Static Synchronous Series Compensators (SSSC)
Overview
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.
Generally, two technology variants can be distinguished:
- Coupling transformers (the conventional SSSC) connected to the power line through a transformer.
- A transformer-less SSSC, also known as Modular SSSC (M-SSSC), with multilevel inverters connected in a series to the power line.
The M-SSSC design has the following advantages over a SSSC with coupling transformer:
- It eliminates the need for a bulky, heavy, expensive system component (the transformer), given the direct connection at line voltage into the circuit.
- It offers higher efficiency, faster response to disturbances and improved controllability and stability within the electrical system.
- Its modularity allows one or more devices to be seamlessly deployed as one to achieve the desired power flow control.
- It allows the same unit to be manufactured for the global market and used at every voltage level up to 550kV.
Benefits
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.
- A major difference between SSSC systems and other series compensation technologies, such as thyristor-controlled series compensation or phase shifting transformers, is their 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.
- Efficient energy management leads to more effective integration of RES. An indirect benefit of SSSC is the reduction network constraints resulting from large power flows from increased integration of renewable energy sources as it controls and stabilises power flow to the system.
Challenges
Some of the challenges for utilising SSSC technology are listed below:
- 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.
- The service experience in EU especially in EHV is very limited. The verification of performance y as well as CAPEX and OPEX require pilot projects in representative networks.
Current Enablers
The following enabling points are beneficial to increase trust in SSSC technology and empower its application:
- Identification of the applications, where SSSC especially beneficial comparing the conventional technology (Phase Shifting Transformer)
- Increase the number of pilot projects and share openly the service experience and use cases
- Develop a standardised guideline for verification of interactions with active series component and perform extended stability studies in different environments
Applications
DSO
TSO
| Location: Colombia | Year: 2023 |
|---|---|
| Description: In 2023, the successful completion of a large-scale project using digital power flow control to enable the connection of over 400 MW of distributed renewable generation was announced. This supports demand growth in Medellín and the surrounding Antioquia region and helps reduce electricity tariffs for consumers. [1] | |
| Design: Single-phase, MSSSC that injects a voltage in quadrature with the line current to synthesise a capacitive (-Ωs) or inductive (+Ωs) reactance. | |
| Result: Recently, the number of SSSC applications has been growing significantly worldwide. This is particularly true of MSSSCs. Several recent applications for these devices can be found in [2]. This technology is becoming increasingly common in transmission power systems. | |
| Technology Readiness Level (TRL): TRL 9 | |
| References: | |
| Location: Great Britain | Year: 2022 |
|---|---|
| Description: National Grid: Northern England projects. The first set of projects used an advanced power flow control solution at three substations to balance power flows across five circuits, unlocking 1.5 GW of transmission capacity for renewable energy and providing a quickly deployable, flexible, scalable solution that could be easily expanded as network needs evolve over time. In 2022/2023, two of the deployments were expanded, with additional solutions to meet the increased need for power flow control following the closure of a nearby power station, unlocking an additional 500 MW of capacity. [1] | |
| Design: Single-phase, MSSSC that injects a voltage in quadrature with the line current to synthesise a capacitive (-Ωs) or inductive (+Ωs) reactance. This means it can push power off overloaded lines or pull power onto underutilised lines. | |
| Result: Following this success, National Grid has included advanced power flow control in many network options as part of its grid development process and the (network options assessment) NOA for future years. | |
| Technology Readiness Level (TRL): TRL 9 | |
| References: | |
| Location: New York, US | Year: 2019 |
|---|---|
| Description: Three MSSSCs were installed on the 115 kV Sturgeon Pool-Ohioville line owned by Central Hudson in New York State. Central Hudson sought to gain experience with the technology before a larger installation planned for 2021. This larger installation will add 21% series compensation on a 345 kV line, enabling full capacity deliverability of interconnecting generation. The Electric Power Research Institute (EPRI) observed the 2019 installation and evaluated the technology's functionality. [1] | |
| Design: Use of modular MSSSC to enable real-time power flow control on grids. | |
| Result: Following this success, National Grid has included advanced power flow control in many network options as part of its grid development process and the (network options assessment) NOA for future years. | |
| Technology Readiness Level (TRL): TRL 9 | |
| References: | |
| Location: Austria | Year: 2021 |
|---|---|
| Description: The Austrian transmission system operator Austrian Power Grid (APG) completed a demo to evaluate how the grid availability and operation reliability in Austria can be improved with modular power flow control, to ensure the security of electricity supply. The demo took place at the end of 2021.   | |
