Enhancing power transfer capability through flexible AC transmission system devices: a review

Global demand for power has significantly increased, but power generation and transmission capacities have not increased proportionally with this demand. As a result, power consumers suffer from various problems, such as voltage and frequency instability and power quality issues. To overcome these problems, the capacity for available power transfer of a transmission network should be enhanced. Researchers worldwide have addressed this issue by using flexible AC transmission system (FACTS) devices. We have conducted a comprehensive review of how FACTS controllers are used to enhance the available transfer capability (ATC) and power transfer capability (PTC) of power system networks. This review includes a discussion of the classification of different FACTS devices according to different factors. The popularity and applications of these devices are discussed together with relevant statistics. The operating principles of six major FACTS devices and their application in increasing ATC and PTC are also presented. Finally, we evaluate the performance of FACTS devices in ATC and PTC improvement with respect to different control algorithms.


Introduction
Power generation capacity has not increased proportionally with consumer demand for power. This demand can be met by building new power generation stations and transmission lines. However, the construction of new transmission systems is hindered by many factors, such as ecological considerations, financial difficulties, and unavailability of space in overpopulated areas (Ahmad et al., 2014a;Albatsh et al., 2015b). Instead of building a new power system network, the total power transfer capability (PTC) of an existing transmission network could be enhanced. Enhancing PTC can also improve the available transfer capability (ATC), on which the restructuring of power systems is usually based. These improvements also provide an economical business solution to the deregulated power market (Ren et al., 2009;Pandey and Chaitanya, 2012;Ahmad et al., 2014b).
ATC is the measurement of the transfer capability that remains in the transmission system network for further commercial use. Given that restructuring power systems is based completely on ATC, system operators and planners use ATC to determine the capability and strength of the transmission system. These properties are evaluated to estimate the total Frontiers of Information Technology & Electronic Engineering www.zju.edu.cn/jzus; engineering.cae.cn; www.springerlink.com ISSN 2095-9184 (print); ISSN 2095-9230 (online) E-mail: jzus@zju.edu.cn transfer capability (TTC), the transmission reliability margin (TRM), and the capacity benefit margin (CBM) (Khaburi and Haghifam, 2010a). Thus, ATC can be expressed mathematically as Ou and Singh (2002). ATC = TTC − TRM − Existing Transmission Commitments (including CBM). ATC can be a very dynamic quantity because it is a function of variable and interdependent parameters which depend on network conditions. Thus, the accurate calculation of ATC relies heavily on the completeness and correctness of available network data.
Enhancing ATC requires extensive control over power flow in an interconnected system. It also requires measuring effective stability progress by using the features of transmission lines to achieve an economical solution. Flexible AC transmission systems (FACTS) are devices that meet these requirements. Various FACTS devices are used to control dynamically the bus voltages, line impedance, and phase angle of HVAC transmission lines, thereby enabling them to operate near their thermal capacity (Ahmad et al., 2014d;Albatsh et al., 2015a) and increasing transmission capacity. Given the significance of FACTS devices, many researchers are investigating how they can be used to enhance ATC (Hamoud, 2000;Hingoranl and Gyugyi, 2000;Abido, 2009;Venkatesh, 2011).
Two types of FACTS controllers are available: one based on thyristor-controlled switches and another on voltage source converters (VSCs) (Section 2). Given that VSCs offer reactive power compensation and control the flow of active power, VSCbased FACTS controllers are used widely to enhance the ATC and PTC of congested transmission (Xia et al., 2008). These controllers are used mainly to provide shunt or series compensation. The exact location, number, and parameter settings of FACTS controllers are based on the optimal performance of these devices in enhancing ATC and reducing real power losses (Chansareewittaya and Jirapong, 2012). Many efficient heuristic techniques have been used to solve complex optimization problems. These techniques include genetic algorithm (GA) (Goldberg and Holland, 1988;Leung and Chung, 2000;Gerbex et al., 2001;Panda and Padhy, 2008;Gitizadeh and Kalantar, 2009), particle swarm optimization (PSO) (Eberhart and Kennedy, 1995;Eberhart and Shi, 2001;Moraglio et al., 2007), the bees algorithm (Idris et al., 2009a;Yousefi-Talouki et al., 2010;Naidu et al., 2014), evolutionary programming (EP) (Yang et al., 1996;Yuryevich and Wong, 1999), tabu search (TS), and simulated annealing (SA) (Burke et al., 1999;Bhasaputra and Ongsakul, 2002;Ongsakul and Bhasaputra, 2002;Chansareewittaya and Jirapong, 2012). Conventional methods such as AC load flow and performance index-based methods have also been used for optimization. All these methods for improving ATC and PTC are presented in different sections of this review.
In this paper, an overview of FACTS devices and their classification is presented. The six major FACTS devices and their effects on improving ATC and PTC are discussed, based on various controller techniques. In addition, a critical analysis of the performance of these controllers is presented.

