Series Compensation of Transmission Systems: A Literature Survey: A Literature Survey

energies Review Series Compensation of Transmission Systems: A Literature Survey Camilo Andrés Ordóñez 1,∗, Antonio Gómez-Expósito 2,∗and José María Maza-Ortega 1,∗   Citation: Ordóñez, C.A.; Gómez-Expósito, A.; Maza-Ortega, J.M. Series Compensation of Transmission Systems: A Literature Survey. Energies 2021,14, 1717. https://doi.org/10.3390/en14061717 Academic Editor: Anna Richelli Received: 3 February 2021 Accepted: 15 March 2021 Published: 19 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Department of Electrical Engineering, University of Sevilla, 41004 Sevilla, Spain 2Laboratory of Engineering for Energy and Environmental Sustainability, University of Sevilla, 41004 Sevilla, Spain *Correspondence: [email protected] (C.A.O.); [email protected] (A.G.-E.); [email protected] (J.M.M.-O.) Abstract: This paper reviews the basics of series compensation in transmission systems through a literature survey. The benefits that this technology brings to enhance the steady state and dynamic operation of power systems are analyzed. The review outlines the evolution of the series compensation technologies, from mechanically operated switches to line- and self-commutated power electronic devices, covering control issues, different applications, practical realizations, and case studies. Finally, the paper closes with the major challenges that this technology will face in the near future to achieve a fully decarbonized power system. Keywords: series compensation; Flexible AC Transmission Systems (FACTS); Distributed-FACTS (D-FACTS); Fixed Series Capacitor (FSC); Thyristor-Switched Series Capacitor (TSSC); Thyristor- Controlled Series Capacitor (TCSC); Static Synchronous Series Compensator (SSSC); Distributed Static Series Compensator (DSSC) 1. Introduction FACTS is the acronym for Flexible Alternating Current Transmission System. FACTS is an evolving technology-based solution to help the utility industry and power system operators to deal with changes in the power delivery business [ 1 ]. The concept of FACTS was first introduced by Dr. N.G. Hingorani in 1988 [ 2 ]. In Reference [ 1 ], the flexibility of the system is defined as the ability to accommodate changes in the electric transmission system or operating conditions while maintaining sufficient steady state and transient margins. In this regard, FACTS technology, defined as AC transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability, may bring such flexibility to the power system. FACTS can be classified according to different criteria. In case of their connection to the power system, they can be grouped into series, shunt or shunt-series devices. In addition, it is possible to group them depending on the used power-electronic switch into line-commutated (e.g., thyristors) and auto-commutated (e.g., Insulated-gate bipolar transistor (IGBTs)) devices. Figure 1presents the different FACTS with their acronyms and names, classified according with these categories. Some of the principal benefits of using a FACTS are [3]: • Power flow control. • Increase of transmission and loading capability. • Increasing the power system security through stability improvement. • Provide secure tie line connections. • Voltage control and flicker mitigation. • Reactive power compensation and reduction of reactive power flows. • Power quality improvement and power conditioning. • Provide flexibility in interconnecting new renewable and distributed generation, as well as energy storage devices. Energies 2021,14, 1717. https://doi.org/10.3390/en14061717 https://www.mdpi.com/journal/energies Energies 2021,14, 1717 2 of 28 • Increase utilization of lower cost generation assets through enhancement of transmission capacity. Figure 1. Flexible AC Transmission System (FACTS) classification by topology and technology. This survey paper focuses on series compensation, including series capacitors and series FACTS, their technological evolution through the revision of the principal published literature. The survey is motivated because series capacitors are becoming a key element in power transmission systems around the world. This subsystem is characterized by long high-voltage (HV) AC lines connecting major generation and demand areas in a meshed fashion, to increase as much as possible the overall system reliability. In such HV-AC systems, the power transmission between two nodes is primarily limited by the line series reactance. In addition, the power flows in the meshed network fulfill the Kirchhoff equations, irrespective of physical constraints (ampacity or nodal voltage limits) which may lead to loop flows or network congestions [ 4 ]. The use of series capacitors may prevent these adverse situations by partly compensating the line reactance to either increase the transmittable power or modify the power flow patterns in the meshed network. The