Upfc

IEEE Transactions on Power Delivery, Vol. 10, No. 2, April 1995

1085

THE UNIFIED POWFBFLOW CONTROLLER: A NEW APPROACH TO POWER TRANSMISSION CONTROL
L Gyqyi .

s. L willirms . Westinghouse Science t Technology Center i

C. D. Schauda

T R Rietman . D. R T o ~

A Edris .
o
Electric Power Research Institute

Western Area Power Administration

Abstract - l& paper s h m that theUnified Power Flow Controller l

(UPFC) is able to control both the transmitted real power and, independently, the reactive power flows at the sending- and the recebing-end of the transmission line. The unique capabilities of the UPFC in multiple line Compensation are integrated into a generalized power flow controller that i able to maintain prescribed, and independently s controllable, realpower and reactive p e r f o in the line. The paper lw describes the basic concepts of the proposed generalized P and Q controller and compares it to the more conventional, but related power flow c o n t r o l e ~ such as the l%y&br-Controlled Series Capacitor and Thyristor-ControlledPhase Angle Regulator. The paper also presents results of computer simulations showing the performance of the UPFCunder different system conditions. Keywords - AC transmission, FACTS, power flow controller, line compensation, seriescapacitor, phase angle regulator, thyristor, GTO, power converter, inverter. INTRODUCI'ION The Unified Power Flow Controller (UPFC) was proposed' for real-time control and dynamic compensationof ac transmission systems, providing the necessary functional flexibility required to solvemany of the problems facing the utility industy. The Unified Power Flow Controller consists of two switching converters, which in the implementations considered are voltagesourced inverters usinggate tzun-off (GTO) thyritorvalves, as illustrated in Fig. 1. These inverters, labelled "Inverter 1"and "Inverter 2" in the figure, are operated from a common dc link provided by a dc storage capacitor.This arrangement functions as an ideal a c t o ac power converter in which the real power can freely flow in either direction between the ac terminals of the two inverters and each inverter can independently generate (or absorb) reactive power at its own ac output terminal. Inverter 2 provides the main function of the UPFC by injecting an acvoltage vw with controllable magnitude Vm (OsV,sV& andphase angle Q ( O S Q S ~ ~ ) ,at the power frequency, inserh with line via an insertion transformer. This injected voltage can be considered essentially as a synchronousac voltage source'. The transmission line current flows through this voltage source resulting in real and reactive power exchange between it and the ac system. The real power exchanged at the ac terminal (i.e., at the terminal of theinsertion transformer) is converted by the inverter into dc power which appears at the dc ln as positive or negative real power demand. The reactive ik power exchanged at the ac terminal is generated internally by the inverter. The basic function of Inverter 1 is to supply or absorb the real 94 SM 474-7 PWRD A paper recommended and approved by the IEEE Transmission and Distribution Committee ofthe IEEE Power Engineering Society f o r presentation e t t h e IEEE/PES 1994 Summer Meeting, San 28, 1994. Manuscript subFrancisco, CA, July 24 mittedDecember 28, 1993; made available f o r printing May 9, 1994.

power demanded by Inverter 2 at the common dc link. This dc link power is converted back to ac and coupled to the transmission line
lnnrmirrlon Une
su@y Transformer

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Inwrler 2w

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Inverter 1

L
Measured Variables
Panmeter

Satlngr

T u <
cnntml

'Re1 ZRef 0IW 'Ref

Fig. 1. - Basic circuit arrangement of the Unified Power Flow Controller

via a shunt-connected transformer. Inverter 1 can also generate or absorb controllable reactive power, if it is desired, aild thereby it can provide independent shunt reactive compensation for the line....