Introduction
FACTS stands for "flexible AC transmission systems" (IEEE). FACTS are "alternating current transmission systems incorporating power electronics-based and other static controllers to enhance controllability and PTC of transmission lines" (Ramey and Henderson, 2007). In the late 1980s, the Electric Power Research Institute (EPRI) in the USA conducted the first study of FACTS to maintain the flexibility and stability of power systems by employing electronic power controllers. That study was presented at IEEE meetings, forums, and workshops, and at the international conference organized by EPRI in Cincinnati, Ohio, USA in Sept. 1990 (Spee andZhu, 1992;Asare et al., 1994). The concept of FACTS controllers was clearly discussed by Hingorani (1993) and Hingoranl and Gyugyi (2000). FACTS devices control power flow through a transmission network by obeying the command of the control center. These devices also facilitate the operation of transmission lines closer to their maximum thermal limits and the control over the line impedances of a transmission system, the voltage magnitude, and the phase angle of buses. u n e d i t e d
These two types of FACTS controllers vary in terms of their operation and performance. The first group uses thyristor switches to control the ON and OFF times of the reactor and capacitor banks, thereby varying the reactive impedance. By contrast, the second group engages self-commutated DC converters with AC converters, which can internally produce reactive power for transmission line compensation without using reactor or capacitor banks. VSC-based FACTS controllers are preferable to current source inverters because of their economic and performance advantages (Acha et al., 2004;Sood, 2004). VSCbased devices can be used uniformly to control transmission line impedance, angle, and voltage by providing reactive shunt and series compensation as well as phase shifting, or to directly control the real and reactive power flow in the line (Albatsh, 2009;Venkatesh, 2011). FACTS controllers are classified into three categories according to their connection to the system: shunt controllers (e.g., thyristorcontrolled reactors (TCR), SVC, and STATCOM), series controllers (e.g., TCSC and SSSC), and series-shunt controllers (e.g., TCPST or thyristorcontrolled phase angle regulators and UPFC) (Hingoranl and Gyugyi, 2000;Watts et al., 2007); this classification is illustrated in Fig. 1. Table 1 lists the functions of major FACTS devices.

Statistics on FACTS research
This literature review is an extensive survey of  (Hingoranl and Gyugyi, 2000), this survey covers articles only from 1990 to 2012, for convenience. The articles were divided into four groups according to their year of publication: 1990-1994, 1995-1999, 2000-2004, 2005-2009, and 2010 to present. This survey covers almost all publications on the use of FACTS in power systems. The survey results are summarized in Fig. 2. Since the 2000s, FACTS applications have significantly increased. VSC-based FACTS have also become more popular than thyristor-controlled switches. SVC and STAT-COM are the most widely used first-and secondgeneration FACTS controllers, respectively. Both generations have been applied to different areas in power system studies, including optimal power flow (Gyugyi et al., 1995;Ge and Chung, 1999;Li et al., 2000;Zhang and Handschin, 2001;Venkatesh et al., 2004), economic power dispatch, voltage stability (El-Sadek et al., 1997;Haque, 2004), power system security (Visakha et al., 2004), and power quality (Sun et al., 2002;Sannino et al., 2003). The use of FACTS controllers along with increasing interest in their ability to enhance ATC are illustrated in Fig. 3.

