Static Synchronous Series Compensator

The static synchronous series compensator (SSSC) is a power quality FACTS device that employs a VSC connected in series to a transmission line through a transformer or multilevel inverters. The SSSC works like the STATCOM, except that it is serially connected instead of a shunt. Its output is a series injected voltage, which leads or lags the line current by 90°, thus emulating a controllable inductive or capacitive reactance. The SSSC can be used to reduce or increase the equivalent line impedance and enhance the active power transfer capability of the line. Moreover, the SSSCs are highly controllable devices and can provide further functionalities and services to the energy system.

Technology Types

SSCCs are part of the family of series controllers within FACTS devices. Two variants are possible:

  • The conventional SSSC, connected to the transmission line through a transformer
  • The transformerless SSSC, connected to the transmission line through multilevel inverters (such as modular transformerless SSSCs).

Components & enablers

Conventional SSSCs are also known as advanced series compensators (ASCs) or GTO-CSC, being the evolution of controlled series compensation (SC) devices. The SSSC consists of a coupling transformer, a GTO VSC and a DC circuit. They act as a controllable voltage source whose voltage magnitude can be in an operating area controlled independently of the line current. The SSSC can be considered functionally as an ideal generator that can be operated with a relatively small DC storage capacitor in a self-sufficient manner to exchange reactive power with the AC system or, with an external DC power supply or energy storage, to also exchange independently controllable active power, analogously to a STATCOM.

Transformerless SSSC solutions typically comprise a single-phase, modular-SSSC injecting a leading or lagging voltage in quadrature with the line current, but include a built-in-bypass to avoid damage of the power electronics resulting from high currents e. g. during a network fault. It can increase or decrease power flows on a circuit and perform dynamic services.

Advantages & field of application

The use of SSSCs provides the typical advantages of load flow control that can also be realised by other technologies such as PST, TCSC and partly by a series reactor.

However, both conventional and transformerless SSSCs also offer additional special functions such as:

  • Better controllability of power flow, as SSSCs possess the inherent capability to decrease as well as to increase (real) power flow almost linearly in the circuit.
  • Receiving end voltage regulation of a radial line: In short circuit weak networks by controlling the degree of series compensation to keep the end-voltage constant in the face of changing load and load power factor.
  • Power oscillation damping: the SSSC is a controlled device and can be used to damp wide area low frequency power oscillations.
  • Regional network voltage regulation by the inherent networking of devices applied at different locations in a grid.
  • Phase balancing: Modular SSSC as a single phase device can independently alter the effective phase impedance of a circuit[s] to rebalance power flows.

In addition to these range of applications, the modular SSSC are designed to be easily relocated if needed. Moreover, the amount of load flow shift can be adjusted by adding or removing modules, depending on the network needs. A little effort redeployability for is expected.

Technology Readiness Level

Conventional SSSC: TRL 7 – System prototype demonstration in operational environments

Transformerless SSSC up to 110 kV: TRL 9 – System prototype demonstration in operational environments

Transformerless SSSC above 110 kV: TRL 7 – System prototype demonstration in operational environments

Research & Development

Current fields of research: Generic research related to FACTS includes a variety of domains, from power electronics to applications. Of note are: power electronic topologies and control; exploration of new type of semiconductors replacing silicon; mitigation of power quality impacts of large scale power electronics application; more user-friendly interfaces; standardisation; evaluation of cost and benefits through demonstrations; coordinated control of multiple FACTS devices and relocatable FACTS.

In addition to these generic topics, when focusing more particularly on SSSC, research is conducted to detect in advance the interactions such as sub synchronous resonance (SSR) events with other network components. Some research is focusing on enhancing the low voltage ride through the capability of wind turbines using a combination of SSSC and controllable series braking resistor.

Focusing specifically on modular transformerless SSSC, real time congestion management, power quality management, offshore network control, phase angle correction and ancillary services from short term active power provision / management are the fields to be further developed in the future.

Best practice performance

Conventional SSSC:

  • Rated system voltage: 220 kV
  • Rated reactive power: 100-400 MVAR

Transformerless SSSC:

  • Rated system voltage: up to 550 kV
  • Rated reactive power: modular units are designed to be operated in combination, allowing any reactive power rating to be possible, i.e. 1 – 10 MVAr in size for each module.