first transmission series capacitor was installed in 1928 by the General Electric on a 33-kV line (1.2 Mvar) of the New York Power & Light system, in Ballston Spa, New York, NY, USA [ 5 ]. Some improvements were published in the next few years [ 6 , 7 ], all the applications relying on mechanically operated switches. The advent of power electronics in the last quarter of the 20th century has driven the change from the conventional mechanicallyswitched capacitors [ 8 , 9 ] to the brand new series compensators, under the umbrella of FACTS. The Thyristor-Controlled Series Capacitor (TCSC), the Thyristor-Switched Series Capacitor (TSSC) [ 10 ], the GTO-Controlled Series Capacitor (GCSC) [ 10 , 11 ], the Static Energies 2021,14, 1717 3 of 28 Synchronous Series Compensation (SSSC) [ 12 ], and the Distributed SSSC (DSSC) [ 13 , 14 ] are the most prominent topologies within this FACTS category. It has been demonstrated that variable series compensation is a cornerstone of FACTS technology [ 3 ], owing to its effectiveness in both controlling line power flows and improving system stability. The series FACTS allows maximizing the utilization of transmission assets by controlling the power flow in the lines, preventing loop flows and, with the use of fast controllers, minimizing the effect of system disturbances, thereby reducing traditional stability margin requirements [ 15 ]. In this regard, the current power system transition towards a fully decarbonized paradigm will definitely require the additional flexibility that series FACTS may bring, with the support of power electronics technologies [16]. This paper presents a literature survey of the series compensation technologies deployed so far in transmission systems. The aim is to provide an overview of each technology, pointing out its fundamentals, applications from an operation and planning perspective, benefits, and drawbacks, supported by an extensive and exhaustive review of the state of the art. For this purpose, the methodology outlined in the next section, aimed at performing a taxonomic classification of published works, has been elaborated, followed by a number of sections devoted to each relevant topology. Particularly, the analyzed arrangements have been grouped according to the way the series compensation is implemented (a generic “Series FACTS” category has been added, for convenience, to account for those papers in which no particular technology is discussed): first, those technologies that modify the line impedance by the insertion of series capacitors, either mechanically or through power electronic devices; then, those technologies based on self-commuted switches, which behave as a voltage source. The paper closes with the main conclusions, challenges, and recommendations to foster the deployment of this technology. 2. Taxonomic Methodology for the Literature Review Several survey papers have been published, both in journals and conferences, about series compensation in transmission systems. Most of those papers, however, are restricted to general aspects of the series FACTS, rather than providing a systematic classification and holistic overview [ 17 ]. For this reason, it is yet difficult to answer major questions, such as where the research focus has been concentrated, which technologies are most promising, which problems have been addressed, and which commercial realizations are already available in the market. The aim of this paper was to answer those questions, while, at the same time, performing a comprehensive literature review. For this purpose, the following set of the most recognized journals and international conferences in the field have been systematically searched: • IEEE Transactions on Power Systems. • IEEE Transactions on Power Electronics. • IEEE Transactions on Power Delivery. • IEEE Transactions on Industrial Electronics. • IEEE Transactions on Industry Applications. • IEEE Transactions on Power Apparatus and Systems. • IET Power Electronics. • IET Generation, Transmission & Distribution. • International Journal of Electrical Power & Energy Systems (Elsevier). • Electric Power Systems Research (Elsevier). • Journal of Modern Power and Clean Energy Systems (SGEPRI). • Energies (MDPI). • Energy Procedia (Elsevier). • CIGRE Publications (reports, papers, and technical brochures). In order to identify the key publications, the databases related to those journals and conference proceedings have been explored, by applying complex queries analyzing metadata, keywords, title and abstract contents. Different concepts have been searched, and the results have been clustered into the following categories: Fixed Series Capacitor Energies 2021,14, 1717 4 of 28 (FSC), series FACTS, Distributed-FACTS (D-FACTS), TSSC, TCSC, GCSC, SSSC, and DSSC (all the acronyms are provided at the end of the paper). Table 1shows the number of publications found within each category. As expected, there is