04-09-2014, 11:02 AM
Unified Power Flow Controller (UPFC): Modeling and Analysis
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INTRODUCTION
As the load increases, power utilities are looking for ways
to maximize the utilization of their existing transmission
system.
In recent years. advances in the high power solid-state
switches. e.g. Gate Turn Off (GTO) thyristors, have led to
the development of transmission controllers that provide
controllability and flexibility for power transmission. A new
technology program is known as Flexible AC Transmission
System (FACTS) is currently sponsored by (EPRI) [l].
This technological program has resulted in successful
demonstration of a couple of FACTS Controllers: 208 Mvar,
500 kV Thyristor Controlled Series Capacitor (TCSC) at
BPA's power system and + 100 Mvar. 161 kV Static
Synchronous Compensator (STATCOM) at TVA's power
system [31. The Unified Power Flow Controller (UPFC) is
the latest FACTS controller [2]. UPFC provides a dynamic
control of transmission parameters, voltage, line impedancc
and phase angle..
American Electric Power (AEP). in a collaborative R&D
project with EPRI and Westinghouse is implementing a
+160 MVA Unified Power Flow Controller (UPFC)
EQUATIONS AND NUMERICAL, METHOD
This section describes the equations needed to model the
two busses network. The new numerical method is based on
d-q-transform. This method is well suited for small
networks.
Equation q f one branch, using the d-q-transfiwni
The circuit shown in Fig. 2 is divided into different
branches as described on Fig. 1. The d-q-transform, [5]. is
applied to the branch composed of a voltage source in series
with an R-L element representing a transformer and
transmission line. (d-q) coordinates represent the direct and
quadrature projection of vectors.
Network equations
Fig. 2 shows the UPFC in the considered two-bus
network. Detailed calculation is given in the Appendix.
- RI,XI=LIO& Xs=Lsm are calculated considering short
circuit power reference, Appendix A.
- Vector voltage Vnetl and VneW are calculated to ensure a
given P2 and Q2 flows in initial conditions without UPFC.
Appendix B.
In Fig. 2 the UPFC shunt part is represented by a parallel
reactive current source. The UPFC series part is represented
by a series voltage, (R, X,) are for UPFC series transformer
leakage reactance and (RI, X,) for line parametcrs.
The current component in phase wilh the voltage injectcd
by the series part determines the real power exchanged. This
real power is exchanged with the shunt part. The Matlab
model ensures an active power balance between shunt and
series part
Convergence criteria
The convergence property is an important issue for any
numerical method and time computation. The convergence
criteria adopted in (2) is based on a local convergence for
both variables, voltage VZd and current in the series part i3.
The voltage VZd checking ensures proper convergence of the
reactive current injection. The current i3 checking ensures
proper convergence of the UPFC series voltage injection
REACTIVE COMPENSATION IMPACT ON A TWO BUS SYSTAM
Based on an actual installation,[6],[8],the impact of the
UPFC shunt part on the Power System is evaluated.
RI.I$.XI.X, are calculated using short circuit strength at
the busses. V,,etl ,Vnet2 are calculated for initial power flows
BUS SYSTEM
CONCLUSION
This paper presents a Matlab modeling tool to determine
the impact of UPFC on a two bus system. This tool is useful
to rate. evaluate or understand the characteristic
peribrmance of UPFC on power systems. A new numerical
method tested with Matlab model has been successfully
validated with EMTP. Through real examples. the Matlab
model has been used to give an overview of the versatile
UPFC capabilities. The Matlab code given in the paper
allows to perform fast parametric studies for the application
of the UPFC
A numerical inethod has been developed and the step by
step associated codes are given. Through examples, the
UPFC impact on the power system is evaluated. Validation
based on an actual system study performed on TNA has
confirmed the validity of the approximations and the
developed technique