Static Synchronous Compensation (STATCOM)

A STATic synchronous COMpensator (STATCOM) is nowadays a fast-acting device capable of providing or absorbing reactive current and by that regulating the voltage at the point of connection to a power grid. It is categorized under Flexible AC transmission system (FACTS) devices. The technology is based on voltage source converter (VSC) with semiconductor valves in a modular multi-level configuration. The dynamic reactive current output range is symmetrical (during normal disturbed network conditions), however non-symmetrical designs are possible by introducing mechanically or thyristor switched shunt elements with unified control systems to cover most conventional applications. The STATCOM design and fast response makes the technology very convenient for maintaining voltage during network faults (as STATCOMs are capable to provide fast fault current injection limited to the rated current), enhancing short term voltage stability. In addition, STATCOMs could provide power factor correction, reactive power control, damping of low-frequency power oscillations (usually by means of reactive power modulation), active harmonic filtering, flicker mitigation and power quality improvements. Typical applications are in the electric power transmission, electric power distribution, electrical networks of heavy industrial plants, arc furnaces, high-speed railway systems and other electric systems, where voltage stability and power quality are of utmost importance.


Technology Types

A typical STATCOM configuration consists of multi-level VSC based on IGBTs, phase reactors and step-up transformer. It is shunt-connected to the grid. The reactive current is provided or absorbed by producing a controlled internal voltage waveform. Most STATCOMs available in the market today operate as grid following converters and require a grid voltage reference to operate (with a defined level of grid strength). The voltage waveform is adjusted in its response with reference to the grid connection point voltage. In general, the STATCOMs operate as AC current controlled device, although the control of the output current is achieved via the regulation of the STATCOM internal voltage (behind the phase reactor) in amplitude, while the angle is close to 90 degrees with respect to the grid connection point voltage. If the STATCOM voltage amplitude is higher than the system voltage amplitude capacitive reactive power is provided to the grid. If vice versa current flows from the system to the STATCOM and inductive reactive power is provided. The amount of reactive current depends on the transformer short circuit reactance and the voltage difference and is limited to the thermal limits of the IGBTs. In normal operation, i.e. the system voltage is within certain limits, both voltage amplitudes are equal and no reactive power is exchanged with the grid. An established control is if the grid voltage is above the threshold value, STATCOM control will decrease the amplitude of the STATCOM voltage waveform, making the STATCOM act as an inductive element and absorb reactive power from the grid. When the grid voltage is above the threshold value, the magnitude of the voltage waveform will be increased, making the STATCOM act as a capacitive element and providing reactive current to the grid.


Components & enablers

Typical components of a STATCOM installation are:

  • high-voltage AC circuit breaker,
  • step-down transformer,
  • coupling reactors,
  • 3 converter branches connected in delta,
  • converter-branch sub-modules, consisting of H-bridge installation of DC capacitor, IGBT and diode,
  • control and protection,
  • auxiliary system,
  • cooling system,
  • high-frequency filters,
  • additional capacitive or inductive shunt elements for asymmetrical control range (MSC, MSR, TSC, TSR)

Advantages & field of application

Modern designs are modular and allow for high level of scalability and flexibility ensuring the total required dynamic and steady state rating. By addition of shunt elements, the symmetrical output range of the pure STATCOM device can be adjusted to meet also non-symmetrical performance requirements. For conditions, where a fast non-symmetrical dynamic range is required, on the one hand, thyristor-switched reactors and capacitors can be operated in parallel to form hybrid solutions. On the other hand, mechanically switched reactors and capacitors can be added to optimize slow response performance and provide additional steady-state capacity as required by e.g. typical intra-day load flow changes.


Technology Readiness Level

TRL 8 – System ready for full scale deployment if classical design and control is used system ready for full scale deployment.


