FACTS devices

Among the available FACTS devices, the Unified Power Flow Controller (UPFC) is the most versatile one that can be used to improve steady state stability, dynamic stability and transient stability. The UPFC can independently control many parameters since it is the combination of Static Synchronous Compensator (STATCOM) and SSSC. These devices offer an alternative mean to mitigate power system oscillations. UPFC can improve stability of single machine infinite bus (SMIB) system and multi machine system. The inter-area power system has special characteristic of stability behaviour. The improvement of transient stability of a two-area power system with a UPFC is investigated. A Matlab/Simulink model can be developed for a two-area power system with a UPFC. The performance of UPFC can be compared with other FACTS devices such as SVC, TCSC, and SSSC respectively.

According to IEEE definition it is defind as “: Alternating current transmission systems incorporating power-electronic based and other static controllers to enhance controllability and increase power transfer capability. The need for more efficient and fast responding electrical systems has given rise to innovative technologies in transmission using solid-state devices. These are called FACTS devices which enhance stability and increase line loadings closer to thermal limits. The development of power semiconductor devices with turn-off capability(GTO, MCT) opens up new perspectives in the development of FACTS devices. FACTS devices are the key to produce electrical energy economically and environmental friendly in future.

Interconnected electrical network comprises of numerous generators, transmission lines, variety of loads and transformers. The term Flexible Alternating Current Transmission System (FACTS) devices or controllers describe a wide range of controllers, many of which incorporate large power electronic converters that can increase the flexibility of power systems making them more controllable and stable. FACTS devices stabilize transmission systems with increased transfer capability and reduced risk of line trips [A. Kumar and S. B. Dubey 2013]. The major problem in power system is upholding steady acceptable system parameters like transients and voltage under normal operating and anomalous conditions, which is usually referred as voltage regulation problem and regaining synchronism after a major fault [C. Makkar and L. Dewan 2010]. This results in system overloading. Overloading may also due to faults, heavy loading, long transmission lines with uncontrolled buses at the receiving end, radial transmission lines, and shortage of local reactive power, intrinsic factors, and small generation reserve margins[A. Satheesh and T. Manigandan 2013] .

Voltage stability improvement is an important issue in power system planning and operation.In this regard, this paper presents a comparison of FACTS devices for static voltage stability study. To achieve this, the performance of Shunt Capacitor, Static Var Compensator (SVC), and Static Synchronous Compensator (STATCOM) are compared under normal and contingency conditions. Result reveals that the correct position of STATCOM and SVC will increase voltage stability and power transfer capability.The paper provides a guide for utilities to have an appropriate choice of FACTS device for enhancing static voltage stability.

10. K.Elango., and S.R.Paranjothi “Power Transmission Congestion Management in Restructured Power System by FACTS Devices, Generation Rescheduling and Load Shedding using Evolutionary Programming” European Journal of Scientific Research,ISSN 1450- 216X Vol.56 No.3 ,2011, pp.376-384.

SSSC is a type of FACTS devices which is connected in series with transmission line. Depending upon voltage source inverter, SSSC is used as a reactive power compensator. SSSC can operate in capacitive and inductive modes. SSSC consists of voltage source inverter, series transformer, capacitor and control block as explained in Fig- 5.

The main objective of this paper is to develop an LFB formulation of power balance equations for analyzing a radial distribution system that will efficiently incorporate embedded series and shunt FACTS devices. The LFB equations use bus voltage magnitudes and line power flows as independent variables and directly relate the FACTS device variables with system operating conditions. The line loss terms are the only nonlinear terms in the formulation. By adding them to bus power injections, the coefficient matrix of LFB equations is rendered linear. A preliminary Breadth-First-Search (BFS) ordering of the branches transforms the coefficient matrix structure to strictly upper/lower diagonal and leads to simple backward/forward substitution for calculating real and reactive line power in each branch and voltage at each bus. The FACTS device models are described first, and the development of LFB equations follows. Numerical examples, including multiple FACTS devices in the standard IEEE systems, illustrate the power of the new approach. The procedure exhibits good convergence characteristics, high reliability, and computational efficiency. A balanced distribution feeder modeled by the positive sequence impedance is used in the paper, since the aim of this paper is to demonstrate the advantages of the LFB formulation in handling the embedded FACTS devices. FACTS devices can be assumed to be cost-effective when deployed on the main distribution feeder.

