15-01-2013, 03:02 PM
Understanding Power Quality Based FACTS Using Interactive Educational GUI Matlab Package
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Introduction
The electricity is invisible and the complexity of mathematical models deviate the graduate
students attention from well understanding the underlying main concepts. Interactive
educational power system software has become a fundamental teaching tool because it
helps in particular the undergraduate students to assimilate theoretical issues and complex
models analysis through flexible graphic visualization of data inputs and the results (Abur
et al., 2000), (Milano, F., 2005). From the educational point of view software developed for
educational purposes should be flexible and interactive, easy to use and reliable. In
particular, software for power system education should contain a user interface not only to
allow graduate student to analyse and understand the physical phenomena, but also to
improve the existing models and algorithms (Mahdad, B., 2010 ).
Flexible AC Transmission Systems (FACTS) philosophy was first introduced by Hingorani
(Hingorani N. G., and Gyugyi L, 1999) from the Electric power research institute (EPRI) in
the USA in 1988, although the power electronic controlled devices had been used in the
transmission network for many years before that. The objective of FACTS devices is to bring
a system under control and to transmit power as ordered by the control centers, it also
allows increasing the usable transmission capacity to its thermal limits. With FACTS devices
we can control the phase angle, the voltage magnitude at chosen buses and/or line
impedances.
The avantages of the graphical user interface tool proposed lie in the quick and the dynamic
interpretation of the results and the interactive visual communication between users and
computer solution processes. The physical and technical phenomena and data of the power
flow, and the impact of different FACTS devices installed in a practical network can be
easily understood if the results are displayed in the graphic windows rather than numerical
tabular forms (Mahdad, 2010).
Basic principles of power flow control
To facilitate the understanding of the basic principle of power flow control and to introduce
the basic ideas behind the different type of FACTS controllers, the simple model shown in
Fig. 2 is used (Mahdad, B., 2010). The sending and receiving end voltages are assumed to be
fixed. The sending and receiving ends are connected by an equivalent reactance, assuming
that the resistance of high voltage transmission lines is very small. The receiving end is
modeled as an infinite bus with a fixed angle of 0°.
Role of FACTS devices in power system operation and control
To further understand the strategy of FACTS devices in power system operation and
control, consider a very simplified case in which generators at two different regions are
sending power to a load centre through a network consisting of three lines.
Fig. 7 shows the topology of simple electrical network, suppose the lines 1-2, 1-3 and 2-3
have continuous ratings of 1000MW, 2000MW, and 1250MW, respectively, and have
emergency ratings of twice those numbers for a sufficient length of time to allow
rescheduling of power in case of loss of one of these lines (Hingorani, N. G., and Gyugyi L,
1999).
For the impedances shown, the maximum power flow for the three lines are 600, 1600, and
1400, respectively, as shown in Fig. 7, such a situation would overload line 2-3 (loaded 1600
MW for its continuous rating of 1250 MW), and there for generation would have to be
decreased at unit 2, and increased at unit 1, in order to meet the load without overloading line
2-3. The following simplified studies cases demonstrate the main objective of integration of
FACTS technology in a practical power system to enhance power system security.
Capacitive Series Compensation at line 1-3
If the dynamic series FACTS Controller (type capacitive)installed at line 1-3 adjusted to
deliver a capacitive reactance, it decreases the line’s impedance from 10Ω to 4.9919Ω, so that
power flows through the lines 1-2, 1-3, and 2-3 will be 250 MW, and 1750 MW, respectively.
Fig. 8 illustrates the per cent loading of lines. It is clear that if the series capacitor is
adjustable, then other power flow levels may be realized in accordance with the ownership,
contract, thermal limitations, transmission losses, and wide range of load and generation
schedules. Fig. 8 shows clearly the effect of series capacitive compensation to control the
active power flow with another degree of compensation ( 6 C X = Ω).