FACTS Devices and their Controllers: An Overview

468 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 FACTS Devices and their Controllers: An Overview S. K. Srivastava, S. N. Singh and K. G. Upadhyay Abstract: In this paper some developed FACTS devices and their control features have been critically reviewed. The fast development of power electronic technology has made flexible AC transmission systems (FACTS) a promising for power system performance enhancement. The idea behind the FACTS concept is to enable the transmission system to be an active element in increasing the flexibility of power transfer requirements and in securing stability of integrated power system. It may also be effective in transient stability improvement, power oscillations damping and balancing power flow in parallel lines. This paper also highlights some underdeveloped FACTS devices and their controllers, which are in testing and R & D stage. Index Terms: Flexible AC transmission systems, FACTS controllers, deregulated power system. R I. INTRODUCTION apid advances in high power semiconductor devices and control technology, recently made it possible to provide fast voltage support by dynamic reactive compensation of the transmission system and power flow control in transmission corridors. In the present pace of power system restructuring, transmission systems are being required to provide increased power transfer capability to accommodate a much wider range of possible generation patterns. There are several problems associated with power transmission network expansion. Hence, there is an increased interest in better utilization of available power system capacities by installing new device such as Flexible AC Transmission systems (FACTS). The FACTS devices can reduce the flow of power in heavily loaded lines, resulting in an increased loadabilty, low system loss, improved stability of the network, reduced cost of production. A number of FACTS controllers are proposed [5-7] and implemented in order to achieve these objectives. The increased interest in these devices is essentially due to two reasons. Firstly, the recent development in high power electronics has made these devices cost effective and secondly, increased loading of power systems, combined with deregulation of power industry, motivates the use of power flow control as a very cost-effective means of dispatching specified power transactions. Several emerging issues in competitive power market, namely, as congestion management, enhancement of security and available transfer SK Srivastava is with Department of Electrical Engineering MMM Engineering College Gorakhpur, India (telephone: 0551-270063, email: sudhirksri@yahoo.co.in). SN Singh is with Department of Electrical Engineering Indian Institute of Technology, Kanpur, India (telephone: 0512-597009, email: snsingh@iitk.ac.in). KG Upadhyay is with Department of Electrical Engineering MMM Engineering College Gorakhpur, India (telephone: 0551-273731, email: kgupadhyay@rediffmail.com). capability of the system, transmission pricing, etc. have been restricting the free and fair trade of electricity in the open power market. FACTS devices can play a major role in these issues. Moreover, it is important to ascertain the location for placement of these devices because of their considerable costs. The flexible AC transmission system is akin to high voltage DC and related thyristor developments, designed to overcome the limitations of the present mechanically controlled AC power transmission systems. By using reliable and high-speed power electronic controllers, the technology offers five opportunities for increased efficiency of utilities. Greater control of power so that it flows on the prescribed transmission routes. Secure loading of transmission lines to levels nearer their thermal limits. Greater ability to transfer between controlled areas. Prevention of cascading outages. Damping of power system oscillation. Static Var compensator (SVC) improves the system performances by controlling the magnitude of voltage. Thyristor controlled phase angle regulator (TCPAR) controls the phase angle of voltage, while thyristor controlled series compensator (TCSC) changes the effective impedance of transmission line to the system performance. The unified power flow controller (UPFC) offers to combine all three functions in one device. The control of system parameters can be carried out concurrently or sequentially with transfer from one type control (phase shift) to another one (series compensation) in real time. A UPFC generates all the reactive power needed for phaseshift without requiring reactive power to be transmitted over the line, unlike TCPAR. UPFC is also able to supply independently controlled shunt reactive compensation equivalent to a static condenser (STATCON). The other devices of FACTS controller family are static compensator (STATCOM), static synchronous series compensator (SSSC), generalized unified power flow controller (GUPFC) and interline power flow controller (IPFC) etc. [10, 12]. In this paper, an overview of some developed FACTS controllers and their control features have been critically reviewed. A simple and suitable GUPFC controller has been suggested for the deregulated market place to enhance its capabilities such as to increase the transmission capability by capacitive reactance compensation, or to span large voltage phase angles between the sending and receiving ends while operating as a phase shifter or to reverse the direction of power flow. This paper also highlights some FACTS controller at R & D stage.

INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 469 II. OVERVIEW OF DEVELOPED FACTS DEVICES Static var compensator, composed of thyristor-switched capacitor (TSC) and thyristor-controlled reactor (TCR) with proper co-ordination of the capacitor switching and reactor control, the VAR output can be varied continuously between the capacitive and inductive ratings of the equipment. In TCSC the degree of series compensation is controlled by increasing or decreasing the number of capacitor banks in series. To minimize the switching transients and utilize natural commutation, the operation of thyristor valve is coordinated with voltage and current zero crossings. The TCSC can be effective [11] in transient stability improvement, power oscillation damping and balancing power flow in parallel lines. Working principle of TCPAR is identical with a phase shifting transformer with a thyristor tap changer and could be applied to regulate the transmission angle to maintain balanced power flow in multiple transmission paths, or to control it so as to increase the transient and dynamic stabilities of the system. III. CONVERTER BASED FACTS CONTROLLERS A. STATic CONdensor (STATCON) The working of STATCON is based on the use of Gate Turn Off thyristors (GTO) [7] in building a voltage source inverter driven from a voltage source across DC storage capacitors. EPRI and Tennessee Valley Authority (TVA) had developed and installed ±100 MVAR STATCON at the Sullivan substation on TVA power system in NewYork. B. Static Synchronous Compensator (STATCOM) STATCOM [7-9] is a static synchronous generator operated as a shunt connected static var compensator and is able to control its output current over the rated maximum capacitive or inductive range independently of the AC system voltage. STATCOM may have an increased transient rating in both inductive and capacitive operating regions. The available transient rating of STATCOM is dependent on the characteristics of the power semiconductors used and the junction temperature of operating devices. A new approach for the dynamic control of a current source inverter based STATCOM is reported in [21]. The d-q frame model and steady state characteristics of the CSI based STATCOM are proposed which results rapid non oscillatory dynamics of ac current without overshoot or steady state error. The control scheme of a STATCOM [15] is shown in Fig. 1. The output of static var generator or STATCOM is to be controlled at the point of connection in transmission system. A generator with rotor angle d, internal voltage V and a source impedance z represents the power system at terminal of the compensator. Figure 1: Control scheme of static synchronous generator The output of static var generator is controlled so that the amplitude of reactive current I 0 follows the current reference I QRef. The amplitude of terminal voltage V T is measured and compared with the voltage reference V Ref. The error?v T is passed and amplified by a PI controller to provide the current reference and the var generator. I 0 is closed loop controlled via I QRef so that V T is maintained at the level of reference voltage V ref in the case of power system load change. C. Static Synchronous Series Compensator (SSSC) The SSSC [4,5,7] can be considered as a FACTS controller acting like a controlled series capacitor. It compensates the inductive voltage drop in the line by inserting capacitive voltage in order to reduce the effective inductive reactance of the transmission line. In contrast to series capacitor, the SSSC is able to maintain a constant compensating voltage in case of variable line current or controls the amplitude of the injected compensating voltage independent of amplitude of line current. The function of series capacitor is simply to produce an appropriate voltage at AC system frequency to increase voltage across the inductive line impedance and thereby increasing line current and transmitted power. If this voltage is injected in series with the line, series compensation equivalent by a series capacitor at fundamental frequency is obtained. The source voltage expression is given by, Vc = jkxi (1) where Vc is injected compensating voltage phasor, X is series reactive line impedance, K is degree of compensation. For normal capacitive compensation, the output voltage lags the line current by 90 degree. The output voltage of the SVS can be reversed by simple control action to make it lead the line current by 90 degree. In this case the injected voltage decreases the voltage across the inductive line impedance and the series compensation has the same effect as if the reactive line impedance was increased. A generalized expression for the injected voltage Vq can be given as,