740 The main objective of this paper is to develop an LFB formulation of power balance equations for analyzing a radial distribution system that will efficiently incorporate embedded series and shunt FACTS devices. The LFB equations use bus voltage magnitudes and line power flows as independent variables and directly relate the FACTS device variables with system operating conditions. The line loss terms are the only nonlinear terms in the formulation. By adding them to bus power injections, the coefficient matrix of LFB equations is rendered linear. A preliminary Breadth-First-Search (BFS) ordering of the branches transforms the coefficient matrix structure to strictly upper/lower diagonal and leads to simple backward/forward substitution for calculating real and reactive line power in each branch and voltage at each bus. The FACTS device models are described first, and the development of LFB equations follows. Numerical examples, including multiple FACTS devices in the standard IEEE systems, illustrate the power of the new approach. The procedure exhibits good convergence characteristics, high reliability, and computational efficiency. A balanced distribution feeder modeled by the positive sequence impedance is used in the paper, since the aim of this paper is to demonstrate the advantages of the LFB formulation in handling the embedded FACTS devices. FACTS devices can be assumed to be cost-effective when deployed on the main distribution feeder.

Abstract: - Nowadays, power systems are facing new challenges, such as increasing penetration of renewable energy sources, in particular wind generation, growing demands, limited resources,and competitive electricity markets. Under these conditions, thepower systems has had to confront some major operating problems in voltage regulation, power flow control,transient stability, and damping of power oscillations, etc. Flexible AC transmission system (FACTS) devices can be a solution to these problems. This paper investigates the application of FACTS devices on a 12-bus multimachine benchmark power system including a large wind farm. A STATCOM and an SSSC are added to this power network to provide dynamic voltage control for the wind farm, dynamic power flow control for the transmission lines, relieve transmission congestion and improve power oscillation damping and transient stabilit

The determination of the best sizing as well as placement of FACTS device are considered as one of the important issues in the distribution network. In addition, the optimal positioning and sizing of the FACTS sources are intensely impacted by the losses of power in a distribution network. Several research works is performed on optimal location as well as sizing of the FACTS devices in distribution systems to attain different objectives, concurrently. Here, the voltage profile is the vital objective for the development of the quality power, which is presented by the deviation of the voltage from the voltage base. To enhance the voltage stability, the FACTS devices are positioned in the receptive buses. A multi-objective problem based on the improved reliability, enhanced voltage profile as well as reduction loss of power is needed to determine the optimal placement of the FACTS devices. Some FACTS devices require batteries to avert fluctuations of power, which results from deviations of the weather condition.

The solution to improve the power quality at the load side is of great important when the production processes get more complicated and require a bigger liability level, which includes aims like to provide energy without interruption, without harmonic distortion and with tension regulation between very narrow margins. The devices that can fulfill these requirements are the Custom Power; a concept that we could include among the FACTS, but that is different to them because of their final use. In fact the topologies that they employ are identical to the ones in the FACTS devices with little modifications and adaptations to tension levels; therefore they are most oriented to be used in distribution networks of low and medium tension, sometimes replacing the active filters.

The Static Synchronous Series Compensator (SSSC), one of the key FACTS devices, consists of a voltage-sourced converter and a transformer connected in series with a transmission line. The SSSC injects a voltage of variable magnitude in quadrature with the line current, thereby emulating an inductive or capacitive reactance. This emulated variable reactance in series with the line can then influence the transmitted electric power. In our demo, the SSSC is usedto damp power oscillation on a power grid following a three-phase fault. The power grid consists of two power generation substations and one major load center at bus B3. The first power generation substation (M1) has a rating of 3000 MVA, and second 2500 MVA.

