Advances in FACTS Technology for Transient Stability in Power Systems

1. Introduction

Integrating renewable energy sources into the electrical grid is one of the challenges that arise from the world’s transition to cleaner and more sustainable energy sources. As more wind and solar power are used as renewable energy sources (RES), the electrical grid will face significant challenges regarding system stability. Renewable energy is different from traditional power sources because it can be intermittent and variable, so the grid must be able to handle sudden changes in supply of electricity. Furthermore, the inertia of power systems additionally decreases as more and more converter-interfaced generation (CIG) units are connected to the power system [1]. The intermittent nature of RES requires a reevaluation of grid control strategies and methods, as current traditional communication and control strategies are better suited to handle traditional power systems. Optimal integration of RES requires the grid to be flexible and that it can adapt to fast changes in electricity generation. In a system with reduced inertia, after a disturbance, a larger rotor angle instability occurs that requires a greater restoring force to return the system to equilibrium. Managing this could strain synchronous generators in the system, potentially resulting in significant instability issues. Renewable energy sources are often located far away from load centers, which places additional stability strain on the power system. Modern electrical grids are going to have to maintain stability and reliability despite displaced centers of electricity production and supply fluctuations. As penetration of distributed generation in the power system increases, these challenges are going to become more prominent. Large renewable energy like photovoltaic (PV) systems located in remote areas may overload the transmission system that connects them to load centers [2]. As installation of PV systems in the transmission grid has becoming more frequent, additional research was being conducted in field of general performance analysis with high-penetration PV systems [3], and further regarding transient stability [4], [5]. Researchers in [3] observed that system inertia and frequency regulation is reduced as conventional generation is replaced by PV. It was shown in [4] that large-scale PV systems, that increase PV penetration levels from 20% to 50%, while replacing conventional synchronous machines, have decreased voltage stability. The study in [5] investigated the dynamic performance of the system with a varying level of PV penetration, namely small-signal stability, voltage stability, and time-domain contingency assessments. Eigenvalue analysis revealed that PV systems had a negligible impact on the overall small-signal stability of the system. Conversely, voltage stability and transient stability analysis indicated that a distributed approach could significantly enhance system stability compared to centralized PV farms. A simulation of a test system with high penetration of PV for transient stability analysis was conducted in [6] and the low-voltage ride-through characteristics are examined. It was shown that high penetration of PV lowers transient stability under a large disturbance. Tripping of low voltage protection designed to safeguard PV inverters, during a voltage sag in the system, can potentially lead to a cascading fault since disconnecting PV systems during a fault can exacerbate the drop in system voltage. There is a need for comprehensive research to identify an optimal low voltage ride-through characteristic for PV systems and develop control schemes that enhance power system stability under large disturbance scenarios. Additional analysis [7] for dynamic behavior of synchronous generations with high penetration of PV was done by numerical simulation. Research in the field leads to the conclusion that high penetration of renewable sources will need a solution regarding system stability [2]. Fast control of system parameters like node voltage and phase angle, line reactance/current and shunt impedance can increase the flexibility of the power system [8]. Fast development of power electronics [9], [10] and extensive research in the field of Flexible AC transmission systems (FACTS) give an option for increased controllability of system parameters [11],[12].

2. Power System Stability

Power system stability is defined as the ability of the power system to maintain synchronism and equilibrium under normal operating conditions and to remain in equilibrium after a disturbance [13]. In [13], power system stability was classified into three broad categories – rotor angle stability, voltage stability and frequency stability, Figure 1.

Rotor angle stability refers to the ability of a synchronous machine to remain in synchronism after a disturbance. This stability relies on the system's capability to sustain equilibrium between the electromagnetic and mechanical torques of each synchronous machine. Instability that may develop manifests as escalating angular swings in certain generators, potentially causing them to lose synchronization with other generators in the system. Voltage stability refers to the ability of a system to maintain a predefined voltage range at all system buses after a disturbance. Voltage stability relies on the system’s capability to sustain equilibrium between electricity supply and demand. Instability that may develop manifests as a gradual decline or increase in voltages at certain buses. Voltage instability could potentially lead to consequences such as load shedding in a specific area, or the activation of protective systems causing the tripping of transmission lines and other components, ultimately resulting in cascading outages. These two broad categories can be additionally divided into four subcategories based on the physical process and time frame. Time frame refers to transient and long-term effects. When a disturbance happens, it can cause two types of effects: transient and long-term. Transient effects are fast and severe, and they happen right after a large disturbance. Long-term effects are slow and moderate, and they happen over time after a small or gradual disturbance. Physical processes refer to rotor or voltage dynamics. Rotor dynamics is concerned with electro-mechanical oscillations of a machine due to torque-angle variations, while voltage dynamics is concerned with voltage variations due to reactive power changes. Lastly, frequency stability refers to the ability of a power system to maintain a steady operational frequency value following a large disturbance that causes a substantial imbalance between power generation and load. This imbalance, if not resolved quickly, can lead to frequency swings that can result in tripping of generators or loads.