470 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 I Vq = ± jvq(ξ ) (2) i where Vq(ξ ) is the magnitude of the injected compensating voltage in the operating control range ( 0 Vq( ξ) V q max) and ξ is a chosen control parameter. The transmitted power P versus transmission angle d, characterizing an SSSC compensated two machine system can be expressed with injected voltage Vq as such, 2 2 V V δ P = sin δ + Vqcos (3) X X 2 From power angle characteristics of SSSC it can be shown that the series capacitor increases the transmitted power by a fixed percentage of that transmitted by the uncompensated line at a given d, and by contrast, the SSSC increases it by a fixed fraction of the maximum power transmittable by uncompensated line and independent of d in the operating π range of (0 δ ). 2 The SSSC has wider control range than the controlled series capacitor of same MVA rating for practical application point of view in steady state power flow control or stability improvement. Also in [16], the on-line fuzzy control of SSSC has been reported in order to improve the transient stability limit, damping out the system oscillation, control the voltage regulation and overall enhancement of power transfer capacity. In this scheme, the heuristics and Direct Lyaponov Stability method has considered to define the Fuzzy rules. According to this the SSSC control signal u has to assume values, so that the time derivative of energy function is negative definite where u is the control function of SSSC. Line-1 Line-2 SSSC1 Figure 2(a): IPFC controller SSSC-2 D. Interline Power Flow Controller (IPFC) The IPFC schemes provide [12] the independent control of reactive power as well as transfer of real power between the compensated lines. It consists of two converter based SSSCs, connected back to back for real power transfer. The converter of each SSSC provides a controllable AC output voltage at fundamental frequency, which is synchronized to the voltage of transmission line. IPFC provides economical solution of power flow problems in a multilane transmission system and also used to properly equalize power flow among lines, to transfer power from overloaded lines to under-loaded lines. Multi-line IPFC is shown in Figure 2(a). The control structure of the IPFC is shown in Fig.2 (b) with two converters. Converter-1 controls the real and reactive power independently. The operation of converter-1 is synchronized to line current i 1 and converter-2 to line current i 2 by two independent phase locked loops. The input to first converter control is either the desired real and reactive line powers P 1 and Q 1 or it could be the desired quadrature and real compensating voltages V 1q and V 1p (shown as internal references V 1q *,V 1p *). Figure 2(b): IPFC control structure The internally desired references are compared to the actual voltage components (V 1q, V 1p ) derived from measured line current and injected voltage vectors. The error signals after amplifications provide the input for the computation of V 1pq and angle? 1 of injected voltage vector. The reference input to control of converter-2 is the desired quadrature compensating voltage, V 2qRef. The reference voltage is compared to voltage component V 2q derived from measured injected voltage. From amplified error signals, the magnitude V 2pq and angle? 2 are derived to generate the output voltage of converter-2. E. Unified Power Flow Controller (UPFC) The UPFC was proposed [1-3] for real time control and dynamic compensation of AC transmission systems, providing the necessary fundamental flexibility required to solve many problems facing the electricity utilities. It consists of a shunt voltage source converter which provides reactive and real power support through DC link and a series voltage

INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 471 source converter which offers phase shifting and/or series compensation. 1) UPFC Controllers and their Structures The UPFC control system is to be divided into two parts, internal control and functional operation control. Internal control provides gating signal to the converter valves in order to properly respond of converter output voltages with internal reference variables, i 1ref, i qref, and v pqref as shown in Fig.3. The series converter responds directly and independently to the demand for series voltage injection. The shunt converter operates under a closed-loop current control structure where the real and reactive power components are independently control. Figure 3: Structure of UPFC controller In functional operation control, the functional operating mode of UPFC is defined which is responsible for generating the internal references, V pqref and i qref for series and shunt compensation. It is to be set manually or by an automatic optimization system as shown in Fig. 3, to meet specific operating and contingency requirements. The UPFC circuit structure may also be decoupled of two converters (separating DC terminals) to provide independent reactive shunt compensation (STATCOM) and reactive series compensation (SSSC) without real power exchange. 2) Control Structure of Shunt Converter The shunt converter is operated so as to draw a controlled current i sh from line. One component i shp is automatically determined by requirement to balance the real power of series converter. The other component of current i shq is reactive and can be set to any desired reference level. Reactive power (VAR) control and automatic voltage control modes of operation can be performed. In reactive power control mode, the reference input may either inductive or capacitive VAR on request. The shunt converter control translate the VAR reference in to the corresponding shunt current on request and adjust the gating of converter to establish the desired current. Whereas in voltage control mode, the shunt converter current is automatically regulated to maintain the transmission line voltage to a reference value at the point of connection. 