The static synchronous series compensator (SSSC) is a device that employs controllable power electronic components for series reactive power compensation. For this reason, the technology is categorised as a flexible alternating current transmission system (FACTS). It outputs a series-injected voltage that leads or lags the line current by 90°, thus emulating a controllable inductive or capacitive reactance. The SSSC is most commonly used to provide series compensation in power transmission lines.

Generally, two technology variants can be distinguished:

• Coupling transformers (the conventional SSSC) connected to the power line through a transformer.
• A transformer-less SSSC, with multilevel inverters connected in a series to the power line.

The transformer-less SSSC design has the following advantages over conventional models with a coupling transformer:

• It eliminates the need for a bulky, heavy, expensive system component (the transformer).
• It offers higher efficiency, faster response to disturbances and improved controllability and stability within the electrical system.

SSSCs provide several benefits to power systems, enhancing efficiency, safety and flexibility while supporting environmental objectives:

• The SSSC is used to reduce or increase equivalent line impedance, thereby modifying the power flow in the electrical line. The system operator can manage load flows in the grid to improve network efficiency, increasing network capacity by reducing transmission losses.
• SSSCs are highly controllable devices, offering additional functionalities and services to the energy system, such as damping load flow oscillations, balancing phase currents, preventing overloads and compensating for balanced/unbalanced voltage sags.
• SSSC systems optimise power flow by changing transmission line impedance, which can lower network development costs and reduce both CAPEX and OPEX.
• A major difference between SSSC systems and other series compensation technologies, such as thyristor-controlled series compensation or phase shifting transformers, is their stabilising behaviour as a voltage source and very low impedance (passivity) during a short-circuit event.
• SSSCs are often modular, allowing users to add or upgrade devices without significant workload, making it easier to scale up or modernise a system over time.
• SSSCs can also be applied at the distribution level to avoid loop currents, such as when a group is energised by two transformers with a small voltage angle difference. Additionally, an SSSC can help regulate the receiving end voltage of a radial line in weak networks (i.e. with low short-circuit capacity) by controlling the degree of series compensation to maintain a constant end voltage in case of load and load power factor changes.
• By controlling and stabilising power flow to the system, an SSSC also supports the integration of renewable energy, as it can reduce network constraints resulting from large power flows from increased integration of renewable energy sources (RES). Moreover, efficient energy management leads to more effective integration of RES.

There are many enabling factors for utilising SSSC technology:

• Integrating an SSSC requires studies of various system conditions and fault scenarios, as it is an active device that may interact with other electrical system components (e.g. amplifying resonant frequencies or generating direct current in transformer neutrals). These types of dynamic system studies are extensive and time-consuming.
• Depending on the system operator’s reliability and maintainability requirements, additional components like bypasses, earthing switches, disconnectors and circuit breakers may be necessary. These components significantly increase space requirements for the overall application.
• For significant voltage control, it may be necessary to connect numerous modular devices in a series.

• International standardisation should be improved. Currently, we have the following standards:

  • IEEE 2745.2-2021: “Guide for Technology of Unified Power Flow Controller Using Modular Multilevel Converter–Part 2: Terminology” [1]
  • CIGRE WG B4.40: “Static Synchronous Series Compensator (SSSC)”, Report ELT_242_7 and technical brochure #371 (2009) [2]
  • Modelling of SSSC and its controls is generally not supported by power system simulation applications or data exchange standards such as IEC 61970-600-1:2021 and IEC 61970-600-2, known as CGMES.

Current research related to SSSCs should focus on:

• Defining criteria for the evaluation of wanted (passive) behaviour of SSSC.
• Modelling SSSCs.
• Incorporating SSSCs into the power system networks to enhance security.
• Detecting sub-synchronous oscillations caused by interactions between SSSCs and other network components, such as nuclear power plants.
• Determining the optimal SSSC allocation in power systems based on RES [3].
• Identifying the role of the SSSC in control strategies for automatic generation control in power systems with new energy sources [4].
• Defining the role of transformer-less SSSCs in real-time congestion management, power quality management and ancillary services (e.g. dumping electromagnetic transient states).
• Conducting a comprehensive life cycle cost analysis and comparisons with other technologies for a variety of applications.

The technology is in line with milestone “Demonstration of innovative technologies for power flow control and increasing grid efficiency” under Mission 1, milestone “New and optimised control concepts of reactive power” under Mission 3 and milestone “Technological advancement to tackle interconnectivity issues (i. e. thermal and stability limits, frequency and voltage regulation)” under Mission 5 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

TRL 7 for conventional SSSC.
TRL 9 for transformer-less SSSC up to 110 kV.
TRL 6 for transformer-less SSSC above 110 kV.

1. “IEEE guide for technology of unified power flow controller using modular multilevel converter—Part 2: terminology”, in IEEE Std 2745.2-2021, pp. 1–25, Sept. 2021, doi: 10.1109/ IEEESTD.2021.9525319.

2. “Static synchronous series compensator (SSSC)”, eCIGRE, 2009.

3. S. Galvani, B. Mohammadi-Ivatloo, M. Nazari-Heris and S. Rezaeian-Marjani. “Optimal allocation of static synchronous series compensator (SSSC) in wind-integrated power system considering predictability”, Elect. Pow. Syst. Res., vol. 191, p. 106871, 2021.

4. M. Kuanga, Y. Tian, M. Zhao and X. Win. “A review of control strategies for automatic generation control of power systems containing new energy sources”, Authorea, Aug. 24, 2023.

5. “Smart Wires and EPM complete project unlocking 400 megawatts of grid capacity for renewables in Colombia,”, Smart Wires, Jan. 26, 2023.

6. “National Grid: Northern England projects,”, Smart Wires, April 2, 2024.

7. “Evaluation of SmartValveTM devices installation at Central Hudson, technical report,”, EPRI, Aug. 28, 2020.