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Static synchronous compensator

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Static Synchronous Compensator
A STATCOM located in an Electrical Substation
Component typeActive
Working principle‍Voltage Source Converter
Electronic symbol

A static synchronous compensator (STATCOM), originally known as a static synchronous condenser (STATCON), is a shunt-connected, reactive compensation device used on transmission networks. It uses power electronics to form a voltage-source converter that can act as either a source or sink of reactive AC power to an electricity network. It is a member of the FACTS family of devices.

STATCOMS are alternatives to other passive reactive power devices, such as capacitors and inductors (reactors). STATCOMs have a variable reactive power output, can change their output in terms of milliseconds, and able to supply and consume both capacitive and inductive vars. While they can be used for voltage support and power factor correction, their speed and capability are better suited for dynamic situations or supporting the grid under fault conditions or contingency events.

The use of voltage-source based FACTs device had been desirable for some time, as it helps to mitigate the limitations of current-source based devices whose reactive output decreases with system voltage. However, limitations in technology have historically prevented wide adoption of STATCOMs. When gate turn-off thyristors (GTO) became more widely available in the 1990s[1] and had the ability to switch both on and off at higher power levels, the first STATCOMs began to be commercially available. These devices typically used 3-level topologies and Pulse-Width Modulation (PWM) to simulate voltage waveforms.

Modern STATCOMs now make use of Insulated-Gate Bipolar Transistors (IGBTs), which allow for faster switching at high-power levels. 3-level topologies have begun to give way to Multi-Modular Converter (MMC) Topologies, which allow for more levels in the voltage waveform, reducing harmonics and improving performance.

History

When AC won the War of Currents in the late 19th century, and electric grids began expanding and connecting cities and states, the need for reactive compensation became apparent.[2] While AC offered benefits with transformation and reduced current, the alternating nature of voltage and current lead to additional challenges with the natural capacitance and inductance of transmission lines. Heavily loaded lines consumed reactive power due to the line's inductance, and as transmission voltage increased throughout the 20th century, the higher voltage supplied capacitive reactive power. As only operating a transmission line at it surge impedance loading (SIL) was not feasible,[2] other means to manage the reactive power was needed.

A mercury-arc Valve used for high-voltage power electronics.

Synchronous Machines were commonly used at the time for Generators, and could provide some reactive power support, however were limited in doing so due to the increase in losses it caused. They were also limited in their effect as the system grew and transmission lines continued to be far from the generator. Fixed, shunt capacitor and reactor banks filled this need by being deployed where needed. In particular, shunt capacitors switched by circuit breakers provided an effective means to managing varying reactive power requirements due to load changes.[3] However, this was not without limitations.

Shunt capacitors and reactors are fixed devices, only able to be switched on and off. This required either a careful study of the exact size needed,[4] or accepting less than ideal effects on the voltage of a transmission line. The need for a more dynamic and flexible solution was realized with the mercury-arc valve in the early 20th century. Similar to a vacuum tube, the mercury-arc valve was a high-powered rectifier, capable of converting high AC voltages to DC. As the technology improved, inverting became possible as well and mercury valves found use in power systems and HVDC ties. When connected to a reactor, different switching pattern could be used to vary the effective inductance connected,[5] allow for more dynamic control. Arc valves continued to dominate power electronics until the rise of solid-state semiconductors in the mid 20th century.[6]

As semiconductors replaced vacuum tubes, the thyristor created the first modern FACTs devices in the Static VAR Compensator (SVC).[7] Effectively work as a circuit breaker that could switch on in milliseconds, it allowed for quickly switching capacitor banks, or connected to a reactor switching sub-cycle when to vary the connected reactance. The thyristor also greatly improved the control system, allowing an SVC to detect and react to faults to better support the system.[8] The thyristor dominated the FACTs and HVDC world until the late 20th century, when the IGBT began to match its power ratings.[9]

With the IGBT, the first voltage-sourced converters and STATCOMs began to enter the FACTs world. A prototype 1 MVAr STATCOM was described in a report by Empire State Electric Energy Research Corporation in 1987.[10] The first production 100 MVAr STATCOM made by Westinghouse Electric was installed at the Tennessee Valley Authority Sullivan substation in 1995 but was quickly retired due to obsolescence of its components.[11]