a huge amount of publications presented in conferences, meetings, and symposia, overall representing about 80% of the total published works. The stacked bar chart in Figure 2shows the number of publications in each category, separating journals and conferences. In this figure, the category ‘Others’ refers jointly to series FACTS, GCSC, FSR, TCSR, and TSSC. It is evident that the largest number of published articles are related with TCSC, followed by SSSC. Table 1. Number of publications organized by category and publisher. Category IEEE/IET Journals Elsevier and Energies CIGRE IEEE/IET Conferences Total FSC 68 31 14 93 206 FSR/TCSR 1 0 3 2 7 TSSC 1 0 0 2 3 TCSC 50 70 13 923 1056 GCSC 7 2 1 15 25 SSSC 25 42 2 307 376 DSSC/D-FACTS 13 1 2 56 72 Series FACTS 0 5 0 37 42 Total 165 151 35 1435 1786 0100 200 300 400 500 600 700 800 900 1000 TCSC SSSC FSC Others DSSC 133 69 113 20 16 923 307 93 56 56 Number of publications Journals Conferences Figure 2. Journal and conference papers published, organized by category. The research effort on each of the considered topologies evolves over time depending on the technology maturity. A natural indicator for measuring this trend is the number of publications per year related to each category, represented in Figure 3for the last 60 years. This figure reveals that the FSC was the principal element analyzed during more than 30 years. Since the 1990s, though, the line-commutated power electronic devices, such as the TCSC, have been the principal series FACTS in which academia and R&D efforts have been interested. Since the beginning of the 21st century, with the irruption of the self-commutated switches, an increase of publications dealing with SSSC and DSSC is apparent. In this regard, it is somewhat surprising that the publications dealing with thyristor technology keeps really high. This can be explained by the fact that the main thyristor-based converters manufacturers worldwide have promoted the research sector to continue producing publications related to the TCSC technology, still remaining in force, although new technologies, such as DSSC, are increasingly entering the market. Publications accepted during 2020 has been added to those of 2019 because it has been considered that probably journal databases are not fully updated in the period this research has been issued. Energies 2021,14, 1717 5 of 28 1960 1970 1980 1990 2000 2010 2019 0 10 20 30 40 50 60 70 80 Year Number of publications TCSC SSSC FSC DSSC Others Figure 3. Time evolution of number of publications by category. Finally, this outlook closes with a review of the research focus of the different publications, grouped by the following criteria: • Control strategies to improve the steady-state and dynamic performance of power systems . • Design and modeling issues dealing with modifications proposed to the FACTS design, including new features or additional elements, as well as new models for new power systems software tools. • Operational analysis, assessing the FACTS impact on the power system, including steady-state simulations, real-time and wide-area control applications. • Planning studies, dealing with location and sizing of FACTS. An important research is focused on the placement of series controllers. That includes methodologies to achieve an equivalent objective than if it were located in another points. However, it has to be considered that, sometimes, series FACTS may be located at non-optimal locations if the aim is minimizing costs because the the cost of lands and all the environmental licenses are becoming crucial for projects development. • Protections design and coordination issues, focusing on the interaction of series compensation with new protections schemes and its behavior in transient simulations. • Stability issues dealing with dynamic assessment and new methods to enhance angular, frequency, or voltage stability. • FACTS Installations. The results of this classification are summarized in Table 2. The FACTS control is one of the most researched issues. It has been found that much research has been aimed at improving controllers schemes, using new control techniques (robust controllers, fuzzy algorithms, non-linear controllers, gravitational search optimization, linear quadratic Gaussian control, particle swarm optimization, etc.), in order to reduce the computational processing times, reducing power electronics requirements, etc. After that, papers dealing with modeling, operation, and planning issues are also quite numerous. This topic has also been explored by researchers in the main faculties of electrical and software engineering, since better optimization methods are constantly being proposed, which allow in a more agile or efficient way to find the optimal results (e.g., in location or sizing tools), reduction of computational times, and use of artificial intelligence and metaheuristic techniques to find the optimal solution, among