Utilization of FACTS devices in power systems: A review

Utilization of FACTS devices in power systems: A review Irene N. Muisyo and Keren K. Kaberere Abstract Electricity demand has been increasing over the last three decades, while the expansion of generation and transmission networks has been limited due to environmental and economic constraints of building new generating plants and transmission lines. Consequently, transmission networks are at times driven close to their thermal and stability limits. Over the years, attention has been shifting towards better utilization of existing power system infrastructure, which can be achieved through employment of power electronic technologies such as flexible AC transmission system (FACTS) devices. FACTS devices have been used to solve various power system problems such as voltage regulation, power flow control, transfer capability enhancement, damping inter area modes and enhancing power system stability. This paper presents a literature survey of FACTS devices and their applications in power systems. Keywords FACTS, SVC, TCSC, STATCOM, SSSC, UPFC. T I. INTRODUCTION HE power system is the most complex system built by man. It consists of numerous components such as generators, transmission lines, distribution lines, transformers and a variety of loads. The efficiency of a power system is determined by the ability of transmission lines to optimally transfer electrical power from generating stations to the loads. Power flow control in power systems is increasingly becoming a major concern for system operators as a result of power system restructuring and constrained transmission expansion [1]. Conventional power flow control techniques such as generation rescheduling, use of phase shifting transformers, reactive power support and load shedding are frequently applied in power systems. In deregulated markets, generation rescheduling and load shedding have to be limited since there are existing power transaction contracts between generating companies and their customers. Reactive power support and the use of FACTS devices to enhance power system performance has gained a lot of interest in recent years [2] [3]. This paper presents an overview of FACTS devices, their applications in power systems and their associated costs and benefits. The paper is organized as follows: in Section 2, an introduction to FACTS devices is given whereas the different types of FACTS devices is presented in Section 3. FACTS device applications are outlined in Section 4 while their benefits and costs are given in Section 5. The emerging research areas related to FACTS I. N. Muisyo, Department of Electrical and Electronic Engineering, JKUAT (Mobile Phone: +254721332377; e-mail: muisyoirene@jkuat.ac.ke). device utilization is given in Section 6. Conclusions are drawn in Section 7. II. FLEXIBLE AC TRANSMISSION SYSTEMS The concept of FACTS was first introduced by Hingorani and Gyugyi in 1988. FACTS refer to the application of power semiconductor devices to control electrical variables such as voltage, impedance, phase angle, current, active and reactive power [1]. The active power transferred in a lossless line from bus i to bus j is dependent on line reactance X ij, bus voltage magnitudes V i and V j and the phase angle between sending and receiving end buses δ, as approximated in (1). P ij V iv j X ij sin δ (1) FACTS devices act by modifying the line reactance, injecting or absorbing reactive power (thus influencing bus voltage magnitude) or modifying the phase angle between sending and receiving end buses. FACTS devices are designed using high speed power electronic controllers. This overcomes limitations of mechanically controlled AC power transmission system components which include slow response and wear and tear [3]. Increased utilization of FACTS devices has been made possible due to the ongoing developments in the field of power electronics. In practical systems, some lines become congested if unplanned power exchanges occur. To enhance the security of the power system, appropriate types of FACTS devices should be chosen and suitably installed to redistribute power flow and regulate bus voltages [2] [4]. The optimal location of FACTS devices is a highly constrained and complex optimization problem. Generally, the approaches towards optimal location of FACTS devices can be classified into three categories: classical optimization approaches, sensitivity based approaches and heuristic optimization techniques [5]. FACTS devices can be connected to a transmission line at any appropriate location in series, shunt or a combination of series and shunt. Shunt FACTS controllers include the static VAr compensator (SVC) and static synchronous compensator (STATCOM), series FACTS controllers include the thyristor controlled series compensator (TCSC) and static synchronous series compensator (SSSC) whereas the combined shunt and series FACTS controllers include unified power flow compensator (UPFC) and interline power flow compensator (IPFC) [6] [7]. K. K. Kaberere, Department of Electrical and Electronic Engineering, JKUAT (e-mail: kkanuthu@eng. jkuat.ac.ke). 1

