A FACTS Device: Distributed Power-Flow Controller (DPFC)
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This paper presents a new component within the flexible
ac-transmission system (FACTS) family, called distributed
power-flow controller (DPFC). The DPFC is derived from the unified
power-flow controller (UPFC). TheDPFC can be considered as
a UPFC with an eliminated common dc link. The active power exchange
between the shunt and series converters, which is through
the common dc link in the UPFC, is now through the transmission
lines at the third-harmonic frequency. The DPFC employs
the distributed FACTS (D-FACTS) concept, which is to use multiple
small-size single-phase converters instead of the one large-size
three-phase series converter in the UPFC. The large number of
series converters provides redundancy, thereby increasing the system
reliability. As the D-FACTS converters are single-phase and
floating with respect to the ground, there is no high-voltage isolation
required between the phases. Accordingly, the cost of the
DPFC system is lower than the UPFC. The DPFC has the same
control capability as the UPFC, which comprises the adjustment
of the line impedance, the transmission angle, and the bus voltage.
The principle and analysis of the DPFC are presented in this paper
and the corresponding experimental results that are carried out on
a scaled prototype are also shown.
THE GROWING demand and the aging of networksmake it
desirable to control the power flow in power-transmission
systems fast and reliably . The flexible ac-transmission system
(FACTS) that is defined by IEEE as “a power-electronicbased
system and other static equipment that provide control
of one or more ac-transmission system parameters to enhance
controllability and increase power-transfer capability” , and
can be utilized for power-flow control. Currently, the unified
power-flow controller (UPFC) shown in Fig. 1, is the most powerful
FACTS device, which can simultaneously control all the
parameters of the system: the line impedance, the transmission
angle, and bus voltage .
Two approaches are applied to the UPFC to increase the reliability
and to reduce the cost; they are as follows. First, eliminating
the common dc link of the UPFC and second distributing
the series converter, as shown in Fig. 2. By combining these two
approaches, the new FACTS device—DPFC is achieved.
The DPFC consists of one shunt and several series-connected
converters. The shunt converter is similar as a STATCOM, while
the series converter employs the D-FACTS concept, which is to
use multiple single-phase converters instead of one large rated
converter. Each converter within the DPFC is independent and
has its own dc capacitor to provide the required dc voltage. The
configuration of the DPFC is shown in Fig. 3.
As shown, besides the key components, namely the shunt and
series converters, the DPFC also requires a high-pass filter that
is shunt connected at the other side of the transmission line, and
two Y–Δ transformers at each side of the line. The reason for
these extra components will be explained later.
The unique control capability of the UPFC is given by the
back-to-back connection between the shunt and series converters,
which allows the active power to exchange freely. To ensure
that the DPFC have the same control capability as the UPFC,
a method that allows the exchange of active power between
converters with eliminated dc link is the prerequisite.
ANALYSIS OF THE DPFC
In this section, the steady-state behavior of the DPFC is analyzed,
and the control capability of the DPFC is expressed in
the parameters of the network and the DPFC.
To simplify the DPFC, the converters are replaced by controllable
voltage sources in series with impedance. Since each
converter generates the voltage at two different frequencies,
it is represented by two series-connected controllable voltage
sources, one at the fundamental frequency and the other at the
third-harmonic frequency. Assuming that the converters and the
transmission line are lossless, the total active power generated
by the two frequency voltage sources will be zero. The multiple
series converters are simplified as one large converter with the
voltage, which is equal to the sum of the voltages for all series
converter, as shown in Fig. 8.
To control the multiple converters, DPFC consists of three
types of controllers; they are central controller, shunt control,
and series control, as shown in Fig. 14.
The shunt and series control are local controllers and are
responsible for maintaining their own converters’ parameters.
The central control takes account of the DPFC functions at the
power-system level. The function of each controller is listed
The central control generates the reference signals for both
the shunt and series converters of the DPFC. It is focused on the
DPFC tasks at the power-system level, such as power-flow control,
low-frequency power oscillation damping, and balancing
of asymmetrical components. According to the system requirement,
the central control gives corresponding voltage-reference
signals for the series converters and reactive current signal for
the shunt converter. All the reference signals generated by the
central control are at the fundamental frequency.
An experimental setup has been built to verify the principle
and control of the DPFC. One shunt converter and six singlephase
series converters are built and tested in a scaled network,
as shown in Fig. 17. Two isolated buses with phase difference
are connected by the line. Within the experimental setup, the
shunt converter is a single-phase inverter that is connected between
the neutral point of the Y–Δ transformer and the ground.
The inverter is powered by a constant dc-voltage source. The
specifications of the DPFC experimental setup are listed in the
Appendix (see Table I).
Within the setup, multiple series converters are controlled
by a central controller. The central controller gives the reference
voltage signals for all series converters. The voltages and
currents within the setup are measured by an oscilloscope and
processed in computer by using the MATLAB. The photograph
of the DPFC experimental setup is illustrated in Fig. 18.
This paper has presented a new concept called DPFC. The
DPFC emerges from the UPFC and inherits the control capability
of the UPFC, which is the simultaneous adjustment of
the line impedance, the transmission angle, and the bus-voltage
magnitude. The common dc link between the shunt and series
converters, which is used for exchanging active power in the
UPFC, is eliminated. This power is now transmitted through
the transmission line at the third-harmonic frequency. The
series converter of the DPFC employs the D-FACTS concept,
which uses multiple small single-phase converters instead
of one large-size converter.