Theory

The basis of a STATCOM is a voltage source converter (VSC) connected in series with some type of reactance, either a fixed Inductor or a Power Transformer. This allows a STATCOM to control power flow much like a Transmission Line, albeit without any active (real) power flow.[12] Given an inductor connected between two AC voltages, the reactive power flow between the two points is given by:

A STATCOM consisting of a VSC (bottom) and a fixed inductor (top). The inductor is connected on the other side to an AC system.

where

: Reactive Power

: Sending-End Voltage

: Magnitude difference in and receiving end voltage

: Reactance of the Inductor or transformer

: Phase-Angle difference between and

With close to zero (as the STATCOM provides no real power and only consumes a small amount as losses[13]) and a fixed size, reactive power flow is controlled by the difference between the two AC voltages.[14] From the equation then, if the STATCOM creates a voltage magnitude greater than the system voltage, it supplies capacitive reactive power to the system. If the STATCOM's voltage magnitude is less, it consumes inductive reactive power from the system. As most modern VSCs are made of power electronics that are capable of making small voltage changes very quickly,[15] a dynamic reactive power output is possible. This compares to a traditional, fixed capacitor or inductor, that is either off (0 MVar) or at it's maximum (for example, 50 MVar), while a 50 MVar STATCOM could range from 50 MVar Capacitive to 50 MVar Inductive in as small as 1 MVar steps.

Operation

STATCOM as a voltage source (in red) connected to a transmission line

The reactive power at the terminals of the STATCOM depends on the amplitude of the voltage source. For example, if the terminal voltage of the VSC is higher than the AC voltage at the point of connection, the STATCOM generates reactive current (appears as a capacitor); conversely, when the amplitude of the voltage source is lower than the AC voltage, it absorbs reactive power (appears as an inductor).[16][11]

A voltage droop of 1-10% (usually 3%) is built into STATCOMs.[16]

Application

While they can be used for voltage support and power factor correction, their speed and capability are better suited for dynamic situations or supporting the grid under fault conditions or contingency events. Usually a STATCOM is installed to support electricity networks that have a poor power factor and often poor voltage regulation.[1] There are however, other uses, the most common being to improve voltage stability.

STATCOM vs. SVC

A static VAR compensator (SVC) can also be used to maintain the voltage stability. STATCOM is costlier than an SVC (in part due to higher cost of the GTO thyristors) and exhibits higher losses, but it has a few technical advantages. As a result, the two technologies coexist.

The response time of a STATCOM is shorter than that of a SVC,[10] mainly due to the fast switching times provided by the IGBTs of the voltage source converter (thyristors cannot be switched off in a controlled fashion). As a result, the reaction time of a STATCOM is one to two cycles vs. two to three cycles for an SVC.[17]

The STATCOM also provides better reactive power support at low AC voltages than an SVC, since the reactive power from a STATCOM decreases linearly with the AC voltage (the current can be maintained at the rated value even down to low AC voltage), as opposed to power being a function of a square of voltage for SVC.[18] The SVC is not used in a severe undervoltage conditions (less than 0.6 pu), since leaving the capacitors on can worsen the transient overvoltage once the fault is cleared, while STATCOM can operate until 0.2-0.3 pu (this limit is due to possible loss of synchronicity and cooling).[19]

The footprint of a STATCOM is smaller, as it does not need external inductors and large capacitors used by an SVC.[20]