Practical applications of FACTS
Manufacturers of FACTS devices are searching for ways to increase reliability under contingency conditions, reduce cost, enhance system stability, and improve power quality. The potential of FACTS devices for these purposes has been widely known since 1979 (Zhang, et al., 2012). The first commercialized SVC used to enhance power quality was installed by GE in 1974(Acharya et al., 2005. SVCs have been installed in about 100 places to control voltage by reactive compensation (Ren, et al., 2009) and have been found to enhance ATC and PTC remarkably well. Around 13 SVC projects in 10 countries have been implemented by Siemens. SVC projects were initiated in Canada and France in 2011, and in Saudi Arabia in 2012. The total capacity of installations is higher than 30,000 Mvar, which provides good client support and global experience (Siemens, 2012). Since 2002, Cascade Steel (McMinnville, Oregon, USA) has been operating an ABB SVC in its electric arc furnace-based melt shop (Abb, 2012). China set up an SVC in the South Hebei Power Grid to enhance power quality. SVCs also provide voltage stability in the transmission network (Baofeng et al., 2010). ABB has installed FACTS devices in railways to ensure voltage stability, avoid sagging and fluctuating voltage, and improve power quality in the railway network and in surrounding networks (Abb, 2012). SVC is also used at the Anding traction substation of the Beijing-Shanghai Electrified Railway to enhance power quality (Jianzong et al., 2009).
The second most widely used FACTS device is the TCSC. The first TCSC, installed by ABB at a substation in Kayenta, Arizona, USA in 1992, increased ATC by 30% (Acharya, et al., 2005. TCSC installations can be found in Stöde, Sweden, at the Slatt Substation of the Bonneville Power Administration (USA), and at the Kayenta substation of the  mann et al., 2002;Paserba, 2003). In 1999, ABB installed two TCSC banks in the Brazilian North-South Interconnection (Gama et al., 2000). A TCSC was installed at the SC station in Stöde, Sweden, to offset subsynchronous resonance (SSR); the TCSC significantly mitigated the SSR problem (Holmberg et al., 1998). The first commercial STATCOM (±80 MVA, 154 kV) was installed by Mitsubishi Electric Power Products at its Inuyama substation in Japan in 1991 (Acharya, et al., 2005). Major STATCOM projects can be found in the USA at the Sullivan substation in northeastern Tennessee, the Talega substation of San Diego Gas and Electric, the Essex substation of the Vermont Electric Power Company, and the 115 KV Glenbrook Substation in Stamford, Connecticut (Ren, et al., 2009). In 2011, ABB supplied and commissioned a FACTS with a STATCOM and an SVC for the power transmission system of Transelec in Chile. Austin Energy, the public utility which serves Austin, Texas USA and surrounding areas, has been operating an ABB-supplied STATCOM in its 138 KV power system since 2005. The STATCOM, which is 80 Mvar inductive to 110 Mvar capacitive, replaced the oil-and gas-fired Holly Power Plant near downtown Austin, which was constructed in the 1960s (Oskoui et al., 2006).
There have been very few UPFC projects. The first practical UPFC project, consisting of two 160 MVA voltage source GTO thyristors, was constructed in Inez, Kentucky, USA in 1998 (Renz et al., 1999;Paserba, 2003) to control the real power, reactive power flow and bus voltages of the transmission network. American Electric Power (AEP) applied the 160 MVA UPFC in the Inez area because of the critical need to increase power transfer capability and provide voltage support in that area. Based on a boundary diagram for UPFC capability, the power flow is increased to its maximum real power swing of 80 MW and a maximum reactive power swing of +200/−150 MVAR. These results proved the capability of UPFC to control real and reactive power flow in transmission lines independently. In addition, the voltage profile has been improved and the power loss reduced significantly in the whole network after installing the 160 MVA UPFC. Another UPFC project was built in the Kangjin substation in South Korea in 2003 (Han et al., 2004). The research institute of the Korean Electric Power Corporation (KEPCO) found that power demand was increasing every year. As a result, the power network was suffering from voltage instability and difficulties in power flow through the transmission lines, especially when there was a fault on the surrounding feeders. KEPCO found that UPFC was the best solution to this problem compared to other devices in terms of the system performance and cost of installation. Based on this investigation, KEPCO has designed and implemented an 80 MVA UPFC project to be integrated with the 154 KV transmission network. UPFC revealed its high performance in controlling the power flow in transmission lines and in dealing with fault cases during under-voltage or overloaded conditions. Table 2 illustrates the FACTS devices made and installed in different countries by ABB, Siemens, and GE, among others.

Power flow equations for different FACTS devices
The main concept of FACTS devices can be described by the basic power flow equation for transmission networks (Fig. 4). The real power transmitted between bus a and b in the network depends on the voltage at each end, line impedance, and phase angle. The power flow equation is described as follows: The parameters of this equation can be controlled easily by using different FACTS devices to enhance power flow. Series FACTS devices, such as TCSC and SSSC, control line impedance X to increase the real power through transmission lines. On the other hand, shunt FACTS devices, such as SVC and STATCOM, regulate bus voltage to control reactive power. Series-shunt FACTS devices, such as TCPST, modify the phase and magnitude of the injected series voltage, whereas UPFCs control all power flow

Series compensation
The function of series compensation is to decrease reactive power in transmission lines by controlling line impedance and to increase line voltage to increase line current and real power.
A simple two-bus network with a series capacitor that compensates for the transmission line (Fig. 5) explains the principle behind series compensation. V a is the sending voltage at bus 1, and V b is the ( 2) where X eff =X L −X C and δ=δ a −δ b . Increasing X C reduces X eff and thus increases the transmitted power. We can present Eq.
(2) in terms of voltage on capacitor V c as follows: where X eff =X L −V C /I and V C =jX C I. Eq. (3) confirms that changing capacitor impedance varies capacitor voltage and thus increases real power.