Best practice application



The application foresees the SSSC functionalities validation, the case study definition enabling the equipment behaviour validation (in normal operation and during contingencies in the grid) by using a power flow simulation software. It is complemented by a simplified case study to analyse the behaviour of the SSSC in electromagnetic short-circuit simulations.

The validation of the SSSC solutions behaviour used a reference grid including two transmission lines representing the 400 kV Transmission System with two parallel lines representing the 220 kV lines. A need was identified in the 220 kV lines, due to the differences in impedance and the transmission capacity of the 220 kV lines. The short circuit behavior of the network and its impact on the SSSC was evaluated, to ensure the SSSC could withstand 40 kA of short circuit current across the coupling transformer.

The SSSC can solve some of the overload problems detected in the 220 kV grid of the Spanish Electrical System. It was proven to be particularly convenient in old lines (with low capacity), with a power flow very much influenced by wind power production.



At times of high wind infeed and hydro production, 220 kV high voltage lines were overloaded in Torres del Segre in Spain. To relieve congestion, the system operator was obliged to reduce renewable production output at certain instances of time.

A 50 MVAr conventional SSSC was installed including control equipment, magnetic elements for grid coupling, by-pass switch, thyristor and a local SCADA.

Construction of a new 220 kV circuit has been avoided and the efficiency of the existing infrastructure has been improved, with enhanced dis-patching of power flows and an increase in renewable energy integration.



For the trial installation, three transformerless SSSC units were installed on the Cashla – Ennis 110 kV line. Once the trial validated the safety of the installation process, two transformerless SSSC units were installed on the first tower coming out of the Cashla substation in County Galway. The remaining unit was installed on the first tower coming out of the Ennis substation on the same circuit. This pilot lasted for a year, verifying that communications with the devices did not impact the systems used to report primary faults and can change reactance as specified. The full functionality of the devices was assessed, including switching units from full Capacitive Reactance Injection mode to full Inductive Reactance Injection mode.

Use of modular transformerless SSSC to enable real-time power flow control on grids.

During system faults, the devices performed as expected and entered a bypass mode under fault conditions, with no unexpected interactions with the normal protection system. During the testing, Ireland was hit by the tail end of Hurricane Ophelia on Monday 16 October 2017 (Status Red wind warning – the highest threat level possible). Despite strong gusts of up to 156 km an hour on land, there was no structural damage caused to the devices. The units did not cause any damage to the transmission infrastructure. All units remained fully operational through this period.



Use of SSSC for solving problems associated with the Nigerian 330 kV longitudinal power network, using voltage magnitude as performance metrics.

Modelling of power system and SSSC modelling producing two sets of non-linear algebraic equations solved simultaneously using the Newton–Raphson algorithm method and implemented using MATLAB.

Results of power flow analysis of Nigerian 330 kV transmission network without SSSC showed that there was voltage limit violation of ±10 % at bus 16 Gombe (0.8973 p. u). The results with the incorporation of SSSC showed that the SSSC was effective in eliminating voltage limit violations and reduced network active power loss by more than 5% of the base case (93.87 MW). Therefore, SSSC is effective in solving steady-state problems of longitudinal power systems.

United Kingdom


NGET are proceeding with five installations in 2020, due for completion in 2021. These projects will contain a total of 375 MVAr of power flow control capability, located along the Fourstones to Harker to Stella West, Penwortham to Kirkby, and Lackenby to Norton circuits.

Use of modular transformerless SSSC to enable real-time power flow control on grids.

These projects are anticipated to increase boundary capabilities by 1.5 gigawatts in total across three transmission network boundaries. National Grid ESO has assessed similar projects at a number of further locations, and these are recommended for progression in the Network Options Assessments report for later years.

New York, United States [21]


Three transformerless SSSCs 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 in advance of a larger installation planned for 2021. This larger installation will add 21% series compensation on a 345 kV line to enable full capacity deliverability of interconnecting generation. The Electric Power Research Institute (EPRI) observed the 2019 installation and evaluated the technology’s functionality.