others. Other topics, like protections and stability issues, have been relatively also explored to a lesser extent in the case of FACTS, probably because these topics are normally included in the control-focused articles. In the cases where no articles have been found (most of the topics dealing with TCSR, TSSC, GCSC), probably it is motivated by the technology evolution and the corresponding commercial trend which, without any doubt, influence the academia research. Energies 2021,14, 1717 6 of 28 Finally, the publications were filtered according to their impact level, measured by the number of citations, prioritizing the journal papers. Approximately 222 publications (180 Journals, 37 Conferences, and 5 Chapter Books) were selected and included in the following review. Table 2. Taxonomy of journals by their main topic. Category Control Design/Model Operation Planning Protections Stability Installation Total TCSC 55 13 24 5 10 12 1 120 FSC 20 23 7 11 16 15 7 99 SSSC 42 3 2 14 4 2 0 67 DSSC 3 2 7 2 0 0 0 14 GCSC 2 2 0 4 1 0 0 9 Series FACTS 3 0 1 0 0 1 0 5 TCSR 0 0 0 1 0 0 0 1 TSSC 1 0 0 0 0 0 0 1 Total 126 43 41 37 31 30 8 316 3. Series Compensation by Inserting Capacitors Series capacitive compensation is well known and has been widely applied in transmission grids. The basic principle is to reduce the inductive reactance of the electrical transmission line by means of a series capacitor, leading to an increased power transfer capability and steady-state stability margin, owing to the higher synchronizing power [ 4 ]. Figure 4shows the one-line and phasor diagrams related to the series compensation using a fixed capacitor. This yields an apparent reduction of the line reactance, Xe=XL−XC , and an associated increase in the transmitted power (assuming the voltage magnitudes and angles remain constant), according to the well-known expression: Pe=VAVB XL−XC sin δ≈V2 XL(1−k)sin δ, (1) where VA and VB are the voltage magnitudes of buses A and B ( VA≈VB≈V ), XL is the impedance of the transmission line, XC is the reactance of the series capacitor, k=XC/XL is the degree of compensation, and δ is the angular difference of the busbar voltages. The reactive power injected by the capacitor depends on the compensation degree [ 3 ], as follows: Q≈2V2 XL k (1−k)2(1−cos δ). (2) From those expressions, the active power increases if the compensation k increases, at the cost of the large series capacitor reactive power, which sharply increases with k [ 3 ]. According to the phasor diagram of Figure 4d, this reduces the voltage drop and the angular deviation between the sending and receiving line ends, providing [ 4 ]: increased transmission capacity [ 18 ], possibility of modifying the power flows in the meshed network [ 19 ], improved voltage profile of the grid [ 20 ], enhanced angular stability of the power corridor [ 21 , 22 ], damping of power oscillations [ 23 , 24 ], and the possibility of optimizing power transported by parallel lines [ 25 ]. In addition, it has to be considered that series compensation may interfere with the conventional power system operation. For this reason, researchers have also analyzed the impact of series compensation on voltage sags [ 26 ], radial distribution systems [ 27 ], Wide-Area Measurement Systems (WAMS) [ 28 ], security analysis with massive penetration of renewable generation [ 29 ], and even the modification of spot prices in wholesale energy markets [30]. Energies 2021,14, 1717 7 of 28 Figure 4. Change of line reactance caused by the insertion of a series capacitor: ( a ) one-line diagram, ( b ) phasor diagram, ( c ) one-line diagram with the inserted capacitor, and ( d ) phasor diagram. Based on Reference [4]. The following subsections are devoted to give an overview of the mechanical- and power electronics-based options that can be used for introducing a series capacitor in the transmission system. 3.1. Mechanically Commutated Series Devices The Mechanically Commutated Series Capacitors (MCSC), also called Fixed Series Compensation (FSC) is the most common series compensation equipment currently installed in power systems [ 31 , 32 ]. The MCSC scheme is shown in Figure 5[ 4 ]. The capacitor banks consist of parallel and series arrays of AC capacitors, along with other auxiliary equipment. Metal-oxide varistor (MOV) arresters and spark gaps are installed to protect the capacitors from overvoltages [ 33 , 34 ]. The bypass switch is used for connecting/disconnecting the series capacitor to/from the line, while the disconnector switches provide the required isolation for maintenance operations. Finally, current transformers are used for monitoring and control purposes. Figure 5. Components of the Fixed Series Capacitor (FSC). From Reference [ 4 ], based on Reference [ 35 ]. Energies 2021,14, 1717 8 of 28 Several studies have been published about the impact of series compensation on the power system stability. A bang-bang control