Shunt compensation
The equivalent circuit of a shunt compensator can be presented as a controllable voltage source V sh in series with impedance Z sh . In a shunt compensator, reactive power can be adjusted by regulating the output voltage.
Based on the equivalent circuit of a shunt compensator ( Fig. 6 where g sh +jb sh =1/Z sh . The operating constraint of a shunt compensator for active power exchange can be expressed as

Static var compensator (SVC)
The SVC is a device consisting of any one of the following power electronic devices: thyristorswitched capacitor, thyristor-switched reactor, shuntswitched capacitor, shunt-switched reactor, thyristorcontrolled reactor (TCR), or a shunt-switched resistor. Compared with conventional switching devices, the SVC has a fast response time and low maintenance cost (Noroozian et al., 2003). SVCs with thyristor switches achieve a fast response time by controlling the firing angle of the thyristor (Ambriz-Perez et al., 2000;Abdel-Rahman et al., 2006;Padiyar, 2007). Such SVCs can also control transient stability and damp power oscillations. The SVC works as a shunt-connected variable reactor or capacitor that compensates for the reactive power required in a transmission network and keeps bus voltage magnitude within its limit. Fig. 7 illustrates the different types of SVC. A three-phase, three winding transformer is used to connect the SVC to the transmission network (Schauder and Mehta, 1993;Noroozian, et al., 2003). Rewatkar and Kewte (2009) investigated the effect of an SVC placed in the middle section of a transmission line. Three vital properties of power (Bollen, 2000;Haixue, 2001), namely, voltage sag (Lamoree et al., 1994), voltage swell (Naidoo and Pillay, 2007), and interruption (Ooi et al., 1997), were considered. An SVC with a thyristor-controlled  Komoni et al. (2010b). The Kosovo power system was examined and developed in PSS/E32. Simulations were conducted for steady-state conditions. A proportional-integral (PI) controller was used as a control tool. SVCs in two typical buses increased the PTC of the power line and the bus voltage.

Enhancing ATC by using SVC
The feasibility of installing FACTS devices in southeastern Romania was examined by Bulac et al. (2009). A steady-state SVC model and an algorithm designed using power flow analysis and control (PFAC) software were proposed. Static and dynamic analyses of the SVC revealed the improved overall dynamic performance of the power system. Artificial intelligence (AI) methods have been used by many researchers to enhance ATC through optimal placement of FACTS devices. However, the exact location and accurate parameters of FACTS controllers are difficult to determine because of complicated combinatorial optimization (Mori and Goto, 2000). To overcome this problem, Pham et al. (2006a) and Pham et al. (2006b) proposed the bees algorithm, which was used by Idris et al. (2009b); Idris et al. and (2010) to find an optimal location for the SVC to maximize ATC in a deregulated power system. The proposed algorithm effectively maximized the ATC. Another AI method, particle swarm optimization Venter and Sobieszczanski-Sobieski, 2003;Moraglio, et al., 2007), was proposed to solve the multi-objective optimization of minimizing power loss and maximizing TTC with system constraints, such as power balance, voltage limits, and line thermal limits (Chansareewittaya and Jirapong, 2010;Rao and Kumar, 2011). Constraints were handled by using the penalty function of Parsopoulos and Vrahatis (2002). An SVC with optimal location and rating reduces real power losses and increases TTC compared with a no-SVC case. GA and EP (Yang, et al., 1996;Yuryevich and Wong, 1999) were used to determine the optimal location of an SVC discussed by Cai et al. (2004) and Ongsakul and Jirapong (2005). Optimally placing an SVC using both algorithms increases TTC significantly. Conventional heuristic methods have high CPU times. To solve this problem, Chansareewittaya and Jirapong (2012) proposed a hybrid model of the TS (Burke, et al., 1999;Bhasaputra and Ongsakul, 2002) and SA (Van Laarhoven and Aarts, 1987;Goffe et al., 1994) algorithms (TSSA). This hybrid model was used to determine the optimal number, locations, and parameter settings of SVCs in a power system to transfer maximum power and reduce real power loss. TSSA significantly enhances TTC with less CPU time and outperforms EP. Installing FACTS devices enriches both single-area and multi-area ATC. Venkatesh (2011) analyzed the sustainability and technical advantages of enriching single-area and multiarea ATC by using an SVC in a single device and in three multi-type similar and different device combinations. Another optimization tool was used by Madhusudhanarao et al. (2010) andVara Prasad et al. (2011) to find the location and control the parameters of an SVC based on real-code genetic algorithm (RGA) (Xiong et al., 2004;Tsoulos, 2008). Properly installing SVCs improves not only the voltage profile but also the ATC. Nagalakshmi and Kamaraj (2012b) used PSO, differential evolution (DE) (Price et al., 2006;Qin et al., 2009) and composite differential evolution (CoDE) (Zheng and Wang, 2011) algorithms to improve loadability in transmission networks. The performance of PSO, DE, and CoDE was compared to determine their effect on enhancing loadability with SVC. DE is more effective, easy to  (Zhong et al., 2008) for improved ATC estimation as a decision criterion was proposed by Farahmand et al. (2012). HMPSO combines fuzzy logic (Klir and Yuan, 1995;Elsayed et al., 2013;Albatsh et al., 2014) and the analytical hierarchy process (AHP) (Partovi et al., 1990;Handfield et al., 2002) to model the qualification of each problem objective (Saaty, 1977) and prioritize the objectives. It was implemented by using repeated power flow (RPF) (Chiang et al., 1995) with respect to line thermal and voltage stability limits, and was found to be the most promising approach. ATC can be enhanced significantly through prudent usage of FACTS devices.

