Unified Power Flow Controller(upfc)1

Unified Power Flow Controller(UPFC)
 The Unified Power Flow Controller (UPFC) concept was
proposed by Gyugyi in 1991
 The UPFC was devised for the real-time control and
dynamic compensation of ac AC Transmission systems
 The UPFC is able to control, simultaneously or
selectively, all the parameters affecting power flow in
the transmission line (i.e., voltage, impedance, and
phase angle), and this unique capability is signified by
the adjective "unified” in its name.
 It can independently control both the real and reactive
power flow in the line
 UPFC is the interconnected FACTS device of STATCOM
with SSSC

In the presently used practical implementation, the UPFC consists of two
voltage sourced converters

 These back-to-back converters, labeled "Converter 1"
and "Converter 2" in the figure, are operated from a
common dc link provided by a dc storage capacitor.
 As indicated before, this arrangement functions as an
ideal ac-to-ac power converter in which the real power
can freely flow in either direction between the ac
terminals of the two converters, and
 Each converter can independently generate (or absorb)
reactive power at its own ac output terminal.
 Converter 2 provides the main function of the UPFC by
injecting a voltage Vpq with controllable magnitude Vpq
and phase angle p in series with the line via an insertion
transformer.

The transmission line current flows through this voltage source
resulting in reactive and real power exchange between it and the ac
system.
The basic function of Converter 1 is to supply or absorb the real
power demanded by Converter 2 at the common dc link to support
the real power exchange resulting from the series voltage injection
Shunt Converter can also generate or absorb controllable reactive
power, if it is desired, and thereby provide independent shunt
reactive compensation for the line
Thus, Converter 1 can be operated at a unity power factor or be
controlled to have a reactive power exchange with the line
independent of the reactive power exchanged by series Converter
There can be no reactive power flow through the UPFC dc link.

Unified Power Flow Controller(UPFC
a)Voltage regulation
• UPFC can fulfill all these functions and there
by meet multiple control objectives by
adding the injected voltage Vpq, with
appropriate amplitude and phase angle
• Voltage regulation with continuously
variable in-phase/anti-phase voltage
injection, is shown in Figure 8.5(a) for
voltage increments Vpq=ΔV(p=) 0).
• This is functionally similar to that obtainable
with a transformer tap changer having
infinitely small steps.

b)Series reactive compensation
• Where Vpq=Vq is injected in quadrature with the
line current I
• Functionally this is similar to series capacitive
and inductive line compensation attained by the
SSSC
• The injected series compensating voltage can be
kept constant, if desired, independent of line
current variation, or can be varied in proportion
with the line current.

c) Phase shifting
• Where Vpq= Vσ is injected with an angular
relationship with respect to that achieves the
desired Vs
• Thus the UPFC can function as a perfect Phase
Angle Regulator which, as discussed
• Phase shifter can also supply the reactive power
involved with the transmission angle control by
internal var generation.

d) Multifunctional Power Flow Control
• Multifunction power flow control, executed by
simultaneous terminal voltage regulation, series
capacitive line compensation, and phase shifting
• Where Vpq= ΔV + Vq +Vσ.
• This functional capability is unique to the UPFC.
• No single conventional equipment has similar
multifunctional capability.

Power Flow Control Capability of the UPFC
• The real and reactive power transmission versus transmission
angle characteristics of the simple two-machine is shown in fig
• With reference to this figure, the transmitted power P and the
reactive power -jQr supplied by the receiving end, can be
expressed as follows:
P-jQr = Vr((Vs+Vpq-Vr)/jX)*

In compensated system:
Transmittable real power P is controllable between
Reactive power Qr is controllable between

A phasor diagram, defining the relationship between Vs, Vr, Vx (the
voltage phasor across X) and the inserted voltage phasor Vpq, with
controllable magnitude(0 ≤Vpq ≤ Vpqmax) and angle (0 ≤ ρ ≤ 360),
is shown below
The inserted voltage phasor Vpq is added
to the fixed sending-end voltage phasor
Vs, to produce the effective sending-end
voltage Vseff=Vs+Vq

The rotation of phasor Vpq with angle ρpq modulates both the
magnitude and angle of phasor I/x and, therefore, both the
transmitted real power, P, and the reactive power, Qr,, vary with ρpq
in a sinusoidal manner

Real and Reactive Power Flow Control
• The UPFC can control real and reactive
power flow in the transmission line
• Let it first be assumed that the injected
compensating voltage Vpq, is zero.
• Therefore the normalized transmitted
power,
Po(δ) = {VV/X}sinδ=sinδ,
and the normalized reactive power,
Q(δ) = Qs(δ) = -Qr(δ)= = {VV/X}(1-cosδ}=1-cosδ,
supplied at the ends of the line
• Normalisation factor is VV/X=1

• The relationship between real power Po(δ)
and reactive power Qor(δ) can readily be
expressed with VV/X= 1 in the following
form:
• Now consider Vpq ≠0
• Therefore the real and reactive power
change from their uncompensated values,
Po(δ) and Qor(δ), as functions of the
magnitude Vpq and angle ρ of the injected
voltage phasor Vpq

Unified Power Flow Controller(upfc)1

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