for stabilization of interconnected power systems using a series capacitor was presented in Reference [ 36 ]. Reference [ 37 ] shows a control scheme using fuzzy logic to enhance the overall power system stability, while a transient stability preventive control design has been analyzed in Reference [38]. Additionally, planning studies of FSC have also been published. In Reference [ 39 ], a multistage transmission expansion planning model is proposed considering FSC allocation and N-1 security constraints. The problem is modeled as a mixed-integer linear programming (MILP) approach, and it is solved using branch-and-cut methodology to obtain the optimal solution. In Reference [ 40 ], a methodology is developed for identifying the critical lines and their compensation degree in terms of voltage stability using decision trees methodology. It is possible to find several contributions regarding protection issues related with series compensation. Reference [ 41 ] presents a fault current limiter (FCL) with series compensation based on a fast-closing switch composed of a capacitor bank and a series reactor. Reference [ 42 ] introduces a non-iterative fault-location algorithm for compensated transmission lines, while Reference [ 43 ] presents a non-unit protection scheme using discrete wavelet transform and k-nearest neighbor algorithm. In Reference [ 44 ], an accurate fault-location algorithm is proposed making use of synchronized measurements from both ends of the series compensated line. One of the main concerns related to the series compensation of transmission lines refers to the Sub-synchronous Resonance (SSR), due to the interaction of the series capacitor and line reactance. Note that the natural frequency of a series compensated line is given by: fseries =fsXC XL =f√k, (3) which is lower than the network frequency ( k< 1); therefore, a sub-synchronous oscillation mode can be excited creating dangerous SSR situations. Interactions between a transmission line (compensated with a series capacitor), which oscillates at the natural resonant frequency, and the mechanical oscillations in a generator mechanical system can cause negative damping with the consequent mutual reinforcement of the electrical and mechanical oscillations [ 3 ]. This phenomenon, identified in 1937 [ 45 ], and the possible control actions for preventing SSR in a series capacitor-compensated transmission line were worked widely in the 1990s [ 46 – 48 ]. Following this research line, Reference [ 49 ] provides an overview of the impact of series compensation on SSR and the recommended studies for different applications with SC. The evaluation of SSR risk, like stability and transient torque analysis, and some methods for mitigating SSR problems and the torsional protection and monitoring schemes are included. 3.2. Static-Controlled Series Capacitive Compensation The following subsections are devoted to describe the power electronics-based series compensation. For this purpose, a chronological order has been followed to present the different technologies from the initial TSSC to the GCSC, including the TCSC, which has concentrated most of the research effort in this field. 3.2.1. Thyristor-Switched Series Capacitor (TSSC) The Thyristor-Switched Series Capacitor (TSSC), in which the scheme is shown in Figure 6, consists of a set of series capacitors which are shunted by two anti-parallel thyristors (which can be modeled in series with a small reactor, if the design requires it). This thyristor couple acts as an AC static switch which can bypass the capacitor. The compensation degree of the transmission line is controlled in a step-wise way by increasing the number of the inserted capacitors. A capacitor is inserted by using the thyristor valves, by activating (or not) the gating signals. Conversely, a capacitor is bypassed by Energies 2021,14, 1717 9 of 28 conducting fully the thyristor valves [ 3 ]. A patent related with this technology is presented in References [50,51]. Figure 6. Thyristor-Switched Series Capacitor (TSSC) scheme [4]. From a steady-state point of view, the TSSC is similar to the FSC with mechanical switches. However, the following advantages with respect to this conventional technology can be mentioned [4]: • The commutation with thyristors allows unlimited number of switching operations, maximizing its usability along its life cycle. • The thyristors allow choosing the commutation instants, i.e., the point on the voltage waveform where the transition between the conducting and non-conducting states is produced. In this way, the switching transients are minimized, in contrast to the classical commutation with mechanical switches, which is hardly synchronized. • A very fast response, as typically the interval between the control command and the capacitor operation is less than a cycle. • It does not generate harmonics in contrast to the GCSC and TSSC technologies which will