From literature survey, various researchers have investigated utilization of FACTS devices for different objectives such as maximizing power system security [2] [8], enhancing power system transient stability [9] [10], enhancing voltage stability [11], minimizing active power losses [12], minimizing cost of generation [13] among others. Different metaheuristic optimization techniques such as cuckoo search algorithm (CSA) [2], particle swarm optimization (PSO) [14], nondominated sorting genetic algorithm (NDSGA) [15], improved particle swarm optimization algorithm using eagle strategy (ESPSO) [12], evolutionary algorithm [11], genetic algorithm [13] have been investigated. Classical optimization techniques such as nonlinear programming methods [3] and line flow indices [8] have also been explored. Multi-objective optimization problems for FACTS device location have emerged in the last decade, as outlined in [14] [15]. Emerging areas of FACTS device utilization in power systems have also been explored such as the integration of renewable energy sources (RES) and smart grids [16], and parameter tuning of power system stabilizers (PSS) [17] [18]. We now describe various FACTS devices and their utilization in power systems. III. CLASSIFICATION OF FACTS DEVICES There are three generations of power electronic based FACTS devices: i. First generation FACTS devices which employ thyristor switched capacitor and reactor banks to control on and off periods of the devices. ii. Second generation FACTS devices which use selfcommutated DC-AC converters to generate capacitive and inductive reactive power for transmission line compensation. iii. Last generation FACTS devices also known as D- FACTS which are smaller and less expensive than first and second generation FACTS devices. They are used in distribution systems [16]. The basic principles of operation of FACTS devices will be discussed briefly. A. First generation FACTS devices The first generation FACTS devices are: static VAr compensator (SVC), thyristor controlled series capacitor (TCSC), and thyristor controlled phase shifter (TCPS). 1) Static VAr compensator (SVC) The SVC is the oldest FACTS device whose primary purpose is to improve bus voltages by means of reactive power compensation. The world s first SVC for utility application was installed in 1974 in Nebraska by General Electric (GE). More than 800 SVCs have been installed worldwide, both for utility and industrial applications. ABB remains the pioneer in development of SVC and has supplied 55% of the total market of which 13% were being installed in Asian countries [16]. As a consequence of deregulation in the UK in 1990, voltage control became difficult. To accommodate the risk associated with changing power system conditions, the UK installed relocatable SVCs in the National Grid Company (NGC). By 2004, 12 RSVCs of 60 MVAr each were operational [9]. The basic circuit of SVC is shown in Fig. 1. Fig. 1 Basic circuit for SVC As shown in Fig. 1, the SVC is composed of a fixed capacitor (C) and a thyristor controlled reactor (L). The SVC is usually connected at the midpoint or at the end of a transmission line through a coupling transformer. The equivalent susceptance of the SVC is controlled by adjusting the firing angle of the thyristors. With proper coordination of the capacitor switching and reactor control, the VAr output can be varied continuously between the capacitive and inductive ratings of the equipment. SVCs are used for steady state voltage control, damping oscillations, improving transient stability and reducing system losses [17]. 2) Thyristor controlled series compensator (TCSC) The TCSC is a first generation FACTS device which uses thyristors to manage a capacitor bank connected in series with a transmission line. The world s first three phase TCSC was developed by ABB and installed at Kayenta substation, Arizona in 1992, that raised the capacity of a transmission line by almost 30%. By 2004, 7 TCSC s had been installed around the world such as in Sweden, China and India [16]. The TCSC consists of a capacitor bank C, and a thyristor controlled reactor (L) as shown in Fig. 2. Fig. 2 Basic structure of TCSC The TCSC is connected in series with the AC transmission line whose capacity of power transmission is to be improved. By adjusting the firing angle of the thyristors, the capacitive reactance is smoothly controlled over a wide range, thus the line 2