Power system stability definitions and classifications had to be updated because of recent developments in CIG technologies that are becoming more prevalent in electrical power systems [14]. CIG devices have a fast response to changes in system parameters and is the reason that systems dynamics are affected in new ways compared to traditional power generation technologies. Designing grid-connected power converters used by CIG requires ensuring proper operation under various grid voltage conditions. This involves designing control algorithms that guarantee robust and safe performance, especially in abnormal grid conditions. Resonant controllers on the stationary reference frame present an effective alternative to current controllers based on the double synchronous reference frame, particularly for regulating currents during unbalanced and distorted grid conditions. Additionally, alternative solutions like hysteresis current controllers, direct power control methods, and model-based predictive control can be employed for grid-connected power converters, with hysteresis-based approaches standing out for their robustness and quick dynamic response [15]. Due to their increased use, two new stability categories were introduced in [14].

Converter-driven stability was defined because CIG is usually interfaced with the grid by a voltage source converter (VSC) and it’s expected that in the near future, more than 50% of the generated power may be converter-interfaced [16]. CIG relies on control loops with rapid response times, including the Phase-Locked Loop (PLL) and inner-current control loop. Consequently, the extensive timescale associated with CIG controls can lead to interactions with both the electromechanical dynamics of machines and the electromagnetic transients of the power system. This interaction may give rise to unstable power system oscillations across a broad frequency range, posing a novel challenge for system operators. Converter-driven stability can be divided into slow-interaction and fast-interaction stability, depending on the frequency of the phenomena in question.

Resonance stability refers to sub-synchronous resonance (SSR) and can be divided into torsional resonance and electrical resonance. Resonance happens when oscillations of energy between two points increase due to insufficient energy dissipation, which in turn causes an increase in system parameters like voltage, current or torque. SSR was first defined in [17], as a condition where the power system exchanges significant energy with a turbine-generator at a frequency below synchronous frequency (50 or 60 Hz). Research on SSR began in the 1930s because a series-compensated line caused a turbine-generator shaft failure because of neglected torsional oscillations [18].

3. FACTS technology

FACTS devices are a class of power electronic devices used to regulate system parameters such as voltage, frequency and phase angle in electrical power systems [19], improving the stability and reliability of the grid. Power electronics refers to the processing of electrical power with electronic devices, such as insulated-gate bipolar transistors (IGBT), gate turn-off (GTO) thyristors and integrated gate-commutated thyristor (IGCT) [20]. Typically, FACTS devices are installed in high-voltage electrical power systems to address power quality issues or alleviate congestion in areas of the transmission network [21]. Integrating RES into existing power grids presents several technical challenges. The variability of power generation introduces uncertainties and potential risks to system reliability. Additionally, insufficient transmission capacity hinders the delivery of large amounts of power generated by RES from remote areas to load centers. While constructing new transmission lines is an obvious solution to enhance capacity, these projects are often lengthy and expensive due to long construction times and stringent environmental approvals as they require significant land areas. To address some of these challenges, FACTS devices are designed to regulate power flow over long distances, as RES are frequently positioned far from load centers. Controllability of system parameters inherently allows for higher integration of RES in the grid [22]. This can be particularly important when load centers are geographically far away from RES, as these systems can have significant line losses and stability issues. The feasibility of using FACTS depends on several technical, environmental and financial factors. All three must be carefully considered before deciding to implement them in a power system. FACTS devices are usually installed to optimize existing transmission system assets (instead of installing costly and environmentally problematic transmission lines). Papers [23], [24] have shown that installing them in off-shore wind farms can enhance power quality and improve terminal voltage and power fluctuations. It is worth noting that in [25] a proof of concept showed that an uncontrolled rectifier can be used in an off-shore wind farm with a high voltage DC (HVDC) link as a cost-saving measure, which could eliminate the need for off-shore FACTS controllers. As the complexity of the power system increases, coordination between multiple controllers is going to become a key issue for transmission system operators. For example, coordinated use of optimal reclosing and SVC to enhance transient stability in a multi-machine power system is studied compared to conventional reclosing and SVC, considering both balanced and unbalanced permanent faults at various locations in the system, it is shown that the proposed optimal reclosing and SVC combination significantly improves transient stability and outperforms the conventional reclosing and SVC combination [26], [27].