be reviewed later. This is because TSSC works either in full-conduction or blocking modes, i.e., the thyristors act as a conventional but fast-acting conventional switch. • TSSC modules can be quickly inserted or bypassed if SSR oscillations are detected. In this way, the SSR frequency can be controlled and moved away from the critical turbine-generator resonance frequencies. Additionally, TSSCs have been applied for damping power system oscillations [ 52 , 53 ], including inter-area modes, considerably improving the transient stability of the power system. Particularly, Reference [ 54 ] proposes an interesting application where an adaptive damping controller estimates the parameters of a power system by using a two-area simplified power system model. However, Direct Current (DC)-offset voltages can appear in the transmission line, resulting from the insertion of series capacitors. 3.2.2. Thyristor-Controlled Series Capacitor (TCSC) The TCSC, originally called and patented [ 55 ] as Rapid Adjustment of Network Impedance (RANI) by Vithayathil et al. in 1986 [ 56 ], is shown in Figure 7. The TCSC consists of a series capacitor shunted by a Thyristor-Controlled Reactor (TCR). The basic objective of the TCSC is to provide a variable-reactance capacitor by canceling (partially) the effective capacitance injected by the TCR. The equivalent impedance as a function of the thyristor firing angle αis presented in Reference [3] and can be expressed as: XTCSC(α) = XC 1−XC XL(1−2α−sin α π), (4) where XC is the fixed capacitor impedance, and XL is the inductance impedance. In the most common TCSC arrangement, the TCR reactance XL is smaller than the capacitor one, XC , and the TCSC has two operation ranges depending on the firing angle α : capacitive and inductive. According to Equation (4) , a parallel resonance is created between the capacitor and the inductor for the firing angle αr , which clearly separates the different operation modes. Due to the fact that the TCSC impedance sharply varies around αr , the capacitive and inductive operating modes are practically constrained in the ranges [ 0, αL,lim] and [αC,lim,π/2], respectively, as shown in Figure 8. Energies 2021,14, 1717 16 of 28 the switch SM , which is normally-closed. When it is opened, the insertion of the desired impedance is allowed. The switch S1 is closed to inject all the inductance, while S2 is closed to inject the capacitance XC[202]. The scheme in Figure 13c was first explored by Dr. Frank Kreikebaum in 2010 [ 204 ] and consists of the series transformer with magnetizing reactance Lm , two AC switches ( S1 , S2 ) realized using IGBT, an AC condenser ( C ), two LC filter components ( Lf , Cf ), a relay ( Sm ) for operation in case of failure, and the elements associated with controls and communications. The transformer is set by using its air-gap to generate a (designed) magnetizing inductance, avoiding the use of an additional inductance used previously in the DSI approach. With the switches S1 on and the S2 off, the equivalent parallel circuit of Lm and C is inserted in series with the transmission line, conversely with S1 off and S2 on, Lm is injected in series with the line, and switching S1 and S2 in a complementary fashion produces an effective AC capacitance. The feasibility and impact of this technology, including prototype development and system simulations, are presented in Reference [205]. Figure 13. Different schemes for Distributed SSSC (DSSC): ( a ) Distributed Series Reactor (DSR) [ 202 ], ( b ) Distributed Series Impedance (DSI) [202], (c) Inverter-less Static Series Compensator [204], and (d) DSSC with inverter [13]. This new concept allows a cost-effective approach to achieving global objectives of operating with high renewable penetration [ 206 ]. The design considerations for implementing Energies 2021,14, 1717 17 of 28 distributed power control solutions on the power grid are discussed in References [ 207 , 208 ], including the single-turn coaxial series transformer design, DSSC simulations, and the first laboratory prototype of the DSSC. The DSSC, as well as the DSR [ 209 ], have been evolving and the first commercial solution was installed in the United States Grid in 2012. Other schemes have been theoretically proposed [ 210 ]. Today, a transformer-less solution (see the patent of Reference [ 211 ]) has been commercialized, in which the transmission line must be opened to pass through the Modular SSSC (M-SSSC or DSSC). The M-SSSC solution is cost-competitive with FSC installations, while providing the additional benefits, including: inductive or capacitive compensation, continuously controllable compensation, and ability to efficiently scale or upgrade the installation to meet changing demand [212]. In References [ 213 , 214 ], a multilevel converter (with seven levels in cascade H-bridge) DSSC was implemented to limit the short-circuit of a line additionally to the principal