reactance is modified. The TCSC is increasingly finding use in long transmission lines for functions such as increasing line loadability, mitigating sub-synchronous resonance, damping power oscillations and enhancing transient stability [13] [19]. B. Second generation FACTS devices The second generation FACTS devices are: static synchronous compensator (STATCOM), static synchronous series compensator (SSSC), unified power flow controller (UPFC) and interline power flow controller (IPFC). 1) Static synchronous compensator (STATCOM) The STATCOM is a gate turn off (GTO) based voltage source converter (VSC) capable of generating or absorbing real and reactive power at its output terminals. The world s first commercial STATCOM (80 MVA, 154 kv) was installed in Inuyama substation in Japan in 1991 by Mitsubishi Electric Power Products Inc. A STATCOM with a capacity of 225 MVAr was also established in East Claydon 400kV substation, UK in 2001, whose purpose was to provide dynamic reactive compensation. Another STATCOM of 100 MVAr was constructed in SDG & E Talega substation, USA in 2003, with the aim of providing dynamic VAr control during peak load conditions. In 2013, a STATCOM with capacity of 150MVAr at 275kV was installed in Turkey too. There are more than 20 STATCOMs operating successfully around the world [16] [18]. The basic structure of a STATCOM is shown in Fig. 3. Fig. 4 Basic structure of SSSC From Fig. 4, the SSSC is based on a DC capacitor fed VSC which generates a three phase voltage at fundamental frequency, which is then injected in a transmission line through the series transformer. The SSSC injects a voltage with controllable magnitude and phase angle at the line frequency, hence it controls the active and reactive power flow in the network. The main advantage of SSSC over TCSC is that it does not remarkably interfere with the transmission line impedance hence there is no danger of having resonance problems [10]. 3) Unified power flow controller (UPFC) The SSSC and a STATCOM can be combined to produce a unified power flow controller (UPFC). The first utility UPFC was installed at the Inez substation of American Electric Power in 1998. In 2004, an 80 MVA UPFC was also constructed at Gangjin substation in South Korea. The basic structure of UPFC is shown in Fig. 5. Fig 3 Basic structure of STATCOM As shown in Fig. 3, the STATCOM is a shunt connected static VAr compensator which converts DC input voltage into AC output voltage in order to compensate the active and reactive power needed by the system. The STATCOM is able to control its output voltage independently of the AC system voltage, hence can provide better reactive power compensation at low grid voltages [9] [18]. 2) Static synchronous series compensator (SSSC) The SSSC is made up of a voltage source converter (VSC) serially connected to a transmission line through a transformer. The SSSC is not yet in commercial operation as an independent controller. Its basic structure is shown in Fig. 4. Fig. 5 Basic structure of UPFC From Fig. 5, the UPFC consists of two AC/DC converters, one connected in series with the transmission line and the other in parallel with the transmission line. The DC side of the two converters is connected through a common capacitor which provides DC voltage for the converter operation. The series converter injects an AC voltage with controllable magnitude and phase angle in series with the transmission line via a series transformer. The shunt converter injects or absorbs the real power demand of the series converter at the common DC link. It also generates or absorbs reactive power to provide shunt compensation for the line. The UPFC thus allows exchange of real and reactive power with the transmission line, hence improving steady state and transient stability of the system [20] [21]. 3

C. D-FACTS devices A new concept of smaller and less expensive FACTS devices is being explored, known as distributed FACTS devices. The adoption of D-FACTS will enhance performance of power systems at a lower installation cost. D-FACTS will also be used to solve some challenges arising in smart grids and micro grids such as voltage fluctuations and interruptions. They will enhance controllability, reliability and improve end user power quality with little environmental impact. D-FACTS devices include D-STATCOM, D-SSSC and D-UPFC [16] [22]. Next we outline some FACTS device applications. IV. FACTS DEVICE APPLICATIONS FACTS devices find applications in all three states of the power system namely: steady state, transient and post transient steady state. A. Steady state applications of FACTS devices The steady state applications of FACTS devices include: i. Congestion