Thyristor controlled series compensator (TCSC)
The TCSC is a series FACTS controller used to provide series compensation for transmission line impedance in a continuous, swift, and controllable way. The TCSC has great potential for increasing ATC through the transmission line. Features such as automatic control of the thyristor have been integrated into the TCSC. Therefore, the TCSC can be employed to enhance transient stability, mitigate SSR, and damp power oscillations (Perkins and Iravani, 1997;Kakimoto and Phongphanphanee, 2003;Pilotto et al., 2003;Jovcic and Pillai, 2005). Fig. 8 shows a schematic diagram of a TCSC (Del Rosso et al., 2003) in which a TCR is connected in parallel to a fixed series capacitor. Naik et al. (2010) and Srinu Naik et al. (2010) used a method based on an Mvar-corrected MW limit to improve ATC by using a TCSC. This method accounted for changes in reactive power flow through the line to calculate ATC. The limit of real power transfer was determined by solving the base cases. The ATC was significantly improved.

Enhancing ATC by using TCSC
PTC was enhanced by Yang, et al. (1996) and Yuryevich and Wong (1999) by selecting an optimal maximum number of FACTS devices using EP. The same objective was achieved by Chansareewittaya and Jirapong (2011) not only by optimally locating the TCSC, but also by setting its parameters using search space management. Split search space management helped to minimize the operating point interval of the FACTS controller. Using EP and split search space management for the TCSC increases the PTC of the system to a promising value. Manikandan (2010) analyzed ATC boosting with a TCSC, and determined the optimal location and parameters of the TCSC using PSO and GA. ATC was significantly enhanced by the TCSC. The CPU execution time required by PSO to improve ATC was shorter than that required by GA.
An optimization kit which combines GA and RGA (Xiong, et al., 2004;Tsoulos, 2008) was used by Vara Prasad, et al. (2011) to enhance ATC by determining both the optimal location and control parameters of the TCSC. RGA effectively determines the optimal location of TCSCs by considering the aim of ATC enhancement. A statistical analysis was conducted by Manohar and Amarnath (2012) to reduce active power losses by implementing a TCSC to enhance ATC. Placing the TCSC on the line in a direct and simple way reduces losses and enhances ATC. By minimizing active power losses using a TCSC, Rashed et al. (2012) achieved optimum ATC. DE (Price, et al., 2006;Qin, et al., 2009) and GA were used to determine the optimal location and parameter settings of the TCSC.
Khaburi and Haghifam (2010b) used a probabilistic analysis to analyze the effect of a TCSC on enhancing TTC. TTC was calculated by employing RPF method (Chiang, et al., 1995). The performance of the proposed algorithm was evaluated against the IEEE Reliability Test System (Subcommittee, 1979). The algorithm robustly enhanced TTC.  Alabduljabbar and Milanović (2010) to find the optimal location of a TCSC. The objective functions in this algorithm were based on cost functions, including installation and maintenance cost, the cost of both active and reactive power, and the cost of FACTS devices. The TCSC significantly increased ATC by reducing the generation cost of both real and reactive power. Sensitivity analysis (Saltelli et al., 2000) was implemented by Rashidinejad et al. (2008) under steady-state conditions to enhance ATC with respect to control parameters by using a TCSC. These parameters were optimized through a hybrid heuristic approach of AHP, fuzzy logic and RGA. ATC improved when the TCSC was connected to the line. Farahmand, et al. (2012) proposed a novel HMPSO (Zhong, et al., 2008) method consisting of standard PSO, fuzzy logic, and AHP to enhance ATC using a TCSC. The novel HMPSO method can be employed to significantly enhance ATC by using the TCSC precisely. Arzani et al. (2008) and Chawla et al. (2009) discussed the optimized use of a TCSC to improve ATC. The principle of transmission line reactance compensation was employed to enhance network ATC. Installing a TCSC improved ATC by 15.3%.
Many studies on SVC have also employed a TCSC to enhance ATC by using different artificial techniques. Idris, et al. (2009b) and Chansareewittaya and Jirapong (2010); Venkatesh (2011); Nagalakshmi and Kamaraj (2012b) determined the optimal locations of both devices by using the bees algorithm and PSO, respectively. TSSA was used by Chansareewittaya and Jirapong (2012) along with search space management to determine the optimal location and number of TCSC and thus enhance PTC. Nagalakshmi and Kamaraj (2012b) proposed enhancing ATC by using both DE and CoDE.