action of controlling the power flow. Moreover, it includes a control strategy using variable DC bus voltage, which can reduce the Total Harmonic Distortion (THD) of the injected series voltage and the losses of the DSSC. SSR damping using DSSC has also been studied in Reference [ 215 ], where two novel controllers (PSO-based Conventional and Fuzzy Logic- Based Damping Controllers) were designed and implemented. The DSSC modules, shown in Figure 14 (and Figure 13d), can be distributed and can achieve the desired power flow control functionality by effectively and coordinately injecting a series voltage at several places. Figure 14. DSSC circuit schematic. Based on Reference [14]. Few years after the DSSC concept and prototypes were developed, some authors started to research the impact, operation and location methodologies of this technology. The impact of D-FACTS by studying the linear sensitivities between some variables, like voltage magnitude and angle, power injections, line power flow, and losses, with respect to line impedance, were analyzed in References [ 216 , 217 ]. With this, non-linear problems, such as real power loss minimization and voltage control, are solved, and appropriate locations to install DSSCs are determined. In Reference [ 218 ], a method for linearizing a transmission network model allows deriving the relationships between the network operating points and the injected reactances. This information facilitates the optimal deployment of DSSCs. The work in Reference [ 219 ] develops an optimal power flow framework that incorporates both variable-impedance series FACTS and transmission switching, i.e., opening and closing the transmission lines, to Energies 2021,14, 1717 18 of 28 study the link between the two power flow control technologies, finding that the operation of FACTS devices affect the location and frequency of transmission switching actions. Determining the location and amount of distributed series compensation to be employed is an important problem. For this purpose, a PSO technique for the optimal deployment of D-FACTS has been analyzed in Reference [ 220 ]. In Reference [ 221 ], the study uses the DC load flow model, and the optimization problem is solved using mixed-integer linear programming (MILP). In recent years, Dr. Xiaohu Zhang has published relevant advances in transmission expansion planning considering DSSC. In Reference [ 222 ], a multi-stage transmission expansion planning with the DSSC considering the N-1 security constraints is formulated as an MILP model. In Reference [ 223 ], a bi-level optimization model is proposed. The upper level problem seeks to minimize the investment cost on series FACTS, the cost of wind power curtailment and load shedding. The lower level problem captures the market clearing under different operating scenarios. In References [ 224 , 225 ], the original Mixed-integer non-linear programming (MINLP) model is transformed into MILP model to enable the planning model to be directly applied to practical large-scale systems. A similar approach was presented in Reference [ 226 ], in which an approximation method to linearize the model in the structure of the Benders decomposition algorithm is proposed. Other approaches have been published or presented in the last two years related with power systems with wind energy integration. In Reference [ 227 ], the model minimizes the generation and investment costs, while guaranteeing that the robust solution is customized to cover the wind uncertainty interval. In Reference [ 228 ], an AC power flow model is proposed and applied to a real power system problem, and the resulting MINLP is solved using MATLAB routines; in Reference [ 229 ], an MILP is applied initially to find the optimal number, locations and set points of multiple DSSCs, and then a fuzzy algorithm is used to select the most preferred solution. Finally, about the DSSC operation, a heuristic algorithm is developed to control a set of D-FACTS devices connected to all transmission lines of a power system in Reference [ 230 ]. The changes required in line impedances, which will be achieved by D-FACTS, are decided by the proposed algorithm. In this regard, the patent of Reference [ 231 ] proposes a method for enabling intelligent control of large number of DSSC distributed over the grid. 6. Conclusions As discussed in this paper, different technologies for series compensation have being developed over almost the last 100 years of history, some of them effectively installed in power transmission grids, and some still at the theoretical or concept stage. After the proliferation of FSC in the power transmission systems, two approaches to power electronics-based series compensators (series FACTS) have been proposed and investigated (and successfully installed in some cases): one family, employing thyristor-switched reactors and thyristor-controlled condensers to realize a variable reactive