling the injected voltage. VSC using IGBT-based PWM inverters. This type of inverter uses Pulse-Width Modulation (PWM) technique to synthesize a sinusoidal waveform fro m a DC voltage with a typical chopping frequency of a few kilohertz. Harmon ics V SIMULATION A ND RESULT 84 Journal of Applied Engineering (JOAE), 2 (5), May-2014 (Volume-II, Issue-V) capacitive reactance in series with the transmission line. The transition fro m one mode of operation to the other mode takes place in a sub-cycle time. The operation of the model is verified by connecting the model in series with a simp le transmission line which can easily be rep laced by the utility’s existing mo re co mplex power system network. . VI. REFERENCES [1]. N.G. Hingorani, “Flexible AC transmission”. IEEE Spectrum, v. 30, n. 4, pp. 40-44, 1993. [2]. N.G. Hingorani and LGyugyi, “Understanding FACT S Concepts and Technology of Flexible AC Transmission Systems”, IEEE Press, New York, 2000. [3]. Y.H. Song and A.T. Johns, Eds., “Flexible AC T ransmission Systems (FACT S)”, IEE Press, London, 1999. [4]. R.M. Mathur and R.K. Varma, “Thyristor-Based FACT S Controller for Electrical Transmission Systems”, IEEE Press and Wi-ley Interscience, New York, 2002. Vqinj Vqref (pu) 0.2 0.15 [5]. R. Gru¨nbaum, M. Noroozian, and B. T horvaldsson, “ FACT S Powerful systems for flexible power transmission”, ABB Rev., May 1999, 4–17. 0.1 0.05 0 -0.05 [6]. K. Stahlkopf and M. Wilhelm, “Tighter controls for busier systems”, IEEE Spectrum, 1997, 34(4), 48–52. P_B2 (MW) 900 850 800 [7]. K.K.Sen, “SSSC-static synchronous series compensator: theory, modelling and applications”, IEEE Trans. on Power Delivery, v. 13, n. 1, 1998, pp. 241-246. 750 700 650 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 [8]. K.R. Padiyar, “Power System Dynamics - Stability and Control”,Second Edition, Hyderabad: B.S. Publications, 2002. [9]. G. Guo, Y. Wang and D.J. Hill, “Nonlinear output stabilization controlof multi-machine power systems”, IEEE Trans. on Circuits and Systems,Part I, v. 47, n.1, 2000, pp. 46-53. [10]. K.R. Padiyar, “Analysis of Sub-synchronous Resonance in PowerSystems”, Kluwer Academic Publishers, Boston, 1999. [ 11 ]Transmission Line Dynamic Impedance Compensation System,L. Gyugyi and C. D. Schauder, US Patent No. 5,198,746. [12] Static Synchronous Series Compensator: A Solid-stateApproach to the Series Compensation of Transmission Lines, L. Gyugyi, C. D. Schauder and K. K. Sen, 96 WM 120-6 PWRD, IEEE PES Winter Meeting, 1996. V. CONCLUSION The SSSC wh ich is a voltage source inverter injects an sinusoidal voltage in series with the transmission line. This almost injected voltage is almost in quadrature with the line current, thereby emulating an inductive reactance or a capacitive reactance in series with the transmission line. The power flow in the transmission line always decreases when the injected voltage by the SSSC emu lates an inductive reactance in series with the transmission lime and the power flo w in the transmission line always increases when the injected voltage by the SSSC emulates a 85