The powerflowcontrol in the line is changed by series compensation, shunt compensation and phase angle regulation. All series controllers will inject a voltage in series in the line. If the phase difference between the line voltage and injected voltage is 90˚, series controllers will absorb/consumes reactive power. Shunt controllers can deliver/absorb reactive power demanded by the load .Phase shifter is considered to be sinusoidal ac voltage source with the controlled amplitude and phase angle. The phase angle regulator will inject a voltage which will change the effective phase angle at the sending end and thereby changing the powerflow in the line. Third generation of FACTS controllers contains the Unified PowerFlow Controller (UPFC) and Interline powerflow controller (IPFC).They will have two Voltage Source Converters (VSC) connected though a DC link. By using UPFC, many FACTS devices distributed in the line can be avoided, thereby the problem of unwanted interactions between the devices can be eliminated up to a great extent . UPFC proposed by Dr.Gyugyi in 1991 is the most versatile and powerful device that can provide effective means of controlling the powerflow and improving the stability of a power network.
The UPFC can provide simultaneous control of transmission voltage, impedance and phase angle of transmission line. It consists of two switching converters as shown in fig1. These converters are operated from a common d.c link provided by a d.c storage capacitor. Converter 2 provides the powerflowcontrol of UPFC by injecting an ac voltage with controllable magnitude and phase angle in series with the transmission line via a series transformer. Converter one is to absorb or supply the real power demand by the converter 2 at the common d.c link. It can also absorb or generate controllable reactive power and provide shunt reactive power compensation
Abstract—The unified powerflow controller (UPFC) is one of the most promising flexible AC transmission systems (FACTS) devices for the powerflowcontrol. The main objective of powerflowcontrol for UPFC is to improve the powerflow distribution and increase the transmission capacity over the existing transmission corridor or line. This paper presents a practical engineering methodology of embedding the powerflowcontrol model of UPFC into the commercial software -- power system analysis software package (PSASP) based on its user program interface (UPI) function. In the proposed methodology, the interface currents of UPFC series side and UPFC shunt side between the UPFC device and the network are used to control the transmission line powerflow and UPFC bus voltage, respectively. In UPFC series side, the current of UPFC series branch is calculated from the target equation of the powerflow in the controlled line. In UPFC shunt side, the decoupling control between the active power and reactive power is implemented and the reactive current of UPFC shunt side is used to control the bus voltage. In this study, simulations are performed on a typical two-area power system and a practical 496-bus power system and simulation results show that the proposed methodology can be efficiently applied to the engineering research and analysis of the real power grid with UPFC with good convergence and with only one control parameter needed to be prescribed.
In the case of the parallel branch of the UPFC, the active power exchanged with the system primarily depends on the phase shift of the converter output voltage with respect to the system voltage, and the reactive power is controlled by varying the amplitude of the converter output voltage. However, the series branch of the UPFC controls active and reactive power flows in the transmission line by the amplitude and phase angle of the series injected voltage. Therefore, the active power controller can significantly affect the level of reactive powerflow and vice versa. To improve the performance of the UPFC, the interaction between the active and reactive powerflowcontrol system must be reduced. In recent years, a number of investigations have been carried out to demonstrate the capabilities of UPFC as an effective powerflowcontrol , providing voltage control , transient stability enhancement , and oscillation damping [7-9]. It has been reported in the literature that a strong dynamic interaction exists between the active and reactive power flows through a transmission line when they are controlled by series injected voltage v s of the UPFC. Furthermore, the UPFC is highly nonlinear because it consists of converters, power transformers, filters, and surge arrestors. Uncertainties in the power system make it difficult to model the transmission system accurately.