Thyristor controller phase shifting transformer (TCPST)
TCPST is a FACTS device that can modify the phase angle between bus voltages and the magnitude of series injected variable voltage to enhance power flow. Regulating power flow reduces low-frequency oscillations (Abido, 1999;Hashmani et al., 2001).
TCPST can also provide series compensation to increase system stability by speeding up the response of the phase shifter. The TCPST can control the frequency positively if it is connected in series with the tie-line (Abraham et al., 2007). The TCPST can easily alter the conventional power system stabilizer (PSS) (Wang et al., 1997). The basic construction of a TCPST is shown in Fig. 9.

Enhancing ATC using a TCPST
A conventional thyristor-switched capacitor / reactor (Paserba, 2003), the TCPST is less popular than the SVC and TCSC (Fig. 3). Thus, research on this controller for ATC enhancement has been limited. Most studies have used TCPST only for comparison with other controllers. Idris, et al. (2009b), for instance, used the bees algorithm to find the best location of TCPST, SVC and TCSC for enhancing ATC. Based on the thermal, voltage, and operational limits of FACTS controllers, the RPF algorithm was used to find the most feasible ATC of a system (Grijalva and Sauer, 2001). The rates of ATC enhancement of all devices were higher with the bees algorithm than with GA. Alabduljabbar and Milanović (2010) placed TCPST optimally in a multi-machine power system to enhance ATC. To perform allocation, optimal power flow and GA-based AI techniques were manipulated. Nagalakshmi and Kamaraj (2012a) compared the ability of PSO, DE, and CoDE to enhance the ATC of power systems with TCPST.

Static synchronous series compensator (SSSC)
The SSSC is an advanced controlled series compensator that functions as a controllable voltage It is connected through a transformer in series with a transmission line. The SSSC mainly injects voltage with a variable magnitude quadrature with the line current to compensate for the voltage drop in the transmission network (Sen, 1998;Zheng et al., 2013). In steady-state operation, the SSSC transfers both reactive and real power within the power system network. As the SSSC has its own DC capacitor, it does not draw reactive power from the transmission network and thus enables it to control active and reactive power, and regulate bus voltage. The basic construction of an SSSC is shown in Fig.  10. Fig. 11 illustrates the equivalent circuit of the SSSC, which combines voltage source , transmission line resistance , and reactance . If the DC side has no energy source and the losses of the converter are neglected, the real power in steady-state operation can be expressed as where V c is in quadrature with I. When V c lags I by 90°, the operation is capacitive. Thus, the current in the transmission line and therefore the active power are increased. However, when V c leads I by 90°, the operation is inductive, and the current and active power are reduced.

Enhancing ATC using an SSSC
A series-connected FACTS controller SSSC was used by Nimje et al. (2011) to enhance PTC along with the required active and reactive power flow through a transmission line. PTC was enhanced at an injected voltage magnitude of 0.2 per unit within an angle variation of 0° to 90°. Zhang and Zhang (2006) used a new power injection SSSC model to analyze power flow. This model included the complex impedance of the series coupling transformer and the charging susceptance of the line. Because the SSSC has multi-control capability, it was used by Iwamoto and Tamura (1981) to enhance ATC, and power flow was calculated using the Newton-Raphson method. To maximize ATC, an exhaustive analysis based on the DC load flow method was presented by Menniti et al. (2006) to find the optimal location for an SSSC. Suitable lines for SSSC placement were determined by obtaining a merit order list with respect to the maximum load increase.
Ajami and Armaghan (2013) used an SSSC to lower the congestion of transmission lines and thus maximize the ATC between desired network buses. Harmony search (HS) algorithm (Mahdavi et al., 2007) incorporated in a new method was employed to confine the number of lines to speed up convergence. The PSO algorithm was also used for optimization. The results of the HS algorithm were compared with those of the PSO algorithm to determine the effectiveness of the proposed method in locating and sizing optimization problems.
Other studies on SSSC (Esmaeili and Esmaeili, 2012;Kumar and Kumar, 2013) are discussed in the next two sections along with UPFC and STATCOM.

Operating principles of STATCOM
A STATCOM is composed of a selfcommutated switching power converter, a coupling transformer connected parallel to the transmission line, and a DC link. The construction of a STAT-COM is illustrated in Fig. 12. The STATCOM controls its current magnitude and impedance, and the voltage magnitude of the source and remote bus. It also provides reactive power and controls active power flow, thereby improving the PTC of congested transmission lines (Shakarami and Kazemi, 2010). The exchange of real power between the transmission network and the STATCOM can be neglected in steady-state analysis. Thus, only reactive power can be exchanged between them .

