Control and performance of a unified power flow

controller
Dr.N.Devarajan*1,R.Rajaganapathi*2
*
Department of Electrical and Electronics Engineering,
Government College of Technology, Coimbatore, India

Abstract----- The Unified Power Flow Controller (UPFC) is the late 1980s, the Electric Power Research Institute (EPRI)
the most versatile and complex power electronic equipment introduced a new approach to solve the problem of designing
that has emerged for the control and optimization of power and operating power systems; the proposed concept is known as
flow in electrical power transmission system. This paper Flexible AC Transmission Systems (FACTS) [1]. The two main
presents real and reactive power flow control through a objectives of FACTS are to increase the transmission major
transmission line by placing UPFC at the sending end using impact on the development of the concept itself A new
computer simulation. Without UPFC, real and reactive generation of FACTS controllers has emerged with the
power through the transmission line cannot be controlled. improvement of Gate Turn-Off (GTO) thyristor ratings (4500V
This paper presents control and performance of UPFC to 6000V, 1000A to 6000A). These controllers are based on
intended for installation on that transmission line to control voltage source converters and include devices such as Static Var
power flow. A control system which enables the UPFC to Compensators (SVCs), Static Synchronous Compensators
follow the changes in reference values like AC voltage, DC (STATCOMs), Thyristor Controlled Series Compensators
voltage and angle order of the series voltage source (TCSCs), the Static Synchronous Series Compensators (SSSCs),
converter is simulated. In this control system, space vector and the Unified Power Flow Controllers (UPFCs). The UPFC is
pulse width modulation technique is used to generate firing the most versatile and complex of the FACTS devices,
pulses for both the inverter. Simulations were carried out combining the features of the STATCOM and the SSSC. The
using MATLAB software to validate the performance of the UPFC can provide simultaneous control of all basic power
UPFC. system parameters, viz., transmission voltage, impedance and
phase angle. It is recognized as the most sophisticated power
flow controller currently, and probably the most expensive one.
In this paper, a UPFC control system that includes both the
shunt converter and the series converter has been simulated. The
Keywords----- Flexible AC Transmission Systems (FACTS), performance of the UPFC in real and reactive power flow
feedback control, PWM Inverters, Voltage Regulation, through the transmission line has been evaluated This paper is
Unified Power Flow Controller (UPFC). organized as follows. After this introduction, the principal
operation and also the mathematical equations of a UPFC
I. INTRODUCTION connected to a network are presented.

Today’s power systems are highly complex and require careful

design of new devices taking into consideration the already
2. OPERATING PRINCIPLE OF UPFC
existing equipment, especially for transmission systems in new
deregulated electricity markets. This is not an easy task
considering that power engineers are severely limited by The basic components of the UPFC are two voltage
economic and environmental issues. Thus, this requires a review source inverters (VSIs) sharing a common dc storage capacitor,
of traditional methods and the creation of new concepts that and connected to the power system through coupling
emphasize a more efficient use of already existing power system transformers. One VSI is connected to in shunt to the
resources with out reduction in system stability and security. In
transmission system via a shunt transformer, while the other one the var reference into a corresponding shunt current request and
is connected in series through a series transformer. A basic adjusts gating of the inverter to establish the desired current. For
UPFC functional scheme is shown in fig.1. this mode of control a feedback signal representing the dc bus
voltage, Vdc, is also required.

Automatic Voltage Control Mode:

The shunt inverter reactive current is

automatically regulated to maintain the transmission line voltage
at the point of connection to a reference value. For this mode of
control, voltage feedback signals are obtained from the sending
end bus feeding the shunt coupling transformer. The series
FIG.1 basic Functional scheme of UPFC inverter controls the magnitude and angle of the voltage injected
in series with the line to influence the power flow on the line.
The actual value of the injected voltage can be obtained in
several ways.
The series inverter is controlled to inject a symmetrical three
phase voltage system (Vse), of controllable magnitude and
phase angle in series with the line to control active and reactive
power flows on the transmission line. So, this inverter will
exchange active and reactive power with the line. The reactive Direct Voltage Injection Mode:
power is electronically provided by the series inverter, and the
active power is transmitted to the dc terminals. The shunt
inverter is operated in such a way as to demand this dc terminal The reference inputs are directly the
power (positive or negative) from the line keeping the voltage magnitude and phase angle of the series voltage.
across the storage capacitor Vdc constant. So, the net real power
absorbed from the line by the UPFC is equal only to the losses
of the inverters and their transformers. The remaining capacity
of the shunt inverter can be used to exchange reactive power
with the line so to provide a voltage regulation at the connection Phase Angle Shifter Emulation mode:
point. The two VSI’s can work independently of each other by
separating the dc side. So in that case, the shunt inverter is
operating as a STATCOM that generates or absorbs reactive The reference input is phase displacement
power to regulate the voltage magnitude at the connection point. between the sending end voltage and the receiving end voltage.
Instead, the series inverter is operating as SSSC that generates
or absorbs reactive power to regulate the current flow, and
hence the power flow on the transmission line. The UPFC has
many possible operating modes. In particular, the shunt inverter
is operating in such a way to inject a controllable current, i sh into Line Impedance Emulation mode:
the transmission line. The shunt inverter can be controlled in
two different modes: The reference input is an impedance value
to insert in series with the line impedance.

VAR Control Mode:

Automatic Power Flow Control Mode:
The reference input is an inductive or
capacitive VAR request. The shunt inverter control translates
 The reference inputs are values of P and Q
to maintain on the transmission line despite system changes.