The series inverter is controlled to inject a symmetrical three phase voltage system (Vc), of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point. The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM (Static Synchronous Compensators) that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC (Static Synchronous series compensators) that generates or absorbs reactive power to regulate the current flow, and hence the power flows on the transmission line.
holds a B.Eng, M.Eng and Ph.D Degrees in Electrical/Electronic Engineering and Electrical Power and Machines respectively from the University of Benin, Nigeria. He is a member of the Nigerian Society of Engineers (NSE), and a registered engineer with Council for the Regulation of Engineers (COREN). His research areas include power system optimization using artificial intelligence and application of Flexible Alternating Current Transmission System (FACTS) devices in power systems. He is a Lecturer in the Department of Electrical/Electronic Engineering University of Port Harcourt,Rivers State, Nigeria with twelve (12) papers published both in reputable national and international journals. He has also presented his findings in over five (5) conferences nationally and internationally.
System (FACTS)—a new technology based on power electronics—offers an opportunity to enhance controllability, stability, and power transfer capability of ac transmission systems. Pioneers in FACTS and leading world experts in power electronics applications, Narain G. Hingorani and Laszlo Gyugyi, have teamed together to bring you the definitive book on FACTS technology. Drs. Hingorani and Gyugyi present a practical approach to FACTS that will enable electrical engineers working in the power industry to understand the principles underlying this advanced system. Understanding FACTS will enhance your expertise in equipment specifications and engineering design, and will offer you an informed view of the future of power electronics in ac transmission systems. This comprehensive reference book provides in- depth discussions on: Power semiconductor devices Voltage-sourced and current- sourced converters Specific FACTS Controllers, including SVC, STATCOM, TCSC, SSSC, UPFC, IPFC plus voltage regulators, phase shifters, and special Controllers with a detailed comparison of their performance attributes Major FACTS applications in the U.S. Understanding FACTS is an authoritative resource that is essential reading for electrical engineers
Abstract - Flexible AC transmission system (FACTS) devices uses power electronics components to maintain controllability and capability of electrical power system FACTS controller includes unified powerflow controller (UPFC), Static synchronous compensators(STATCOMs), Thyristor controlled series compensators (TCCs), Static series synchronous compensators (SSSCs) and Static VAR compensators (SVCs), are able to modify voltage, phase angle and impedance at particular bus in a power system. The (UPFC) is the most versatile and complex power electronic equipment that has emerged for the control and optimization of powerflow in electrical power transmission system. In this paper we see how the UPFC increases the transmission capacity and reduce the power congestion in the transmission line. This paper shows the results that we obtained by implementing a Transmission system in MATLAB Simulink by using UPFC and without using UPFC.
The unified energy float controller (upfc) is one in all themost extensively used facts controllers and its major functionis to control the voltage, section angle and impedance of thepower device thereby modulating the road reactance andcontrolling the power waft inside the transmission line.the basic components of the upfcare voltagesource inverters (vsis) linked by a not unusual dc storagecapacitor that's related to the energy gadget thru acoupling transformers. One (vsis) is attached in shunt tothe
together with a set of active and reactive nodal power injections at each end of the series reactance. These powers are expressed as function of terminal, nodal voltages, and the output voltage of the series source which represents the UPFC series converter. The UPFC injection model has implemented into a full Newton-Raphson program by adding the UPFCpower injections model and by derivatives the elements of jacobian matrix with respect to the AC network state variables, i.e. nodal voltage magnitude and angles, at the appropriate locations in the mismatch vector and Jacobian matrix. The mismatch vectors original dimension and Jacobian matrix are not altered at all. The UPFC stability to regulating powerflow through a transmission line and to minimize power losses, without generation re-scheduling, is shown by numeric examples. The attraction of this formulation is that it can be implemented very easily in existing powerflow programs. Another model of UPFC in terms of powerflowcontrol is presented in .In that model state variables of UPFC are incorporated inside the Jacobian matrix and mismatch equations, leading to vary the iterative solutions. UPFC controls the active and reactive power simultaneously and the voltage magnitude also. At initial conditions a set of analytical equations has been derived to provide UPFC. The losses of the UPFC coupling transformers have taken into account. UPFC have the capability to regulate the powerflow and minimizing the power losses simultaneously . M Tomay and A.M. Vuralin the power injection model of (UPFC) the operational losses also taken into account and the effects of UPFC location on different power system parameters are entirely investigated . A general sequential powerflow algorithm based on an injection model of FACTS devices has been presented in . The algorithm is compatible with Newton- Raphson and decoupled algorithms. It is important to ascertain the location for placement of UPFC which is suitable for various contingencies. An effective placement strategy for UPFC is proposed . The method uses Line Stability index which is sensitive to line flow to screen down the possible locations for UPFC.