FACTS devices can be categorized into four categories based on type of connection and two based on operating principle. FACTS devices can have a series, shunt, series-series and series-shunt connection to the power system [28], as seen in Figure 2. Thyristor-controlled FACTS devices and VSC-based FACTS devices are both used to enhance the controllability and efficiency of transmission systems, but they differ in their underlying technologies and characteristics. Thyristors are typically slower in response compared to IGBTs, which makes the control capability of thyristor-based FACTS devices limited to a certain extent. Based on research in [29], it was demonstrated that IGBT-based rectifiers operate at higher switching frequencies using Pulse Width Modulation (PWM), while thyristor-based rectifiers function at lower frequencies using the phase-shifting method. Simulation results demonstrated that implementing IGBT-based rectification systems results in fewer power quality issues and reduced losses, but reduced reliability and higher costs compared to thyristor-based systems. Furthermore, VSC-based FACTS devices: are more modular and scalable, allowing for easier expansion or modification of the system. In summary, the main difference is underlying technology, which is reflected in their control capabilities, dynamic performance, and modularity. Thyristor-controlled FACTS devices may have limitations in speed, but come ahead in cost, while VSC-based FACTS devices leverage better control capability and dynamic performance but for a higher cost and reduced reliability.

The most notable series-connected FACTS device is the Static Synchronous Series Compensator (SSSC), which directly injects variable voltage in quadrature with the current into the connected line. The most notable shunt-connected devices are the Static VAR Compensator (SVC) and Static Synchronous Compensator (STATCOM). Both devices have seen wide installation in today’s power systems. The interline power flow controller (IPFC) [31] and the unified power flow controller (UPFC) [32] represent the series-series and series-shunt devices, respectively. Shunt-connected devices are better at reactive power compensation and voltage control [33]–[39], while series-connected devices are more suitable for active power flow control and increasing transient stability [40]–[42]. Furthermore, shunt-connected devices improve transmission capacity indirectly by providing local reactive power compensation, meaning that less reactive power has to be transferred by the grid. On the other hand, series-connected devices modify line reactance and directly improve transmission capacity [43], [44]. Series-connected devices were shown to help with reducing or postponing the need for new transmission lines, which have significant environmental and geographical constraints [45].

Based on operating principle, FACTS devices can be categorized into thyristor-controlled and VSC-based FACTS devices. Thyristor-controlled devices like the SVC operate as a controlled reactive admittance, while VSC-based devices like the STATCOM operate as a synchronous voltage source [46].

3.1. Historical development and first applications

One of the first FACTS devices installed was an SVC, which was first demonstrated in 1974 and 1975 by General Electric and Westinghouse, respectively [19]. The SVC provides dynamic shunt compensation by adjusting the firing angle of a thyristor circuit that controls the total reactance a reactor or capacitor bank provides into the grid. Today, the majority of installed static var compensation is by ABB [47], [48]. According to the data presented in Figure 3, the installation of FACTS devices experiences a steady increase, reflecting the growing energy demand [49]. Furthermore, it can be concluded from steeper rise in installed power in recent years in Figure 3 that more transmission-level FACTS projects are being developed. As transmission system operators gain more experience with these technologies, less resistance to their implementation is expected. It is further expected that with the growth of renewable sources in the power system, the installation of FACTS technology will follow an upward trend. The increasing complexity of the power system necessitates a demand for more precise and improved controllability. A pioneering paper on reactive power compensation with thyristor-controlled capacitors [50] paved the way for further research in the following decade [51]–[54].

Furthermore, Figure 4 shows that the North American region has had the most experience with FACTS technology, which is supported by the fact that the US and Canada’s large geographical area warrants significant investment in grid stability [55].