The TCPAR allows increasing or decreasing the power flowing in the line where it is inserted. It does not inject any active power in the network whereas in case of TCSC, the node bus-TCSC injects active power. A disadvantage of TCPAR is the voltage drop, that causes in the network. To compensate, this voltage drop, SVC is inserted in the network which enhance the cost of entire system whereas the TCSC is cheap device comparatively. TCSC can control the dynamic power flow of the network. So, it is concluded that TCSC as nodal bus inject power in the network and can control the power flow with small cost comparatively TCPAR because SVC unit is additional device in later technique. Hence TCSC is more useful than TCPAR and other FACTS devices.

It is well documented in the literature that the effectiveness of FACTS controllers mainly depends on their locations [4]. According to the characteristics of FACTS devices, various criteria have been considered in the allocation problem. Some of the reported objectives in the literature are: static voltage stability enhancement [5]-[8], violation diminution of the line thermal constraints [9], network loadability enhancement [10], power loss reduction [11], voltage profile improvement [10], fuel cost reduction of power plants using optimal power flow [12], dynamic stability improvement [13], and damping power swings [14]. It should be noted that each of the mentioned objectives improves the power system network operation and reaching these objectives is desirable in all power system networks. But improvement in one objective does not guarantee the same improvement in others. For instance, in order to improve the voltage stability, as considered here, voltage magnitude alone may not be a reliable indication of how far an operating point is from the collapse point [2]. Hence, satisfying the voltage magnitude constraint does not guarantee the satisfaction of the security margin requirement. By proper TCSC and SVC allocation and setting, both the voltage magnitude and Security Margin (SM) may be improved.

The improvements in the field of power electronics have major impact on the development of the Flexible AC Transmission Systems (FACTS) devices, These devices are based on Thyristor Controlled Reactor (TCR) and Voltage- Source Inverters (VSI) such as; Static Var Compensator (SVC), Thyristor Controlled Series Capacitor (TCSC) and Static Synchronous Compensator (STATCOM), Static Synchronous Series Compensator (SSSC), Unified Power Flow Controller (UPFC). These devices are used for controlling the power flows and for compensation of reactive power in the network. In addition of this, they can help to reduce the flows in heavily loaded lines resulting in an increased load ability to reduce system losses to improve stability of the network and to reduce cost of production.

FACTS devices have been defined by the IEEE as “alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability” [1]. There are five well known FACTS devices namely: Static Var Compensator (SVC), Static Synchronous Compensator, Thyristor Controlled Series Capacitor (TCSC), Static Synchronous Series Compensator (SSSC) and Unified Power Flow Controller (UPFC). Each of them have their own characteristics and limitations. It would be very effective if we could improve voltage stability by incorporating the most beneficial FACTS device for a given operating condition.

nodes. Some methods seek to solve voltage instability by the optimal location of FACTS devices close or in the critical nodes [19]; other methods are based on the coordination of FACTS devices, as in 1998, when a method to coordinate thyristor controlled series and shunt compensators (TCSC and SVC) was presented in order to improve angle and voltage stability, using a disturbance response method based on the Disturbances Auto-Rejection Control (DARC) theory [20]. In 2003, a secondary voltage control method was proposed to eliminate voltage violations at the nodes after a contingency, using the coordination of SVC and STATCOM devices to provide reactive power; the secondary voltage control is implemented by a learning fuzzy logic controller [21]. In 2005, the development of a control system and control strategies capable of governing multiple flexible AC transmission system (FACTS) devices in coordination with load shedding was proposed to remove overloads caused by lines outages in transmission networks, based on linearized expressions in steady state [21]. In 2005, a method for coordination of FACTS devices as SVC, TCSC and TCPST was presented, based on the optimal power flow to avoid congestion, to give greater security and to minimize the active losses in transmission lines [22]. FACTS devices location and coordination techniques mentioned above have been based on the voltage stability improvement in an area near the devices, improving voltages in steady state, preventing violations of the voltage limits, coordinating few quantity and types of FACTS devices, they do not allow to handle reactive power in the lines and aim at relocating the devices increasing the costs. Table I shows the comparison among the techniques used for the solution of voltage stability problems after a contingency. The rating of the indexes was done with numbers that indicate the low (1), middle (2) and high (3) levels of the objective functions of the proposed method.