management usually carried out by the system operator. FACTS devices such as TCSC, TCPAR and UPFC are employed to alleviate congestion. ii. Available transfer capacity (ATC) improvement. Low ATC implies that the network cannot accommodate new transactions. FACTS controllers such as TCSC, TCPAR and UPFC can be employed to increase the ATC. iii. Reactive power and voltage controllers like SVC and STATCOM are used for voltage control. iv. Loading margin improvement. Both series and shunt compensators are used to increase the transfer capabilities of power systems [13] [23]. B. Dynamic applications of FACTS devices Dynamic applications of FACTS devices include: i. Transient stability enhancement. FACTS devices can provide rapid response during the first swing to control the voltage and power flow in the system. ii. Oscillation damping. Electromechanical oscillations can be better damped if power system stabilizers (PSS) are coordinated with FACTS devices. iii. SSR mitigation. Sub-synchronous resonance (SSR) associated with conventional series capacitors can be mitigated by controlling the operating modes of series FACTS devices such as used in Stode, Sweden power system. iv. Power system interconnection. With series compensation, bulk AC power transmission over distances of more than 1,000km are a reality today as has been used in Brazil North South interconnection [16] [17]. C. Application in deregulated markets FACTS controllers are finding new applications in deregulated markets. Some of these applications include: i. Controlling loop flows which can reduce the transmission capacity, and hence limit the possible transactions through a given line. ii. Making use of FACTS controllers in tie lines to enable participation in transactions or to shield them from the neighboring wheeling transactions. iii. Optimally locating FACTS devices to ensure economic dispatch and reduction in power system losses [23]. The benefits and costs of utilizing FACTS devices in power systems are presented below. V. BENEFITS AND COSTS OF FACTS DEVICES The investment costs of FACTS devices are huge, as seen from the world s second UPFC, which installed in the year 2004 in Keepco power system, Korea. This was the largest single procurement order ever placed by Keepco power system [16]. The investment costs of FACTS devices therefore have to be computed against anticipated benefits. A. Benefits of FACTS device utilization The benefits of utilizing FACTS devices in power systems are: i. Environmental benefits FACTS devices are installed on existing infrastructure, hence eliminate the issue of encroaching on new public and private land. For example in Sweden, eight 400kV systems run parallel to transmit electricity from the north to the south. Each of these transmission lines are equipped with FACTS devices. Studies have shown that four additional 400kV transmission lines would have been necessary, if FACTS devices had not been installed on the power system [16]. ii. Enhanced transmission system reliability FACTS devices employ power electronic devices to regulate power flow and transmission network voltages through fast control action. This can mitigate dynamic disturbances thus increasing system reliability. iii. Rapidly implemented installations Constructing a new transmission line may take several years, whereas FACTS device installation takes between 12 to 18 months. FACTS device installation also allows the flexibility for future upgrades. iv. Financial benefits Financial benefits from FACTS device utilization comes from additional sales or wheeling charges due to increased transmission capability, delay in investing in high voltage transmission lines and engaging cheaper generation facilities. The enhanced power system stability also reduces the risk of forced outages, thus reducing lost revenue and penalties from power contracts. v. Reduced maintenance cost The overhead transmission lines need to be cleared from the surrounding environment from time to time. In comparison to this, the FACTS maintenance cost is very minimum [16] [23]. B. Drawbacks of FACTS technology The major limitation of FACTS device utilization is their high cost of investment. The investment costs mostly depend on the 4