impedance (including GCSC, TSSC, and TCSC), and the other, employing a switching power converter to realize a controllable synchronous voltage source (VSC) in series with the line (SSSC and DSSC belong to that approach). This paper has presented a literature survey that explores and summarizes the most relevant publications in this field, from the FSC to the latest and most promising series compensation technology, namely the DSSC, including the more conventional, thyristor-based series FACTS. The DSSC is believed to be already cost-competitive with FSC, providing additional benefits, such as inductive or capacitive compensation, continuously controllable compensation, and ability to efficiently scale the installation to meet changing requirements. Furthermore, they have several advantages over TCSC, including increased control capabilities, minimal impact on line protection system, minimal environmental impact, higher resiliency and reliability (because a set of them can be installed at different locations of the same transmission line or operational area), and reduced substation infrastructure Energies 2021,14, 1717 19 of 28 requirements (because of its footprint size). Additionally, they have a shorter installation time than other FACTS, which can be a decisive factor in the case of power grids that need an immediate solution. Variable series compensation is highly effective in both controlling line power flows and improving stability. It can be applied in many applications: to achieve full utilization of transmission assets by controlling the power flow across the lines, preventing loop flows and reducing stability margin requirements, voltage controlling and stability enhancement. More applications and benefits of this FACTS should continue to be investigated, but more importantly, the research and development of university and research centers will continue to give way to the commercial development of cost-efficient technologies for their installation and proliferation in power transmission systems. From the literature review synthesized in Section 2, it can be inferred that future challenges have to be oriented to find novel applications, additional to those already known, so that different series technologies can be increasingly considered by those in charge of planning the expansion of electrical transmission grids and the electrical equipment development. Particularly, future research might be focused on new protection schemes for SSSC and DSSC considering that these new technologies can be massively deployed in the power system, therefore requiring a smooth integration with legacy power system protection strategies. In addition, more control approaches to find additional applications for DSSC and GCSC are of utmost importance in order to widen the possible economic incomes due to the provision of new services to the power system operator. These new control methodologies along with new planning strategies (i.e., location and sizing of series devices) may facilitate the business plan of these technologies characterized by large investment costs. In this regard, it is key to maximize their capabilities once installed by applying convenient wide area coordinated control techniques aiming at optimizing their operation. In this way, power system operators will be able to achieve the required flexibility to reduce the stress in the complex operation of an increasingly renewable-based power system pursuing its fully decarbonization. Author Contributions: Conceptualization, A.G.-E. and J.M.M.-O.; methodology, A.G.-E.; formal analysis, C.A.O., A.G.-E. and J.M.M.-O.; investigation, C.A.O.; resources, C.A.O.; data curation, C.A.O.; writing—original draft preparation, C.A.O.; writing—review and editing, A.G.-E. and J.M.M.- O.; visualization, C.A.O.; supervision, A.G.-E. and J.M.M.-O. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations The following abbreviations are used in this manuscript: CIGRE International Council on Large Electric Systems DPFC Distributed Power Flow Controller DSI Distributed Series Impedance DSSC Distributed Static Series Compensator DSR Distributed Series Reactor FACTS Flexible AC Transmission System FSC Fixed Series Capacitor FSR Fixed Series Reactor GA Genetic Algorithm GSA Gravitational Search Algorithm GCSC GTO Thyristor-Controlled Series Capacitor Energies 2021,14, 1717 20 of 28 IEEE Institute of Electrical and Electronics Engineers IET Institution of Engineering and Technology MINLP Mixed-integer non-linear programming MILP Mixed-integer linear programming PSO Particle Swarm Optimization SSR Sub-synchronous Resonance SSSC Static Synchronous Series Compensation TCR Thyristor-Controlled Reactor TCSC Thyristor-Controlled Series Capacitor TCSR Thyristor-Controlled Series Reactor TSSC Thyristor-Switched Series Capacitor TSSR Thyristor-Switched Series Reactor References 1. 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