| Design: To increase internal grid capacity for outage management, as well as for the long-term use potential, APG tested the use of a truck mounted 'mobile' MSSSC. The mobile demo used the mobile deployment of the nine MSSSC (1Mvar) devices into the 110kV network in Ybbsfeld 220kV station. The selection of the line is the result of the examination of multiple locations. The devices were installed in under three days. The 'mobile' device deployment allowed the demonstration of the necessary speed of deployment for emergency and outage management provided by this type of device. As the MSSSC devices are used for 'fixed' steel structures, this demo could assess the dynamic capability to control power for both outage and long-term power flow control applications. | |
| Result: Joint studies completed for the project showed that a MSSSC could provide greater capacity to the Austrian transmission grid if positioned at a critical point, provided that certain issues are resolved. APG, TU Graz University of Technology and the MSSSC supplier are therefore now taking a closer look at this technology, focusing on open questions concerning protection scheme and induced DC currents. Published press releases and other statements can be found in ref [1, 2, 3], and the findings of the deployment are summarised below:
As an example, one key finding was: ‘… is a modular SSSC (Static Synchronous Series Compensator) that can dynamically counteract grid bottlenecks ... This makes it possible to better control the transmission grid and reduce the number of redispatch measures required due to line bottlenecks.' (APG [1]) | |
| Technology Readiness Level (TRL): TRL 7 for mobile deployed version of the MSSSC. | |
| References: | |
| Location: Greece | Year: 2021 |
|---|---|
| Description: The FARCROSS project (See Reference 1 - 3) validated the capabilities of MSSSC to balance power flows across circuits, installed in Nea Santa 400kV substation in 2021, alleviating congestion on the Nea Santa-Iasmos 150 kV line in Greece by rerouting power flows to the underutilized 400 kV transmission lines. This can create extra capacity for clean energy to flow between the neighbouring countries by exploiting the high wind generation in the area. The technology is connected in series under the line from the termination tower of the Nea Santa-Iasmos 150kV line and the substation busbar (Figure 1). It uses power electronics to harvest power from the electrical supply to generate and inject a reactive voltage into the circuit. This reactive voltage injection is dynamically controllable providing an effective change in impedance of the circuit to increase or decrease the power flow. The FARCROSS project was built on the previous Horizon 2020 project, Flexitranstore [1], which utilized smart grid technologies to address the challenges of large-scale renewable energy integration and increasing grid flexibility. In this project, mobile power flow technology was initially installed in collaboration with the Independent Power Transmission Operator (IPTO) in Greece and was then redeployed to Bulgaria with ESO to explore further uses cases of this technology to address some of the region's most critical needs. The project showcases the potential benefits of rolling out this modular power flow control technology across Europe to enhance the flexibility of power systems and accelerate the integration of renewable energy.  | |
| Design: Following the identification of the future need to increase cross border capacity in Greece, IPTO Deployment of the six MSSSC (1 Mvar) devices (connected two in series per phase) onto the 150kV network. The selection of the line in question was following the examination of four potential locations, each demonstrating potential benefit. The deployment was installed from conception to commissioning in less than a year. | |
| Result: Published Technical results support this as follows: Fields of Application [2]: N -1 Contingency scenarios as Use Cases with the loss of the transmission Line Xanthi - Iasmos out of Service and Komotini Power Plant out of service. Benefits from the deployment [2]:
'This novel technology was shown also to increase cross border flows in two ways: (i) by increasing the utilization of cross-border infrastructure and (ii) by alleviating internal, downstream constraints that limit cross-border flow.' [3] 'The novel M-SSSC system was implemented in the Greek transmission system and both the installation and real-time operation stages were evaluated in practice. Based on the obtained site experience, a scalability and replicability analysis showed that the M-SSSC presents significant advantages compared to conventional FACTS devices and alternative solutions for mitigating congestion constraints. Overall, it is concluded that this kind of modular design of the M-SSSC can overcome most of the significant issues that have limited a wider deployment of conventional FACTS devices in the past.' [4] | |
| Technology Readiness Level (TRL): TRL 8 | |
| References: | |
| Location: Ireland | Year: 2016 |
|---|---|
| Description: The Irish TSO, EirGrid, in 2016 installed a pilot deployment of two 1 Mvar MSSSC devices onto the Cashla - Ennis 110kV line in Ireland. 'In 2024 the tendering process was completed for … a DPFC device in Binbane 110 kV Station and for the establishment of a framework contract for the supply of multiple devices at multiple locations over a five-year period commencing 2025. Two suppliers have been appointed … delivery period is 22 months … with completion of the installation to follow in 2027.' [4]  | |
| Design: Two MSSSC (1Mvar) devices were installed onto the 110kV line. 'Following the SmartValve testing in September, Ireland was hit by the tail end of Hurricane Ophelia ... This caused … a Status Red wind warning - the highest threat level possible. Despite strong gusts of up to 156 km an hour … there was no structural damage caused to the SmartValve units.' (Reference 1) | |
| Result: Published Technical results support this as follows:
| |
| Technology Readiness Level (TRL): TRL 9 | |
| References: | |
| Location: Slovenia | Year: 2026 |
|---|---|