device rating and modifications to be done on existing infrastructure. The higher the device rating, the more expensive the device is. Due to the modifications to be done on existing substations, costs such as civil works, installation, commissioning, insurance and project management will arise. The resulting voltage and current waveforms from FACTS devices are usually distorted due to the switching nature of power electronic converters. Additional interface filters have to be incorporated [5] [6]. VI. RESEARCH GAPS There is a lot of ongoing research in terms of FACTS device utilization ranging from their technologies, optimal location, coordination of FACTS devices with power system components and the economic benefits of FACTS device utilization. A. FACTS device technologies With the history of more than three decades and widespread research and development, FACTS controllers are now considered a proven and mature technology. From various studies, the performance of second generation FACTS devices is seen to be better than for first generation devices, with the UPFC termed as the most versatile FACTS device [4] [8] [9] [10]. From literature survey, very little has been done on the actual AC/DC models of VSC FACTS devices. An accurate representation of VSC FACTS devices in transient stability studies would be beneficial to bring out their behavior in stressed system conditions, or when the devices operate at their limits. The last generation of VSC based D-FACTS devices should be thoroughly investigated to depict how their topologies can be used to enhance performance of micro-grids, standalone DG schemes, RES integration and smart grids [2] [5] [16]. B. Optimal location of FACTS devices Determining the best location and size of FACTS devices in a highly interconnected network is also a complex task. Evolutionary computation and random search algorithms such as GA, PSO, SA, CSO techniques have all been proposed to find the optimal location of FACTS devices. These techniques have some advantages especially when dealing with nondifferential and non-convex problems, but have poor scalability and repeatability. Sensitivity based approaches such as weighted indices are frequently used but cannot ensure optimality. To complement the shortcomings of different optimization techniques, hybridization, especially of metaheuristic algorithms, should be adopted [2] [3]. To simultaneously solve power system problems such as reactive power dispatch, cost minimization, loss minimization, stability enhancement using FACTS devices, multi-objective problem formulation should be adopted [11]- [15]. C. Coordination of FACTS devices with power system components As the number of FACTS devices in a power system increases, interactions among themselves will be a serious concern. Interactions can also take place between the FACTS devices and PSS, HVDC systems or any other system controllers. Therefore, coordination among various FACTS devices and power system components should be investigated in detail, both in steady state and in the event of a disturbance [17] [18]. D. Economic benefits of FACTS device utilization Lastly, very few researchers have addressed the economic benefits of utilizing FACTS devices. Studies should be carried out on existing FACTS projects in terms of installation, maintenance and other associated costs of FACTS device utilization. Costs should be weighed against the actual benefits in terms of wheeling charges, savings on cost of generation, savings due to improved system reliability among others [2] [3]. VII. CONCLUSION This paper gives a review of various FACTS devices and their application in power systems. The benefits and costs associated with FACTS devices, and research gaps in the field of FACTS device utilization are outlined. We observe that accurate models of VSC FACTS devices should be studied in depth, with focus on the last generation FACTS device technologies. In simultaneously solving power system problems, multiobjective optimization problems should be solved using hybridized metaheuristic techniques. Another research gap lies in how FACTS devices interact with other power system components and how their controllers can be simultaneously coordinated for dynamic stability enhancement. Finally we conclude that the actual costs and benefits of FACTS device utilization should be thoroughly investigated, especially on existing projects. References [1] A. Peter, A. Anthony, A. Claudis and A. Abel, "A review of the applications of FACTS devices on Nigerian 330kV transmission system," Journal of Engineering and applied sciences, vol. XII, no. 20, pp. 5182-5185, 2017. [2] T. Kang, J. Yao, T. Duong, S. Yang and X. Zhu, "A hybrid approach for power system security enhancement via optimal installation of flexible AC transmission system (FACTS) devices," Energies, vol. I, no. 10, pp. 1-32, 2017. [3] X. Zhang, K. Tomsovic and A. Dimitrovski, "Optimal investment on series FACTS device considering contingencies," National Science Foundation, 2017. [4] R. Sikka, R. Bahl and V. Verma, "Review on comparison of FACTS controllers for power system stability enhancement," International Interdisciplinary Conference on Science Technology Engineering Management Pharmacy and Humanities, vol. I, no. 1, pp. 316-320, 2017. [5] N. Khatoon and S. Shaik, "A survey on different types of flexible AC transmission systems (FACTS) 5