A comprehensive and analytical approach for mathematical modeling of UPFC for steady state and linearised dynamic stability has been proposed. Several years the power system stabilizer act as a common control approach to damp the system oscillations. However, in some operating conditions, the PSS may fail to stabilize the power system, especially in low frequency oscillations . It is proved that the FACTS devices are very much effective in powerflowcontrol as well as damping out the swing of the system during fault. Among all FACTS devices the UPFC is the most popular controller for effective damping.
In (2010) Murali, and et al. investigates the improvement of transient stability of a two-area power system using UPFC and simulations are carried out in Matlab/Simulink environment for the two-area power system model with UPFC to analyze the effects of UPFC on transient stability performance of the system. The performance of UPFC is compared with other FACTS devices such as Static Synchronous Series Compensator (SSSC), Thyristor Controlled Series Capacitor (TCSC), and Static Var Compensator (SVC) respectively. The simulation results demonstrate the effectiveness and robustness of the proposed UPFC on transient stability improvement of the system . In (2011) Arup Ratan Bhowmik and Champa Nandi investigated the performance of Unified PowerFlow Controller (UPFC) in controlling the flow of power over the transmission line and they dialed with digital simulation of standard IEEE 14-bus power system using UPFC to improve the real and reactive powerflowcontrol through a transmission line by placing UPFC at the sending end using computer simulation .
This arrangement can be operated as an ideal reversible AC-AC switching power converter, in which the power can flow in either direction between the AC terminals of the two converters. The DC link capacitors provide some energy storage capability to the back to back converters that help the powerflowcontrol Replacing the two three-phase inverters by one matrix converter the DC link(bulk) capacitors are eliminated, reducing costs, size, maintenance, increasing reliability and lifetime. The AC-AC matrix converter, also known as all silicon converters, processes the energy directly without large energy storage needs. This leads to an increase of the matrix converter control complexity. In  an UPFC-connected power transmission network model was proposed with matrix converters and in -  was used to synthesize both active (P) and reactive (Q) power controllers using a modified venturini high- frequency PWM modulator.
With the rapid development of power electronics, Flexible AC Transmission Systems(FACTS) devices have been proposed and implemented in power systems. FACTS devices can be utilized to controlpowerflow and enhance system stability. Particularly with the deregulation of the electricity market, there is an increasing interest in using FACTS devices in the operation and control of power systems with new loading and powerflow conditions. A better utilization of the existing power systems to increase their capacities and controllability by installing FACTS devices becomes imperative. Due to the present situation, there are two main aspects that should be considered in using FACTS devices: The first aspect is the flexible power operation according to the powerflow capability of FACTS devices .The other aspect is the improvement of transient and steady-state stability of power systems. FACTS devices are the right equipment to meet these challenges. system operation according to the powerflowcontrol capability of FACTS devices.
In its general form the Interline PowerFlow Controller employs a number of dc to ac inverters providing each series compensation for a different line. In other words, the IPFC comprises a number of Static Synchronous Series Compensators. However, within the general concept of the IPFC, the compensating inverters are linked together at their dc terminals, as illustrated in Fig. 2. With this scheme, in addition to providing series reactive compensation, any inverter can be controlled to supply real power to the common dc link from its own transmission line. Thus, an overall surplus power can be made available from the underutilized lines which then can be used by other lines for real power compensation. In this way, some of the inverters, compensating overloaded lines or lines with a heavy burden of reactive powerflow, can be equipped with full two-dimensional, reactive and real powercontrol capability, similar to that offered by the UPFC. Evidently, this arrangement mandates the rigorous maintenance of the overall power balance at the common dc terminal by appropriate control action, using the general principle that the under loaded lines are to provide help, in the form of appropriate real power transfer, for the overloaded lines.
