PE-282-PWRD-0-12-1997
UPFC - Unified Power Flow Controller:
Theory, Modeling, and Applications
Kalyan K. Sen, Member, IEEE
Eric J. Stacey
Westinghouse Electric Corporation
1310 Beulah Road
Pittsburgh, PA 15235, USA
Abstract - This paper describes the theory and the modeling
technique of a Flexible Alternating Current Transmission Systems
(FACTS) device, namely, Unified Power Flow Controller (UPFC)
using an Electromagnetic Transients Program (EMTP) simulation
package. The UPFC, in this paper, consists of two solid-state
voltage source inverters which are connected through a common DC
link capacitor. Each inverter is coupled with a transformer at its
output. The first voltage source inverter, known as STATic
Synchronous COMpensator (STATCOM), injects an almost
sinusoidal current, of variable magnitude, at the point of connection.
The second voltage source inverter, known as Static Synchronous
Series Compensator (SSSC) injects an almost sinusoidal voltage, of
variable magnitude, in series with the transmission line. This
injected voltage can be at any angle with respect to the line current.
The exchanged real power at the terminals of one inverter with the
line flows to the terminals of the other inverter through the common
DC link capacitor. In addition, each inverter can exchange reactive
power at its terminals independently. The functionalities of the
models have been verified.
(Fig. 1b). The transmission line’s sending-end voltage, V s , leads
the receiving-end voltage, V r , by an angle δ . The resulting current
in the line is I and the real and reactive power flow at the receiving
end are Pr and Qr, respectively. With an injection of Vdq in series
with the transmission line, the transmission line’s sending-end
voltage, V o , still leads the receiving-end voltage, V r , but by a
different angle δ 1 (Fig. 1c). The resulting line current and the
amount of power flow change. With a larger amount of Vdq
injected in series with the transmission line, the transmission line’s
sending-end voltage, V o , now lags the receiving-end voltage, V r ,
by an angle δ 2 (Fig. 1d). The resulting line current and the power
flow now reverse. With this background, a set of power flow
controllers will be described. The objective in this paper is to
describe the fundamentals of a UPFC and its modeling technique
using an Electromagnetic Transients Program (EMTP) simulation
package.
Pr , Q r
V dq
Keywords - AC transmission, FACTS, power flow controller, power
converter, inverter, thyristor, GTO, etc.
VX
I. INTRODUCTION
VX
Vs
Electric power flow through an alternating current transmission
line is a function of the line impedance, the magnitudes of the
sending-end and receiving-end voltages, and the phase angle
between these voltages. Essentially, the power flow is dependent on
the voltage across the line impedance. Fig. 1a shows a single line
diagram of a simple transmission line with an inductive reactance,
XL, and a series insertion voltage, Vd q , connecting a sending-end
Vs
Vo
Vr
φ
V dq
Vdq >0
Vr
δ1
δ
(b)
V dq
VX
Vo
Vr
I
Pr =
Vs
I
Vdq =0
(a)
voltage source V s and a receiving-end voltage source, V r ,
respectively. The voltage across the transmission line reactance, XL,
is
(1)
V X = V s - V r - V dq = I XL
Qr =
Vo Vr
XL
Vo Vr
XL
I φ
sin δ
V
(cos δ - r )
Vo
VX
Vs
Vdq >0
Vr
δ2
Vo
(c)
(d)
Fig. 1 A Unified Power Flow Controller Operating in a Voltage Injection
Mode and the Related Phasor Diagrams
where I is the current in the transmission line.
Flexible Alternating Current Transmission Systems (FACTS)
devices, namely STATic Synchronous COMpensator (STATCOM),
Static Synchronous Series Compensator (SSSC) and Unified Power
Flow Controller (UPFC), are used to control the power flow through
an electrical transmission line connecting various generators and
loads at its sending and receiving ends. The UPFC, in this paper and
existing references, consists of two solid-state voltage source
inverters which are connected through a common DC link capacitor.
Each inverter is coupled with a transformer at its output. The first
inverter, known as STATic Synchronous COMpensator
(STATCOM), injects an almost sinusoidal current, of variable
magnitude, at the point of connection. The second inverter, known
as Static Synchronous Series Compensator (SSSC) injects an almost
sinusoidal voltage, of variable magnitude, in series with the
transmission line. When the STATCOM and the SSSC operate as
stand-alone devices, they exchange almost exclusively reactive
power at their terminals. While operating both the inverters together
as a UPFC, the exchanged power at the terminals of each inverter
can be reactive as well as real. The exchanged real power at the
terminals of one inverter with the line flows to the terminals of the
other inverter through the common DC link capacitor.
The voltage, V X , across the transmission line can be changed by
changing the insertion voltage, Vdq , in series with the transmission
line and, consequently, the line current and the power flow in the
line will change. Consider the case where Vdq = 0
PE-282-PWRD-0-12-1997
A paper recommended and approved by the
IEEE Transmission and Distribution Committee of the IEEE Power
Engineering Society for publication in the IEEE Transactions on Power
Delivery. Manuscript submitted July 31, 1997; made available for printing
December 12, 1997.
 1997 IEEE
I
XL
1