24-07-2012, 11:15 AM
FACTS CONTROLLERS IN POWER TRANSMISSION AND DISTRIBUTION
Power Transmission.pdf (Size: 5.03 MB / Downloads: 225)
Introduction
General
Modern power systems are designed to operate e±ciently to supply power
on demand to various load centres with high reliability. The generating
stations are often located at distant locations for economic, environmental
and safety reasons. For example, it may be cheaper to locate a thermal power
station at pithead instead of transporting coal to load centres. Hydropower
is generally available in remote areas. A nuclear plant may be located at a
place away from urban areas. Thus, a grid of transmission lines operating at
high or extra high voltages is required to transmit power from the generating
stations to the load centres.
Basics of Power Transmission
Networks
A large majority of power transmission lines are AC lines operating at dif-
ferent voltages (10 kV to 800 kV). The distribution networks generally op-
erate below 100 kV while the bulk power is transmitted at higher voltages.
The lines operating at di®erent voltages are connected through transformers
which operate at high e±ciency. Traditionally, AC lines have no provision
for the control of power °ow. The mechanically operated circuit breakers
(CB) are meant for protection against faults (caused by °ashovers due to
overvoltages on the lines or reduced clearances to ground). A CB is rated for
a limited number of open and close operations at a time and cannot be used.
Control of Power Flow in AC
Transmission Line
We may like to control the power °ow in a AC transmission line to (a)
enhance power transfer capacity and or (b) to change power °ow under
dynamic conditions (subjected to disturbances such as sudden increase in
load, line trip or generator outage) to ensure system stability and security.
The stability can be a®ected by growing low frequency, power oscillations
(due to generator rotor swings), loss of synchronism and voltage collapse
caused by major disturbances.
Flexible AC Transmission System
Controllers
General Description
The large interconnected transmission networks (made up of predominantly
overhead transmission lines) are susceptible to faults caused by lightning
discharges and decrease in insulation clearances by undergrowth. The power
°ow in a transmission line is determined by Kirchho®'s laws for a speci¯ed
power injections (both active and reactive) at various nodes. While the
loads in a power system vary by the time of the day in general, they are also
subject to variations caused by the weather (ambient temperature) and other
unpredictable factors. The generation pattern in a deregulated environment
also tends to be variable (and hence less predictable). Thus, the power °ow
in a transmission line can vary even under normal, steady state conditions.
The occurrence of a contingency (due to the tripping of a line, generator)
can result in a sudden increase/decrease in the power °ow. This can result
in overloading of some lines and consequent threat to system security.
Application of FACTS Controllers in
Distribution Systems
Although the concept of FACTS was developed originally for transmission
network; this has been extended since last 10 years for improvement of Power
Quality (PQ) in distribution systems operating at low or medium voltages.
In the early days, the power quality referred primarily to the continu-
ity of power supply at acceptable voltage and frequency. However, the pro-
li¯c increase in the use of computers, microprocessors and power electronic
systems has resulted in power quality issues involving transient disturbances
in voltage magnitude, waveform and frequency. The nonlinear loads not only
cause PQ problems but are also very sensitive to the voltage deviations.
In the modern context, PQ problem is de¯ned as \Any problem
manifested in voltage, current or frequency deviations that result in failure
or misoperation of customer equipment" [5].