The UPFC is the most versatile of the FACTS devices. It cannot only perform the functions of the static synchronous compensator (STATCOM), thyristor switched capacitor (TSC) thyristor controlled reactor (TCR), and the phase angle regulator but also provides additional flexibility by combining some of the functions of the above controllers. The main function of the UPFC is to control the flow of real and reactive power by injection of a voltage in series with the transmission line. Both the magnitude and the phase angle of the voltage can be varied independently. Real and reactive powerflowcontrol can allow for powerflow in prescribed routes, loading of transmission lines closer to their thermal limits and can be utilized for improving transient and small signal stability of the power system.
The developing interest in tools for powerflowcontrol in a power system has increased significantly during the last 10 years. Demand for research in this field is motivated by rapid transformations in both technology and organization of the power system industry . The deregulation and competitive environment in the contemporary power networks will imply a new scenario in terms of load and power flows condition and so causing problems of line transmission capacity. For this reason, Flexible AC Transmission System (FACTS) controllers has rapidly developed to meet the need for increasing transmission capacity and controlling power flows through predefined transmission corridors. Especially, the improvements in the field of power electronics have had a major impact on the development of this technology. The devices of new FACTS generation are based on the use of high power electronic components such as GTO (Gate Turn-Off Thyristor) and IGBT (Insulated Gate Bipolar Transistor) which makes them respond quickly to the control requirements. It is called by the FACTS technologies based on Voltage Sourced Converter (VSC) designs. The wider application of FATCS leads to numerous benefits for electrical transmission system infrastructure, including increased capacity at minimum cost; enhanced reliability through proven performance; higher levels of security by means of sophisticated control & protection; and improved system controllability with state-of-the-art technology concepts. Thereby, these FACTS devices are able to act almost instantaneously to changes in power system . The most powerful in family of FACTS devices is the Unified PowerFlow Controller (UPFC), because it can control simultaneously all three parameters of transmission power line (line impedance, voltage and phase angle). In practice, the UPFC device consists
Modern utilities are beginning to install FACTS devices in their transmission networks to increase the transmission capacities and enhance controllability. In view of their advantages, there is a growing interest in the use of FACTS devices in the operation and control of power systems. There are two main aspects that should be considered while using FACTS controllers: The first aspect is the flexible power system operation according to the powerflowcontrol capability of FACTS controllers. The other aspect is the improvement of stability of power systems. 
In 1980‟s the Electric Power Research Institute (EPRI) gives a theory of improve the stability and reliability in power systems. This technology is named as Flexible Alternating Current Transmission Systems technology. By the using of FACTS Technology it is ability to increase and control as well as to improve the transmission system with the stability of powerflow, stability limits in power systems [3, 4] In 1980s, a different type of FACTS controller techniques introduced as per demand of the power systems . In 1990s introduced designed based on the concept of combined series-shunt FACTS Controller having the. It capability of improve the powerflowcontrol with stability and reliability and also. The ability to simultaneously control all the transmission parameters without affecting the powerflow of transmission line i.e. voltage, line impedance and phase angle, this is known as Unified PowerFlow Controller (UPFC) .
Representative of the FACTS devices are SVC (Static Var Compensators), TCSC (Thyristor Controlled Series Capacitor), STATCOM, the Unified PowerFlowControl- ler (UPFC), a phase shifters, etc. These devices may be connected in shunt/series or a combination of shunt and series. Variety of advantages like improvement in steady state powerflow, transient stability, voltage stability etc. can be derived. Performance of an existing transmission system can be improved. The UPFC is the most versatile and complex of the FACTS devices, combining the fea- tures of STATCOM and the SSSC. The UPFC can provide simultaneous control of all basic power system parameters viz., line voltage, impedance and phase angle .