10-05-2012, 01:16 PM
UPFC (Unified Power Flow Controller )
pratik a.pdf (Size: 332.75 KB / Downloads: 81)
Representative of the last generation of FACTS devices is the Unified Power
Flow Controller (UPFC). The UPFC is a device which can control simultaneously
all three parameters of line power flow (line impedance, voltage and phase angle).
The UPFC combines together the features of the Static Synchronous
Compensator (STATCOM) and the Static Synchronous Series Compensator
(SSSC). In practice, these two devices are two Voltage Source Inverters (VSI’s)
connected respectively in shunt with the transmission line through a shunt
transformer and in series with the transmission line through a series transformer,
connected to each other by a common dc link including a storage capacitor.
The shunt inverter is used for voltage regulation at the point of connection
injecting an opportune reactive power flow into the line and to balance the real
power flow exchanged between the series inverter and the transmission line. The
series inverter can be used to control the real and reactive line power flow inserting
an opportune voltage with controllable magnitude and phase in series with the
transmission line.
Thereby, the UPFC can fulfil functions of reactive shunt compensation, active
and reactive series compensation and phase shifting. Besides, the UPFC allows a
secondary but important function such as stability control to suppress power
system oscillations improving the transient stability of power system.
Flicker Compensation in Arc Furnace Power systems Using the UPFC
Under unbalanced conditions of electrode arcing, there could be
significant amounts of third harmonic and its multiples. Also, fifth and
seventh harmonics that occur under balanced conditions could increase under
unbalanced arcing conditions. Measurements of arc furnace voltage have
indicated a varying harmonic output. The recorded fifth harmonic voltage has
varied from 8%, 6%, and 2.5% of the fundamental voltage during beginning
of meltdown, end of meltdown, and refining, respectively. The process is
optimized to operate around the rated regime, where the active power is
maximum . But one heating is in fact composed of at least three steps:
(1) The bore down, lasts one or two minutes. The electrodes have to dive
deeply into the scrap to heat it, thus inducing a high instability of the arc
(succession of arc extinction and short-circuit between electrodes and scrap).
(2) When the scrap is hot enough, the electrodes arc set higher to begin the
melting phase (about 10 minutes). Due to collapses in the scrap, the arc is still
quite unstable.