controllers," International Journal of Engineering Development and Research, vol. IV, no. 5, pp. 796-814, 2017. [6] G. Barve, "Application study of FACTS devices in Indian power system," International Journal of Computing and Technology, vol. I, no. 1, pp. 57-60, 2014. [7] M. Singh, "Power flow with flexible alternating current transmission systems (FACTS) controller," IJARIIT, vol. 1, no. 1, pp. 180-184, 2018. [8] I. Khan, M. A. Mallick, M. Rafi and M. S. Mizra, "Optimal placement of FACTS controller scheme for enhancement of power system security in Indian scenario," Journal of Electrical Systems and Information Technology, vol. 1, no. 2, pp. 161-171, 2015. [9] M. Verma and S. Jain, "Improving power system transient stability by using FACTS devices," International Journal for Technological research in Engineering, vol. IV, no. 9, pp. 1579-1582, 2017. [10] "Improvement of Power System Transient Stability using TCSC, SSSC and UPFC Internatinal Journal of Scientific & Engineering Research Lokesh Garg; S. K. Agarwal; Viviek Kumar," vol. 8, no. 4, pp. 57-61, 2017. [11] S. d. Nascimento and M. M. G. Jr, "Voltage stability enhancement in power systems with automatic FACTS device allocation," International Conference on Energy and Environment Research, vol. 1, no. 107, pp. 60-67, 2017. [12] H. Yapici and N. Cetikaya, "An Improved Particle Swarm Optimization Algorithm using Eagle Strategy for Power Loss Minimization," Mathematical Problems in Engineering, vol. 1, no. 1, pp. 1-12, 2017. [13] C. H. B. Apribowo, M. H. Ibrahim and F. X. R. Wicaksono, "Optimal power flow with optimal placement TCSC device on 599kV Java-Bali electrical power system using Genetic Algorithm-Taguchi method," AIP conference proceedings, vol. 1, no. 1, pp. 1-10, 2018. [14] P. Ramesh, A. Chiranjeevi and K. Padma, "Application of SVC for Multi-objective Optimization Powerflow Problem using PSO," International Journal of Pure and Applied Mathematics, vol. 114, no. 8, pp. 23-33, 2017. [15] M. Nafar and A. Ramezanpour, "Optimal Allocation of TCSC using Heuristic Optimization Technique," International Journal of Scientific Study, vol. V, no. 5, pp. 250-254, 2017. [16] F. H. Gandoman, A. Ahmad, A. M. Sharaf, P. Siano, J. Pou, B. Hredzak and V. Agelidis, "Review of FACTS technologies and applications for power quality in smart grids with renewable energy systems," Renewable and sustainable energy reviews, vol. 1, no. 82, pp. 502-514, 2018. [17] M. O. Benaissa, S. Hadjeri and S. A. Zidi, "Impact of PSS and SVC on the power system transient stability," Advances in Science, Technology and Engineering Systems Journal, vol. II, no. 3, pp. 562-568, 2017. [18] K. Karthikeyan and P. Dhal, "Optimal location of STATCOM based dynamic stability analysis tuning of PSS using partice swarm optimization," International conference on processing of Materials, Minerals and Energy, vol. 1, no. 5, pp. 588-595, 2018. [19] A. R. Kumbhaj, "A review on power stability improvement using FACTS controllers in power systems," International Journal of Industrial Electronics and Electrical Engineering, vol. V, no. 11, pp. 10-14, 2017. [20] G. B. Jadahav, C. B. Bangal and S. Kanungo, "Transient stability analysis with SVC and STATCOM in multi-machine power systems with and without PSS using Matlab/Simulink," International Journal of Engineering Development and Research, vol. V, no. 4, pp. 1196-1205, 2017. [21] S. V. Patil and K. Mahajan, "A review on implementation of UPFC for improvement of active power flow capability in power system using IEEE 14 bus system," International Research Journal of Engineering and Technology (IRJET), vol. IV, no. 4, pp. 542-547, 2017. [22] P. Tipathi and G. P. Pandya, "A survey on the impact of FACTS controllers on power system performance," International Journal of Engineering Trends and Technology (IJETT), vol. 64, no. 1, pp. 24-29, 2017. [23] D. Kothari and I. J. Nagrath, Modern power system analysis 3rd edition, New Delhi: Tata McGraw Hill Education Private Limited, 2003. [24] I. Khan, M. A. Mallick, M. Rafi and M. S. Mizra, "Optimal placement of FACTS controller scheme for enhancement of power system security in Indian scenario," Journal of Electrical Systems and Information Technology, no. 2, pp. 161-171, 2015. 6

[25] A. Khasdeo, "Transient stability improvement in transmission system using SVC with fuzzy logic control," International Research Journal of Engineering and Technology, vol. IV, no. 4, pp. 173-177, 2017. [26] N. Khan, H. Ahmad and A. Khattak, "Transient stability enhancement of power system using UPFC (unified power flow controller)," International Journal of Engineering Works, vol. IV, no. 2, pp. 33-40, 2017. 7