Research & Development

Current fields of research and development: mitigation of commutation failures, novel reactive power control strategies based on machine learning techniques, power quality improvement by selective compensation of voltage harmonics (active filtering), challenges of applications in distribution grids (D-STATCOM), use of solar PV inverters as STATCOM (PV-STATCOM), enhancement in power oscillation damping, coordinated use of multiple devices and area voltage regulation and others, change of STATCOM control from normal classical current control to an even more grid convenient Grid Forming Control (GFC) to enable STATCOMs to provide more ancillary grid services such as inherent current response to changes in grid voltage or/and amplitude, change in configuration from delta to star connection to integrate energy storage devices (i.e. supercaps), essential to provide instantaneous power reserve (inertia) in a relevant dimension.

Innovation priority: Grid Forming control with energy storage devices to enable inherent response and to improve dynamic performance, stable steady-state and dynamic behavior, semiconductor performance, reactive power output increase, cost reduction, facility footprint minimization, noise reduction and hybrid solutions.

Number of IEEE publications since 2000, more than 3,000.


Best practice performance

Voltage rating: up to ~ 400kV

Reactive power range: up to ±600 Mvar

Continuous rated capacitive current generation at grid voltage lower than 0.2 per unit.

Voltage measurement and control at reference high-voltage nodes more than 160 km away from the facility.


Best practice application

Borken, Hesse, Germany

2018

Description
The first German hybrid STATCOM facility is in operation since 2018 to dynamically support the voltage and enhance the power quality at the 380 kV level.

Design
Hybrid construction of the reactive power compensation system, with two STATCOM branches and a MSCDN (Mechanical Switched Capacitor with Damping Network) providing reactive power compensation within the range of -250 Mvar to +400 Mvar. The MSCDN is used to provide the capacitive base load.

Results
Provides wide reactive control range and enhances power quality.

Texas, United States

2005

Description
In 2005 a STATCOM was installed in Austin to replace the reactive power capabilities of a closed down power plant. Due to noise and EMF emission as well as land use constraints a STATCOM was chosen instead of an SVC.

Design
A ±100 Mvar system was installed at a 138 kV bus along with three 31 Mvar capacitor banks, controlled by the STATCOM. Due to the IGBT converters being housed in a two-floor building land use and noise emissions could be reduced. The STATCOM is designed for continuous operation at 46°C.

Results
Provides high-speed control and decreases costs as compared to other equipment improvements in the transmission infrastructure.

Virginia, United States

2018

Description
World’s first mobile STATCOM to compensate for the load when plants are shut down, which can occur within months of announcement in the USA.

Design
The design and substation infrastructure corresponds to that of a typical facility, however it has been customized to fit into three trailers. The rated power output is ±50 Mvar.

Results
The mobile solution gives a high level of flexibility. Instead of several years of planning and execution for a permanently installed substation, the mobile technology can be installed and commissioned within days anywhere, providing the required reactive power output to stabilize the grid.


References

[1] B. Singh, R. Saha, A. Chandra and K. Al-Haddad, "Static synchronous compensators (STATCOM): a review," in IET Power Electronics, vol. 2, no. 4, pp. 297-324, July 2009. [Link]

[2] Siemens. The best of both – Combining STATCOM with conventional thyristor based Static Var Compensator technology. [Link]

[3] EdgeFX. Compensators – SVC, SSSC and So on using power electronics. [Link]

[4] TSCNET. First Hybrid STATCOM facility in Germany. [Link]

[5] Tennet. Umspannwerk Borken. [Link]

[6] S. A. Kamran and J. Muñoz, "Study of a state-of-the art M-STATCOM," 2015 IEEE International Conference on Industrial Technology (ICIT), Seville, 2015, pp. 2733-2738. [Link]

[7] ABB. SVC Light improving the grid of Austin, Texas. [Link]

[8] X. Wang, Y. Wang, H. Yan and D. Liu, "Research on influences to reduce the commutation failure of different topology structures of STATCOM," 2016 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific), Busan, 2016, pp. 470-473. [Link]

[9] J. Burr, S. Finney and C. Booth, "Comparison of Different Technologies for Improving Commutation Failure Immunity Index for LCC HVDC in Weak AC Systems," 11th IET International Conference on AC and DC Power Transmission, Birmingham, 2015, pp. 1-7. [Link]

[10] Rhea Wessel, Siemens AG, World’s first mobile STATCOM: When grid stabilization gets wheels, [Link]