See also

References

  1. ^ a b Azharuddin, Mohd.; Gaigowal, S.R. (2017-12-18). "Voltage regulation by grid connected PV-STATCOM". 2017 International Conference on Power and Embedded Drive Control (ICPEDC). pp. 472–477. doi:10.1109/ICPEDC.2017.8081136. ISBN 978-1-5090-4679-9. S2CID 26402757.
  2. ^ a b EPRI Increased Power Flow Guidebook—2017: Increasing Power Flow in Lines, Cables, and Substations. EPRI, Palo Alto, CA: 2017. 3002010150
  3. ^ Gujar, Abhilash (2020-11-06). "Reactive Power Compensation using Shunt Capacitors for Transmission Line Loaded Above Surge Impedance". 2020 IEEE International Conference for Innovation in Technology (INOCON). IEEE. pp. 1–4. doi:10.1109/INOCON50539.2020.9298284. ISBN 978-1-7281-9744-9. S2CID 230512769.
  4. ^ Kamel, Salah; Mohamed, Marwa; Selim, Ali; Nasrat, Loai S.; Jurado, Francisco (March 2019). Power System Voltage Stability Based on Optimal Size and Location of Shunt Capacitor Using Analytical Technique. IEEE. pp. 1–5. doi:10.1109/IREC.2019.8754516. ISBN 978-1-7281-0140-8. S2CID 195831344.
  5. ^ Rissik, H., Mercury-Arc Current Converters, Pitman. 1941.
  6. ^ "1954: Morris Tanenbaum fabricates the first silicon transistor at Bell Labs". The Silicon Engine. Computer History Museum. Retrieved 23 August 2019
  7. ^ Owen, Edward L. (August 2007). "Fiftieth anniversary of modern power electronics: The Silicon Controlled Rectifier". 2007 IEEE Conference on the History of Electric Power: 201–211. doi:10.1109/HEP.2007.4510267. ISBN 978-1-4244-1343-0. S2CID 12720980.
  8. ^ Choudhary, Sunita; Mahela, Om Prakash; Ola, Sheesh Ram (November 2016). Detection of transmission line faults in the presence of thyristor controlled reactor using discrete wavelet transform. IEEE. pp. 1–5. doi:10.1109/POWERI.2016.8077268. ISBN 978-1-4673-8962-4. S2CID 23488189.
  9. ^ Iwamuro, Noriyuki; Laska, Thomas (March 2017). "IGBT History, State-of-the-Art, and Future Prospects". IEEE Transactions on Electron Devices. 64 (3): 741–752. Bibcode:2017ITED...64..741I. doi:10.1109/TED.2017.2654599. ISSN 0018-9383. S2CID 36435533.
  10. ^ a b Hingorani, Narain G.; Gyugyi, Laszlo (2017-12-18). "Static Shunt Compensators: SVC and STATCOM". Understanding FACTS. doi:10.1109/9780470546802. ISBN 9780470546802. {{cite book}}: |website= ignored (help)
  11. ^ a b Al-Nimma, Dhiya A.; Al-Hafid, Majed S. M.; Mohamed, Saad Enad (2017-12-18). "Voltage profile improvements of Mosul city ring system by STATCOM reactive power control". International Aegean Conference on Electrical Machines and Power Electronics and Electromotion, Joint Conference. pp. 525–530. doi:10.1109/ACEMP.2011.6490654. ISBN 978-1-4673-5003-7. S2CID 29522033.
  12. ^ Gaur, Bhaskar; Ucheniya, Ravi; Saraswat, Amit (October 2019). "Real Power Transmission Loss Minimization and Bus Voltage Improvement Using STATCOM". IEEE: 236–241. doi:10.1109/RDCAPE47089.2019.8979110. ISBN 978-1-7281-2068-3. {{cite journal}}: Cite journal requires |journal= (help)
  13. ^ Xi, Zhengping; Bhattacharya, Subhashish (July 2008). "Performance improved during system fault of angle controlled STATCOM by current control". 2008 IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century: 1–8. doi:10.1109/PES.2008.4596928.
  14. ^ Miller, Robert H., & Malinowski, James H. (1993). Power System Operation. New York: McGraw-Hill Inc.
  15. ^ Castagno, S.; Curry, R.D.; Loree, E. (October 2006). "Analysis and Comparison of a Fast Turn-On Series IGBT Stack and High-Voltage-Rated Commercial IGBTS". IEEE Transactions on Plasma Science. 34 (5): 1692–1696. doi:10.1109/TPS.2006.879551. ISSN 0093-3813.
  16. ^ a b Varma 2021, p. 113.
  17. ^ Varma 2021, pp. 114–115.
  18. ^ Singh, S. N. (23 June 2008). Electric power generation: transmission and distribution (2 ed.). PHI Learning Pvt. Ltd. p. 332. ISBN 9788120335608. OCLC 1223330325.
  19. ^ Varma 2021, p. 114.
  20. ^ Varma 2021, p. 115.

Sources

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