3) Control Structure of Series Converter The series converter controls the magnitude and angle of the voltage vector V pq injected in series with the line. This voltage injection has influence to the flow of power on the line. The possible operating modes of this converter are, direct voltage injection, line impedance compensation, phase angle shifter and automatic power flow control. In direct voltage injection mode series converter generates the voltage vector V pq with magnitude and phase angle requested by reference input. When the injected voltage vector V pq is kept in quadrature with the line current vector i, it provides purely reactive compensation. In line impedance compensation mode, the magnitude of the injected voltage vector V pq is controlled in proportion to the magnitude of line current, so it emulates a reactive impedance. In phase angle shifter mode, the injected voltage vector V pq is controlled with request to input bus voltage vector V 1 so that the output bus voltage vector V 2 is phase shifted relative to V 1 by an angle specified by the reference input. In automatic power flow control mode, the series injected voltage is determined automatically and continuously by a closed loop control system to ensure that the desired P and Q are maintained despite system changes. So as, these multiple compensating functions can also be determined by; series compensation, shunt reactive power compensation and phase angle shift in coordination with generalized controller of active and reactive power flows. To realize various objectives of power system performance, there have been various papers in literature [17-20] for UPFC control. A fuzzy logic controller for transient stability enhancement has been categorically reported [19-20]. The proposed controller combines a transient fuzzy logic controller and a fuzzy UPFC coordinator, which results the first swing, damping the oscillation at an optimum rate and maximizing the transient stability margin. The single neuron and multi-neuron radial basic function controller (RBFNN) for the UPFC control in single machineinfinite bus (SMIB) and three machine power systems have being suggested in [17]. The single neuron controller uses either the real and reactive power deviations or real power and voltage deviations at the UPFC junction bus to provide better damping performance and transient stability limit as that of existing PI controllers. IV. UNDER DEVELOPED FACTS DEVICES A. Generalized Unified Power Flow Controller (GUPFC) GUPFC [10, 14], also known as multi-line UPFC, can control bus voltage and power flows of more than one lines or even of sub-networks. The simple GUPFC consisting of three converters, as shown in Fig. 4, one shunt connected and two in series with transmission lines, is capable of simultaneously controlling five power system quantities, i.e.

472 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 the bus voltage at substation, real and reactive power flows on two lines of existing the substation. Two converter applications each provide control capability for three power system quantities. The addition of third converter provides two more degree of freedom in control of system. For control of GUPFC, proportional-integral (PI) loops are utilized. In this scheme the gains of controller parameters are being selected to provide stable operation of GUPFC under steady state and faulty conditions. V P line2 Z 3 V se2 γ P line1 V 1 Z 2 V se1 β V 2 sh α Series Con-1 Series Con-2 Shunt Con. V 3 and interlock control logic is to be implemented for automatic changes from one configuration to another. 2) CSC Operation and Control The CSC control structure consists of inner and outer loops. The inner loop controller is designed to provide the magnitude and angle controlled synchronous voltage source, which is utilized for voltage and power regulation. The outer loop is required for damping of power system oscillations. The purpose of damping controller is to increase the resiliency of high voltage transmission system by adding positive damping during severe system contingencies. The control structure of CSC is shown in Fig. 6. HV Lines V dc Power Flow Controller Inverter Inverter V dc V 1r V 2r V 3r P line-1r P line-2r Figure 4: Three converters GUPFC model The GUPFC, as proposed in [10], can also be used in modeling other members of the CSC family in power flow and OPF analysis. The strong control capability of the GUPFC with controlling bus voltage and multi-line power flows offers a great potential in solving many of problems facing the electric utilities in a competitive environment. B. Convertible Static Compensator (CSC) CSC is the latest generation of FACTS controller family [13], providing the flexibility for adaptation to power system control needs and enable unique control capabilities of power systems. The CSC is being installed at NYPA s Marcy 345- KV substation near Utica, New York. It is a combination of FACTS and conventional technologies. On fully implementation, this will provide a long term solution for the power transfer, improving voltage, power flow control, enhance the reliability and resiliency of the network. 