Under new de-regulated environment an open access to transmission system seems to be desired. Determination and enhancement of available transfer capability (ATC) are important issues in deregulated operation of power systems. This paper focuses on the best location and optimal allocation of FACTS devices to improve maximum available transfer capacity (ATC). ATC is computed using repeated power flow method considering voltage profile. Quantum inspired PSO is used for optimization of FACTs requirement for maximizing ATC. The suggested methodology is tested on IEEE 30-bus system.

ABSTRACT:Modern power systems are enormous and interconnected to serve large, remote load regions [3]. In recent years, voltage stability and voltage regulation have received wide attention [3][4]. Voltage control, voltage regulation, reactive power control, steady state stability etc. are important problems of power systems. Flexible AC Transmission Systems (FACTS) controllers can be used for solving these problems. This method is used either when charging the transmission line, or, when there is very low load at the receiving end. Due to very low or no load a very low current flows through the transmission line. Shunt capacitance in the transmission line causes voltage amplification (Ferranti Effect). The receiving end voltage may become double the sending end voltage (generally in case of very long transmission lines). To compensate, shunt inductors are connected across the transmission line. The lead time between the zero voltage pulse and zero current pulse duly generated by suitable operational amplifier circuits in comparator mode are fed to two interrupt pins of the microcontroller where the program takes over to actuate appropriate number of opto-isolators interfaced to back to back SCRs at its output for bringing shunt reactors into the load circuit to get the voltage duly compensated. The microcontroller used in this work is of 8051 families which is of 8 bit. The power supply consists of a step down transformer 230/12V, which steps down the voltage to 12V AC. This is converted to DC using a Bridge rectifier. The ripples are removed using a capacitive filter and it is then regulated to +5V using a voltage regulator 7805 which is required for the operation of the microcontroller and other components . In this paper, work has been done to improve Power Factor usingTSR based FACTS Devices.

et al. [12] studied the impact of UPFC on damping oscillations of the generator rotor. For this purpose, authors proposed system critical modes and residue factor methods for the optimal placement. In addition, they applied particle swarm optimization (PSO) to optimize the parameters of the UPFC. Reference [13] proposed the use of SVC to improve power system transient stability and damp oscillations in case of three-phase short-circuit. In reference [14], the authors demonstrated the performance of UPFC compared to SVC, in terms of improving power system stability. In order to enhance transient stability, the authors in [15] compared the performances of UPFC to different FACTS devices, namely TCSC, STATCOM (Static Synchronous Compensator) and SVC. B. Bhattacharyya et al. [16] were interested in the cost implication and power loss of installing UPFC alongside with SVC and TCSC. After determining the optimal emplacement and parameters of the FACTS using specific algorithms, authors have reported the gain obtained when adding the hybrid compensator to the other FACTS. Likewise, authors in [17] discussed the use of SVC, TCSC, and UPFC in the improvement of dynamic and transient system stability. They compared the three FACTS based on their mathematical models and operation modes. It was found that UPFC provided the most rapid control and the highest performances in stabilizing the system. P. Pandey et al. [18] studied the contribution of the SVC and UPFC in enhancing the voltage profile of a grid connected distributed generation system. They demonstrated through simulations, the satisfactory operation of the two FACTS especially the hybrid device. Reference [19] presented the application of a heuristic based procedure to