| Description: The Slovenian TSO, Eles, is in the progress of deploying 24 MSSSC devices into operation in 2026 into the Slovenian transmission network into the Podlog-Obersielach 220kV line in Podlog 220kV station. This is part of a European project of Common Interest, Greenswitch. 'ELES intend to solve problems with interconnection connection between Austria and Slovenia on 220 kV level with installation of static synchronous series compensator (SSSC) in the 220 kV connection Podlog - Obersielach'[4] 'The GreenSwitch project will remove up to 3300 hours per year of stressful state of transmission grid operation by removing critical N-1 situations in these periods for Slovenian and Croatian and Austrian grid. By this, the partners will also be able to avoid very critical alternative measures taken today on 220 kV level, which are leaning on topological redirection of flows, therefore reducing the grid availability and also putting the grid security in jeopardy.' [2]  | |
| Design: Eles is deploying 24 MSSSC devices in 2025-6 into the Slovenian transmission network into the Podlog-Obersielach 220kV line in Podlog 220kV station. This is part of a European project of Common Interest, Greenswitch. Detailed design, specification, controls required, studies submitted (load flow, harmonic, noise, stability, etc.) and standards applied can be found in tender documentation [4]. | |
| Result: Of the stated objectives of the Greenswitch project the following are directly attributable to the MSSSC:
And these indirectly:
'Remarkable synergies within and among partners have been achieved through a dedicated siting and sizing of infrastructure and operational solutions. As an example, the complex management of power flows on the 220 kV network, which today represents one of the biggest challenges on the transmission network in N or N-1 system states, would require 2 power control units to be invested in Slovenia, 5 units in Croatia and eventually an additional unit in Austria. By introduction of the GreenSwitch project the number of such units will be reduced from 8 to 5, which represents between 29-37% reduction of costs. Additional flexibility of power control can be further achieved by allowing for the construction of the latest modular power control units (SSSC), which will be commonly analysed by the partners and determined in the design phase of the GreenSwitch project.' [1] | |
| Technology Readiness Level (TRL): TRL 8 | |
| References: https://energy.ec.europa.eu/system/files/2021-03/detailed_project_description_0.pdf https://www.enarocanje.si/#/pregled-objav/815885 https://www.enarocanje.si/api/datoteka/get?id=Mjg1NzA1O1JhenBpc25hX2Rva3VtZW50YWNpamFfZUpOLnppcA Podlog ELES project press release announcement (Hyperlink To Be Advised - going to publication now) | |
| Location: Red Eléctrica, Spain | Year: 2014 |
|---|---|
| Description: The Spanish TSO completed the first SSSC installation in Europe in 2014 in the 220 kV substation Torres del Segre. As there was an overhead line that was overloaded when renewable generation was high, the main goal was to maximise renewable production by means of redirecting power flows in the substation. | |
| Design: The design used VSC technology with 4 inverters of 8 MVA. Each inverter has 3 levels, and it is IGCTs based. An additional inductance was used for a more inductive performance. Finally, a bypass using thyristors was proposed to protect the VSC (100 kA peak)  | |
| Result: The Torres del Segre SSSC has been successfully in service since 2014 without major incidents. When the system detects overload in the overhead line, it modifies its impedance. Consequently, power flow is redirected to other lines, improving system efficiency and enabling more renewable integration. | |
| Technology Readiness Level (TRL): TRL 8 | |
| References: | |
R&D Needs
Current research related to SSSCs should focus on:
- Defining criteria for the evaluation of wanted (passive) behaviour of SSSC
- Modelling SSSCs
- Identification of applications where SSSC are more beneficial comparing to other alternative technologies especially in power systems based on RES [4]
- Development of criteria and process for verification of dynamic interactions and identification of mitigation strategies with other network components, such as nuclear power plants, or power electronic devices (e.g. STATCOM, HVDC converter)
- Identifying the role of the SSSC in control strategies for automatic generation control in power systems with new energy sources [5]
- Defining the role of MSSSCs 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, milestones “New and optimised control concepts of reactive power” and “Ensure secure and stable operation of the hybrid AC/DC grid” 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.
The DSO Entity Technical Vision 2025 outlines digitalisation as a foundation for system performance and industry competitiveness, which explicitly identifies the benefits of power flow control. This is expanded in another main area of 'planning and investments' where 'unlocking efficient and timely investments' identifies smartening the network using digitisation technologies as fundamental to achieving timely and efficient network developments. In the area of 'market facilitation and prosumers', congestion management from infrastructure upgrades and the integration of new technologies like that MSSSC offers is identified. In the area of 'operations and maintenance ', identifies needs which will benefit from MSSSC capability to provide 'enhanced system security measures' to respond to grid overloads, security vulnerabilities, increasing the longevity of assets and enabling longer outage windows contribute to 'operational efficiency and advanced maintenance.' In the area of ,resilience and sustainability', fast deployable MSSSC will 'strengthen[ing] infrastructure, operational efficiency, and emergency response capabilities to ensure reliable energy supply for customers' would be beneficial.
Technology Readiness Level (TRL)
For TSO:
TRL 7 for conventional SSSC
TRL 9 for MSSSC up to 110 kV
TRL 7 for MSSSC above 110 kV
References
“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.
“Static synchronous series compensator (SSSC)” eCIGRE, 2009. [Online]
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.
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.