Technical Description of the Unified Power Flow Controller (UPFC) and Its Potential Variations

  • Ram AdapaEmail author
  • Stig Nilsson
  • Bjarne Andersen
  • Yi Yang
Living reference work entry

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Part of the CIGRE Green Books book series (CIGREGB)


The unified power flow controller (UPFC) is a powerful power flow and reactive compensation FACTS controller. It consists of two voltage source converters connected back-to-back with a common DC bus. One of the VSC converters is shunt connected to the AC power system. It is equivalent to a STATCOM, which injects a current into the power system at the point of connection (POC). The other is what is referred to as a Static Synchronous Series Compensator (SSSC), which injects a voltage in series with the transmission line. The injected series voltage can be at any angle with respect to the line current. The injected currents have two parts. First, when the two converters share the same DC bus capacitor, the real power part, which is in phase with the line voltage, delivers or absorbs real power into/from the line. The real power also compensates for the losses in the UPFC. Second, the reactive part, which is in quadrature with the line voltage, emulates an inductive reactance or a capacitive reactance at the point of connection. That is, in an UPFC, the STATCOM can regulate the shunt reactive power at the line connection and also inject or absorb real power to control the DC bus capacitor voltage, thereby facilitating real power transfer between the two converters.

The first installed UPFCs were built with the use of relatively slow switching gate turn-off (GTO) thyristor devices, which were switched at fundamental frequency. This arrangement required the use of harmonically neutralized voltage-sourced converters (HN-VSC) to achieve harmonic cancellation and eliminate or reduce the need for harmonic filters. Currently built VSCs use Modular Multilevel Converters (MMC) that use insulated gate bipolar transistors (IGBTs), which enable design of higher voltage converter valves that eliminate the need for parallel connection of converter modules.

The chapter also provides information about two variations of the UPFC, the Static Synchronous Series Compensator (SSSC) and the Interline Power Flow Controller (IPFC).


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© Springer Nature Switzerland AG 2019 2020

Authors and Affiliations

  • Ram Adapa
    • 1
    Email author
  • Stig Nilsson
    • 2
  • Bjarne Andersen
    • 3
  • Yi Yang
    • 4
  1. 1.Electric Power Research InstitutePalo AltoUSA
  2. 2.Electrical Engineering Practice, ExponentSedonaUSA
  3. 3.Andersen Power Electronic Solutions LtdBexhill-on-SeaUK
  4. 4.State Grid Jiangsu Electric Power Research InstituteNanjingChina

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