3. MATHEMATICAL MODEL OF UPFC

The basic structure and operation of the UPFC can be

represented through the model shown in fig.2. The transmission
line parameters are as shown in Table I.

(2.3)

The maximum limit of δ is chosen according to

Fig. 2 Mathematical model of UPFC

the stability margin [9].

we have considered the UPFC is placed at the centre of a 500km
transmission line. The equations for sending end active and
reactive power can be obtained from the real and imaginary
powers of power equation as follows:
Ps=Re(Vs∟δ×Is) (2.1) 4. SIMULATION SETUP IN MATLAB

Qs=Im((Vs∟δ×Is) (2.2) The simulation model including a power system with a

transmission line. The UPFC installed near the sending end
the currents Is, Ii and Ir are calculated by the following effectively controls the power flow from sending end to the
expressions: receiving end.

Fig. 3 Power system study model

 Fig. 4 STATCOM DC voltage controller

Here, Vs and Vr are assumed to be sending and receiving-end

voltages. This model assumes that sending end corresponds to a
power plant while the receiving end to an electric power
network, i.e., SMIB system. The receiving end voltage may not
cause any phase angle change, because Vr is an infinite bus
voltage. The phase angle of Vs is adjusted according to the
power demand for the power plant. A phase difference of 100
between sending-end and receiving end voltages is simulated.
The circuit parameters are shown in Table I. The main circuit of Fig. 5 STATCOM AC Voltage controller
the series device (SSSC) consists of a three phase PWM
inverter, the ac terminals of which are connected in series to a
transmission line through three single phase transformers. The Two sets of signals, reference and triangular
shunt device (STATCOM) consists of a three phase PWM ones are needed, One set for turning-on and the other for
inverter, the ac terminals of which are connected in parallel with turning-off the MOSFETs. The generated shift and mi
the transmission line via a three phase star-delta transformer. signals are used to develop firing pulses for the six
MOSFETs in the inverter, as shown in the fig. 6, in
MATLAB environment. Space vector pulse width
modulation switching technique is used for pulse
generation.
A. Shunt Inverter Control Circuit:

In this simulation, the shunt inverter operates in automatic

voltage control mode. Fig. 4 shows the DC voltage control
circuit for the shunt inverter. DC link voltage is measured
(VDCm) and compared with the reference value (VDCref),
whose error is fed to PI controller to generate the shift.
Similarly, AC voltage from the sending end bus feeding
the shunt coupling transformer is measured in p.u, (Vpum)
and compared with the AC voltage set point (here 1.0 p.u),
whose error is fed to PI controller to generate modulation
index, mi. Fig. 5 shows the AC voltage control circuit for Fig. 6 Circuit for firing pulse generation
the shunt inverter

B. Series Inverter Control Circuit:

.
In this case, the series inverter operates in the direct
voltage injection mode. The series inverter simply injects
voltage as per the theta order specified. Fig. 7 shows the
series inverter control circuit, which is an open loop phase
angle controller, generates modulation index, mi and shift.
The mi and shift signals are used to develop firing pulses
as shown in fig. 6.
 Fig. 9 Series injected voltage.

By varying the theta order input to the controller the phase and
magnitude of the series injected voltage can be varied.
Fig. 7 Series inverter open loop phase angle controller
When the transmission line is without UPFC, the real and
reactive power flow cannot be controlled. Fig. 10 shows the the
active power flow through line which is controlled by UPFC.
5. SIMULATION RESULTS
Transmission capability of the existing transmission line is
highly improved with the presence of UPFC. The difference
A transmission line of a simple power system with between the sending-end real power and receiving end real
parameters as given in Table I is considered. UPFC is power is high in the transmission line with UPFC. This is due to
placed in series with the transmission line at the center of the increase in transmission losses, which include losses in the
a 500km line. Active power, reactive power and current both converters and coupling transformers.
variations in the transmission line with UPFC. The power
system studied is SMIB system. When UPFC is placed
across the transmission line, the voltage regulation is
improved as per Fig. 8

Fig. 10 sending end and receiving end active power with UPFC

The reactive power flow through the transmission line with

UPFC is shown in fig. 11. The raise in the transmission
capability is noticed from the simulation results. The power
Fig. 8 Sending end and receiving end voltages with UPFC transfer capability of long transmission lines is usually limited
by their thermal capability. Utilizing the existing transmission
line at its maximum thermal capability is possible with UPFC.

In this simulation, the theta order input to the series

inverter control circuit is 5°. The series inverter injects
voltage into the transmission line at point of connection, as
shown in fig. 9.

Fig. 11 Sending end and receiving end reactive power with UPFC
The performance of the UPFC can be justified by its Transmission rating 100 MVA
controller’s performance. AC voltage controller tracking it Capacitance of DC link 2000μF
reference values is shown in Fig. 8. Similarly, DC voltage
controller tracks its reference value, is shown in fig. 12.
Capacitor
DC link voltage 45 kV
Length of the transmission 500 km
line
Resistance of the line 32 μΩ/m
Inductive reactance of the 388.3 μΩ/m
line
Capacitive reactance of the 241.1
line
Fig. 12 DC link voltage in UPFC

The series inverter injects voltage of variable magnitude

and phase into the transmission line at the point of its
connection, there by controlling real and reactive power
flow through the line. The active power through the line is
supplied by SSSC. This real power obtained from the DC
source connected to its DC terminals. The shunt inverter
provides the required power to the series inverter through
the DC link.
7. REFERENCES

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