1) CSC Configuration and Operational Modes The CSC will be able to utilize two inverters in different configurations such as STATCOM, SSSC, UPFC and IPFC. The conceptual structure of CSC is shown in Fig. 5. The CSC can be deployed on the transmission system in 11 configurations. The control mode determines the functionality of CSC in a particular configuration. Sequence Reference Optical Link Control Figure 5: CSC conceptual structure Set point to CSC Capacitor & reactor bank On/Off commands CSC Inner loop Damping Controller CSC Master Control Control mode and gain change commands CSC status and operating point information Optical Link Power System Outer loop System Telemetry via SCADA plus other measurements Figure 6: CSC control structure

INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 473 V. CONCLUSIONS This paper presents a review of developed and underdeveloping power electronics-based FACTS devices and their control features. Various FACTS controller can enhance the power system performance, both static and dynamic, considerably. Series FACTS controllers such as SSSC, IPFC, UPFC, GUPFC and more recently CSC are being utilized in different applications. The acceptability of new concept of multi-line power compensation in the GUPFC or multi-line UPFC, which can control bus voltage and power flows of more than one line or sub-network, is growing rapidly. The CSCs are still to be practically examined for relieving in transmission line congestion, damping out system oscillations at lower frequencies V. REFERENCES [1] L. Gyugi, Power Electronics in Electric Utilities; Static Var Compensators, IEEE Proceedings, Vol. 76, pp. 481-482, April 1988. [2] L. Gyugi, A Unified Power Flow Control concept for Flexible AC Transmission System, IEE Proceedings-C, Vol. 139, No.4, pp. 323-331, July 1992. [3] L.Gyugyi, C.D Schavder, S.L Williams and A..Edris, The Unified Power Flow Controller: A New Approach to Power Transmission Control. IEEE Trans. On Power Delivery, Vol. 10, No. 2, pp. 1085-1097, April 1995. [4] R. Mihalic and I. Papic, Static Synchronous Series Compensator-A Mean for Dynamic Power Flow Control in Electric Power Systems, Electrical Power System Research vol. 48, pp. 65-72, 1998. [5] P. Moore and P. Ashmole, Flexible AC Transmission Systems Part- 4, Advanced FACTS Controllers, Proc. Power Engineering Journal, pp. 95-100, April 1998. [6] P. Moore and P. Ashmole, Flexible AC Transmission Systems Part- 3, Conventional FACTS Controllers, Proc. Power Engineering Journal, pp. 177-183, Aug.1997. [7] Y. H. Song and A. T. Johns, Flexible AC Transmission Systems, IEE London, T.J.I Ltd. 1999. [8] L. Gyugyi, Solid State Synchronous Voltage Sources for Dynamic Compensation and Real Time Control of AC Transmission Lines, IEEE PES Summer Power Meeting Vancover, BC. Canada, 1993, pp. 1-26. [9] G.D Galanos, C.I Hatziadoniu, Y-J Cheng and D. Maratukulam, Advanced Static Compensator For Flexible AC Transmission, IEEE Trans. On Power Systems, Vol. 8, No.1, pp. 113-121, Feb. 1993. [10] Xiao-Ping Zhang and M. M. Yao, Modeling of the Generalized Unified Power Flow Controller (GUPFC) in a Nonlinear Point OPF, IEEE Trans. On Power Systems, Vol.16, No.3, pp. 367-373, Aug. 2001. [11] T.J.E. Miller, Reactive Power Control in Electric Systems, John Willey & Sons, 1982. [12] B. Mwinywiwa, B. Lu, B. T Ooi, F. D. Galiana and G..Joos, Multiterminal UPFC for Power System Deregulation. IEEE Proc. 2000. [13] S. Zelingher, B. Shperling, L. Kovalsky and A. Edris, Convertible Static Compensator Project---Hardware Overview, IEEE 2000 Proc., pp.2511-2517. [14] A. Edris, FACTS Technology Development: An Update, IEEE Power Engineering Review, pp.1-9, March 2000. [15] N.G Hingorani, Understanding FACTS and Devices, IEEE Press, 2000. [16] S.M Sadeghzadeh, M. Ehsan, N.H Said and R. Fevillet. Improvement of Transient Stability Limit in Power System Transmission Lines Using Fuzzy Control of FACTS Devices, IEEE Trans. on Power System, Vol. 13, No. 5, pp. 917-922, Aug. 1998. [17] P.K. Dash, S. Mishra, and G. Panda, A radial function neural network controller for UPFC, IEEE Trans. 0n Power System, Vol. 15, No. 4, pp. 1293-1299, Nov. 2000. [18] K. R. Padiyar and A. M Kulkurni, Control Design and Simulation of a UPFC, IEEE Trans. On Power Delivery, Vol. 13, No.4, pp. 1348-1354, Oct.1998. [19] P. K. Dash, S. Mishra, and G. Panda, Damping Multimodel Power System Oscillations using a hybrid fuzzy controller for series connected FACTS devices, IEEE Trans. On Power System, Vol. 15, No. 4, pp.1360-1366, Nov. 2000. [20] S. Limyingcharoen, U. D. Annakkage, and N. C. Pahalawaththe, Fuzzy logic based Unified Power Flow Controller for transient stability improvement, IEE Proceedings, Part C, Vol. 145, No.3, pp.225-232, May 1998. [21] Dong Shen and P.W. Lehn, Modeling, Analysis and Control of a Current Source Inverter based STATCOM. IEEE Trans. On Power Delivery, Vol. 17, No.1, pp.248-253, Jan. 2002.