Examining the percentage values depicted in Figure 4, alongside the geographical distribution illustrated in Figure 5, it is evident that the North American region exhibits a substantial level of familiarity and engagement FACTS technology. However, a comprehensive understanding of this scenario necessitates consideration of additional factors, specifically, the overall electricity consumption within the region. To address this limitation and offer a more nuanced perspective, we propose the introduction of a novel metric: the FACTS Penetration Index (FPI). This index, when appropriately scaled for the total electricity consumption of a region [56], aims to provide a more accurate reflection of the actual impact and integration of FACTS devices within the regional power infrastructure. By factoring in the corresponding electricity consumption, the FACTS Penetration Index strives to offer a more refined evaluation of the influence of FACTS technology across diverse regions. This nuanced approach is essential for researchers and policymakers seeking a comprehensive understanding of the intricacies associated with the implementation and success of FACTS devices in different geographical contexts.

FPI can be formulated by considering the ratio of the installed FACTS capacity to the total electricity consumption within a given region. The formula for FACTS Penetration Index can be expressed as (1):

$$\mathrm{F}\mathrm{P}\mathrm{I}=\frac{\mathrm{I}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{d}\mathrm{F}\mathrm{A}\mathrm{C}\mathrm{T}\mathrm{S}\mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}$$

(1)

Figure 6 shows total installed capacity by region and with provided FPI for each region. It can be seen that the Australian region is allocating substantial investments to FACTS technology, driven in part by the growing integration of renewable energy sources [57].

STATCOM is a second-generation FACTS device that showed promising control of reactive power, albeit for a steeper price [58]. First installed in 1980 by Kansai Electric Power Co. as a prototype, it showed sufficient protection against system faults which merited a plan for further commercial application [59]. 1991 saw the first commercial use of STATCOM in Japan’s Inuyama station with the reactive power output of ±80 MVAr [60]. SVC and STATCOM were initially created to enhance the power quality in electric power systems that use electric arc furnaces (EAF), as these furnaces produce random and substantial fluctuations in both real and reactive power, which can result in voltage drops and significant voltage variations in brief time intervals [61]. The SSSC was first simulated and installed in Spanish transmission grid in 2015 in order to control power flow and reduce line loading caused by high penetration of RES [62]–[64]. The third generation of FACTS, known as the Unified Power Flow Controller (UPFC), is created by integrating the STATCOM and the SSSC into a single system that uses a common control system. The UPFC has a distinctive capability to control the flow of real and reactive power independently. In 1998, American Electric Power's Inez substation was selected as the site for the first utility demonstration of a UPFC [65]. By 2018, 6 UPFC have been installed worldwide, 2 of which have been decommissioned [66]. Research on improvement of UPFC is still ongoing to provide similar system parameter controllability and reliability, but at a reduced cost [67]. Although UPFC offers technical benefits over SVC, STATCOM and SSSC, its cost poses a challenge for numerous transmission system operators when it comes to installation. In the following chapters, a concise summary will be provided of the technologies currently in use and familiar to transmission system operators.

3.2. Static VAR Compensators (SVC)

SVC is composed of a thyristor-controlled or a thyristor-switched reactor (TCR/TSR), a thyristor-switched capacitor (TSC) and filters. The TCR generates harmonics, which is why filters are needed [8]. The primary advantage of using SVCs to enhance transient stability is the quick and precise control they provide over bus voltage by changing the equivalent susceptance of TCR, which is a function of the thyristor trigger angle. This control can be especially useful during low-voltage situations that often arise during faults, as it can improve power transfer and prevent local generators from accelerating too much [68]. At present, the most commonly employed technology for minimizing the adverse impacts of EAF on the power system is SVC [69].

3.3. Static Synchronous Compensators (STATCOM)

STATCOM’s operating principle is based on producing a controlled internal voltage. The internal voltage uses grid voltage as a reference and adjusts its amplitude, while maintaining a phase angle of ±π/2. Amplitude higher or lower than grid amplitude makes the STATCOM behave as a source or sink of reactive power, respectively [70]. STATCOM is more effective than SVC in improving transient stability of wind farms because it has a faster response time and can provide the needed reactive power during a fault [71]–[73], but at a higher cost. An additional advantage of STATCOM over SVC is that it does not generate higher-order harmonics. According to data provided by ABB [74], more than 70% of STATCOMs were installed because of flicker mitigation. The increased penetration of renewables in recent years has led to a greater use of STATCOMs for enhancing grid stability [75]–[77].