Use of modular transformerless SSSC to enable real-time power flow control on grids.

The installation process was smooth and without incident. The transformerless SSSC operated effectively in capacitive and inductive injection modes. The devices responded through control commands issued locally through the substation-based interface and remotely from the SCADA / EMS. During system faults, the devices performed as expected and entered a bypass mode under fault conditions. There were no un-expected interactions with the normal protection system.


[ 1 ] WASET. Optimal Sizing of SSSC Controllers to Minimize Transmission Loss and a Novel Model of SSSC to Study Transient Response.

[ 2 ] Power Quality In Electrical Systems. Static Synchronous Series Compensator (SSSC) [Link]

[ 3 ] ABB, A matter of FACTS – Deliver more high quality power, 2015 [Link]

[ 4 ] Siemens. Largest Statcom reactive power compensation project in India [Link]

[ 5 ] T. Rajaram, J. M. Reddy and Y. Xu. Kalman Filter Based Detection and Mitigation of Subsynchronous Resonance with SSSC. [Link]

[ 6 ] L. Piyasinghe, Z. Miao, J. Khazaei and L. Fan. Impedance Model-Based SSR Analysis for TCSC Compensated Type-3 Wind Energy Delivery Systems. [Link]

[ 7 ] Energía Eléctrica. Planificación Energética. [Link]

[ 8 ] Tinho LI, Hailian XIE, Nicklas Johansson. Transformer-less static synchronous series compensator and method therefor. [Link]

[ 9 ] REALISEGRID project (2009 – 2012), D1.4.2, Final WP1 report on cost / benefit analysis of innovative technologies and grid technologies roadmap report validated by the external partners.

[ 10 ] Alvira D., Torre M., Bola J., Burdalo U., Marquez M. (Red Eléctrica de España España) Rodriguez, M.A., Chivite, J., Hernandez, A., Álva-rez, S. (INGETEAM): The use of a static synchronous series compensator (SSSC) for power flow control in the 220 kV Spanish transmis-sion network B4_107_2010 CIGRE 2010

[ 11 ] SMART VALVETM [Link]

[ 12 ] Analysis and Synthesis of Smart Wires in an Electric Power System, A Thesis submitted to the faculty of the University of Minnesota by Allan Craig Bekkala, December 2018 [Link]

[ 13 ] Astick P, Asija Divya, Choudekar Pallavi, Rani Nibha, Transmission line efficiency enhancement with inclusion of smart wires and con-trollable network transformers, Amity University Uttar Pradesh 2017 / 07 / 01 [Link]

[ 14 ] Rüberg, Sven; Ferreira, Helder; L’Abbate, Angelo; Häger, Ulf; Fulli, Gianluca; Li Yong,: ‘Improving network controllability by Flexible Alternating Current Transmission System (FACTS) and by High Voltage Direct Current (HVDC) transmission systems’, REALISEGRD deliverable D1.2.1 March 2010 [Link]

[ 15 ] G. A. Adepoju, M. A. Sanusi and M. A. Tijani, Application of SSSC to the 330 kV Nigerian transmission network for voltage control, 2017

[ 16 ] Cigre, WG B4.40, Static Synchronous Series Compensator (SSSC) 2009 [Link1] [Link2]

[ 17 ] Deepak M. Divan, William E. Brumsickle, Robert S. Schneider, Bill Kranz, Randal W. Gascoigne, Dale T. Bradshaw, Michael R. Ingram, and Ian S. Grant, A Distributed Static Series Compensator System for Realizing Active Power Flow Control on Existing Power Lines, 2015 [Link]

[ 18 ] SmartValve, Pilot Project 2016, Smartwires EirGrid [Link]

[ 19 ] Network Options Assessment report 2019 / 20 [Link]

[ 20 ] RWTH Aachen, IAEW, Modular Power Flow Control enhancing available capacity on the German Transmission Grid: an investigation. Aachen, Germany, June 2020

[ 21 ] EPRI, Evaluation of SmartValveTM Devices Installation at Central Hudson, Technical Report, 2020 [Link]