Enhancing ATC using STATCOM
Power transfer distribution factors (PTDFs) were used by Kumar and Kumar (2013) and sensitivity analysis by Jain et al. (2009) to increase ATC using a STATCOM for bilateral and simultaneous/multi-transaction cases with and without line contingency cases. Both methods provided accurate dynamic ATC values when a STATCOM was connected to the line. Static ATC was slightly increased by optimally placing a STATCOM based on the above methods.
A new control framework using the properties of the single-input two-output feedback system was developed for VSC (Jain, et al., 2009). This new control technique enables a STATCOM to adjust active power transfer by using angle control rather than pulse-width modulation (Trzynadlowski et al., 1994;Holmes and Lipo, 2003). The possibility of increasing PTC and controlling line power flow in the Kosovo power system by using a STATCOM was investigated by Komoni et al. (2010a) through a simulation in PSS/E 32. The results of the simulation showed that using a STATCOM increases PTC and controls line power flow in the power system. Another study used transmission line parameters from the Indore-Itarsi transmission corridor (Chawla, et al., 2009) to enhance PTC using a STATCOM and simulated them in the MATLAB Sim Power System. A 48-pulse STATCOM was placed at the center of a transmission system by Singh and Saha (2008) to enhance the PTC of the line. PI controllers were used where system parameters were processed through the d-q axis reference frame. The results of the simulation showed that the PTC of the transmission network was enhanced.
An effective method for sizing and locating a STATCOM to improve TTC was discussed by Esmaeili and Esmaeili (2012). This method aims to increase TTC, reduce line congestion, and minimize losses for optimization. Optimization was performed using the HS algorithm. The results of the HS algorithm were compared with those of PSO and GA. HS had a better convergence rate and greater accuracy than GA or PSO.

Unified power flow controller (UPFC)
The UPFC is a versatile FACTS device because it can individually or sequentially control all power system network parameters, including voltage amplitude, line impedance, and phase angle. The UPFC consists of a STATCOM and an SSSC connected back to back through a DC link capacitor (Fig. 13). The STATCOM is a controllable current source, whereas the SSSC acts as a controllable voltage source (Albatsh et al., 2015c). The STATCOM is connected to the AC system in parallel through a three-phase transformer and mainly generates the real power to be consumed by the SSSC. Moreover, the STATCOM supports the transmission network with reactive power compensation. The SSSC is also connected to the transmission line via a transformer, but in series. The SSSC compensates for voltage drops in the transmission network by injecting an AC voltage with controllable phase and magnitude, thereby improving active and reactive power transmission. Active power can be exchanged between the STATCOM and the SSSC via the DC link capacitor. Each converter can also exchange reactive power independently at its terminal (Papic et al., 1997;Sen and Stacey, 1998;Huang et al., 2000).

Fig. 12
A two bus transmission network with STATCOM Load Fig. 13 A schematic diagram of a UPFC u n e d i t e d