3.4. Static Synchronous Series Compensators (SSSC)

The SSSC, a device based on a voltage source converter like the STATCOM, is connected in series with a transmission line. Its main functions are to regulate voltage, provide reactive power support, and reduce power system oscillations by injecting or absorbing reactive power. Additionally, the SSSC can adjust the impedance of the line, thus enabling control of power flow and reducing the possibility of congestion. The SSSC is recognized for its capability to quickly respond to changes in system parameters and offer damping support during transients. It can function alone or work in tandem with other FACTS devices to achieve more extensive power system control [41], [78]. Further success in improving transient stability was shown in [79]. The effectiveness of SSSC and STATCOM in improving power system stability differs based on the specific scenario. In terms of improving the first swing stability limit, STATCOM is more effective than SSSC. However, when it comes to improving damping in subsequent swings, SSSC is found to be more effective [80]. In [81], different aspects of research on SSSC are shown.

4. Current Developments and Future Directions in FACTS Technology

Initially, SVCs were used mainly for addressing flicker issues in electric arc furnaces, but their usage is now expanding within utilities due to growing concerns over system stability caused by the reduced system inertia resulting from the higher integration of renewable energy sources [82]. Research in [83] conducted a cost-benefit analysis on SVCs in relation to dynamic economic dispatch. By incorporating stability concerns into the calculations, extended analysis could provide further insight into the viability of installing SVCs, which is a factor that often dissuades system operators from considering their use. Advanced models are being researched to represent real systems more accurately in the context of SVC modeling for transient analysis [84], [85]. Furthermore, the control system is being improved in ongoing research aimed at enhancing the low voltage ride through (LVRT) capability of wind farms through the use of SVC [86]–[88]. The location and size of SVC has been a persistent challenge since its initial installation. However, it has been demonstrated that cost-effectiveness in terms of transient stability necessitates additional research [89]–[92].

Novel approaches to STATCOM control are being explored that incorporate considerations for transient behavior [93], [94], and there is ongoing progress in refining control strategies that utilize neural networks [95]. Due to the decreasing initial cost of battery energy storage systems (BESS), there is an increasing consideration for using more STATCOMs with BESS to manage power system transients [96]. Although BESS has demonstrated superior post-fault voltage restoration performance compared to STATCOM individually [97]–[99], a hybrid solution combining both devices has been shown to yield even greater benefits for transient response [100]–[105]. In recent years, there has been growing consideration and testing of PV inverters as a means of supporting transient stability, driven by increasingly stringent grid codes and the desire to maximize the potential of already-installed PV systems [106]–[110].

The current focus of SSSC research is on developing novel control algorithms aimed at enhancing transient stability [111]–[113]. Additionally, modular implementation is being explored and field-tested as a means of overcoming the challenges that have traditionally hindered the widespread adoption of conventional FACTS devices [114]–[116].

5. Conclusions

Although there are many benefits to using FACTS devices, not all are easily quantifiable. Furthermore, the cost of these devices is quite significant and must be weighed against their expected advantages. One of the reasons for the limited adoption of FACTS devices is the lack of evidence of their profitability as many factors must be taken into account to accurately predict installation profitability. It is also challenging to obtain information regarding the total capital needed for the installation and operation of FACTS devices. FACTS devices can prevent potential system failures, which could have severe consequences for other economic sectors. They can also help prevent widespread blackouts, and the opportunity cost of not using them in such situations must be considered. The high cost and potential losses, as well as issues such as appropriate sizing and settings, location, and availability, are major considerations in deploying FACTS controllers. Despite the long history of development, proven technology, and extensive benefits, FACTS controllers are still not widely used due to their higher perceived costs compared to conventional alternatives. It can be concluded that future research in the field of FACTS technology must focus on improving transient stability and cost-effectiveness through advanced control strategies and integration with renewable energy sources, thereby optimizing the utilization of pre-existing installed devices. Standardized cost-effectiveness metrics can be developed to evaluate different FACTS technologies based on objective and consistent criteria, enabling accurate comparisons and informed decision-making regarding implementation in the power system.