Enhancing ATC using UPFC
A simplified steady-state model of a multiterminal UPFC (Fardanesh, 2004;Vasquez-Arnez and Zanetta, 2008) was analyzed by Omoigui et al. (2008) to control both real and reactive power flows through the network. A rotating d-q axis framework controller was used to obtain a time-invariant equation that describes the system. The simulation result confirms that a multi-terminal UPFC is a promising real and reactive power flow controller in transmission lines.
Chengaiah and Satyanarayana (2012) developed a new steady-state UPFC model with both its shunt and series controllers employed to solve operating constraint violations. The Newton-Raphson method was used to solve the power flow problem. Both voltage and power improved significantly when the UPFC was connected to the system.
A method for increasing ATC by using a UPFC was presented by Takasaki (2006) for different power system models. The performance of the UPFC was compared with that of a PSS (Chung et al., 2002;Hashemi et al., 2012). A combined UPFC and generator PSS was also employed to enhance ATC. This combination significantly enhanced ATC stability.
A comprehensive analysis to improve the PTC of transmission lines using the UPFC was presented by Ramesh and Laxmi (2012). A new UPFC control scheme was described for overcoming the limitations of the conventional control scheme, such as the damping of real and reactive power and the attenuation of power fluctuation. Installing the UPFC increased PTC and reduced the magnitude of fault current and excitation voltage oscillations.
The UPFC and Sen transformer (ST) were compared by Kumar and Kumar (2012) in terms of enhancing ATC by using an approach based on optimal power flow for both multi-transaction and bilateral-transaction environments with intact and contingency cases. A model was created to observe the impact of ZIP load (Grigsby, 2012) and compared with the optimal power flow (OPF) model. ATC increased with both the UPFC and ST for all transactions in the line contingency and intact cases. Some studies have used AI techniques to enhance ATC by using the UPFC. A PSO-based algorithm was used by Chansareewittaya and Jirapong (2010) and Venkatesh (2011) to determine the optimal locations, types, and parameter settings of UPFC to enhance PTC and reduce power losses. The results were compared with those of the EP algorithm to determine the efficiency of the UPFC. Esmaeili and Esmaeili (2012) used HS, which has good convergence and accuracy, to improve TTC and reduce line congestion and total power loss. An AHP was used to obtain the priority vector for each alternative. The performance of the proposed method was compared with that of GA and PSO. Simulation results indicated that the proposed algorithm outperformed the other two algorithms.
The optimal location of FACTS devices, including the UPFC, was investigated by Kumar and Kumar (2013) based on the variation pattern of PTDFs (Sookananta et al., 2007) obtained from the Newton-Raphson load flow approach for bilateral and simultaneous/multi-transaction cases with and without line contingency cases. The FACTS devices enhanced ATC in all transaction cases and line contingencies.
A method for improving transient stability using the UPFC was proposed by Masuta and Yokoyama (2006). This method involves conducting an OPF to enhance ATC. ATC calculation accounted for both transient and steady-state stability constraints. The OPF problem was formulated to optimize the size of the UPFC inverters. A gain-phase compensation controller, such as a PSS-type, was used by Motoki and Yokoyama (2004) to improve steady-state stability. Transient stability was enhanced by selecting an appropriate gain Kp value. Cai et al. (2002) discussed the proper installation of a UPFC in a system with parallel transmission lines. The UPFC should always be placed on lines with higher impedance because losses in these lines are greater than in other lines. Such an installation increases total PTC. Sawhney and Jeyasurya (2004) employed a UPFC to increase ATC. The Newton-Raphson load flow method was used to calculate ATC. The results were verified through a continuous power flow program. Power transfer was enhanced by properly allocating the UPFC.
A cross-coupled controller was implemented by Basu (2011) and Chansareewittaya and Jirapong (2011) to increase PTC in the transmission line by considering the variation of the power system parameters. The disadvantage of previous methods was that they did not consider the dynamic performance u n e d i t e d of the DC link capacitor in the implementation of the controllers. Kannan et al. (2004) proposed the use of another controller for UPFC based on coordination control of real and reactive powers and in which the dynamics of the DC capacitor were taken into consideration.
Fuzzy logic based UPFC has been implemented in PSCAD software to increase PTC in transmission lines (Ahmad et al., 2014c).

Critical Analysis
Based on this review of major FACTS controllers and their effect on ATC and PTC, we summarize the features of each controller in Table 3. Some remarks about the behavior of different FACTS controllers are also included. Based on this critical analysis, we conclude that research on using D-Q transformation, artificial neural networks, and fuzzy logic controllers to increase ATC and PTC has been insufficient. The combination of neural network and the fuzzy expert systems (i.e., neuro-fuzzy systems) has good potential for enhancing ATC by using different FACTS controllers. Moreover, several studies have focused on the steady state model of FACTS devices. But all these techniques have adopted the steady state analysis of the FACTS controller which is effective only for the planning and designing stage of power system networks. These models cannot be used to study real time operation of power system networks. Therefore, it is essential to develop a dynamic model of FACTS devices so that the real time analysis of power system networks can be conducted.

Conclusions
In this paper, we have presented an overview of FACTS devices and their classification, and reviewed studies of their applications and their use in enhancing ATC (from 1990 to 2012). The applications of FACTS devices in different countries by reputable companies, such as ABB and Siemens, were also reported.
The six major FACTS controllers and their basic structure and effects on enhancing ATC and PTC were examined. A critical analysis of various control techniques for the main FACTS controllers was tabulated to show their performance in improving ATC and PTC. This survey will be helpful to researchers of ATC and PTC enhancement through the use of FACTS controllers. Advanced SVC models for Newton-Rphson load flow and newton optimal power flow studies. IEEE Transactions on Power Systems, 15(1):129-136. Arzani, A., Jazaeri, M., Alinejad-Beromi, Y., 2008. Available transfer capability enhancement using series FACTS devices in a designed multi-machine power system. 43rd International Universities Power Engineering Conference, p. 1-6. [doi:10.1109/upec.2008.4651434] Asare, P., Diez, T., Galli, A., et al., 1994. An overview of flexible AC transmission systems, Babu, A.V.N., Sivanagaraju, S., Assessment of available transfer capability for power system network with multiline FACTS device, Bachmann, U., Berger, F., Reinisch, R., et al., 2002. Possibilities of multifunctional FACTS application in the european electric power system under the changing conditions of the liberalized electricity market. CIGRE Session, Germany. Baofeng, T., Hui, F., Xiaowei, W., Xiao, Y., Cuiyan, H., 2010.

References
The dynamic simulation research on application of svc in the south hebei power grid. IEEE press, New York, Bulac, C., Diaconu, C., Eremia, M., et al., 2009. Power transfer capacity enhancement using SVC, PowerTech,