V-I CHARACTERISTICS OF STATCOM
Ø A typical
characteristic of a STATCOM is depicted in Fig.
STATCOM can supply both the capacitive and the inductive compensation and is
able to independently control its output current over the rated maximum
capacitive or inductive range irrespective of the amount of ac-system voltage.
STATCOM can provide full capacitive-reactive power at any system voltage—even
as low as 0.15 pu.
characteristic of a STATCOM reveals another strength of this technology: that
it is capable of yielding the full output of capacitive generation almost
independently of the system voltage (constant-current output at lower
voltages). This capability is particularly useful for situations in which the
STATCOM is needed to support the system voltage during and after faults where
voltage collapse would otherwise be a limiting factor.
illustrates that the STATCOM has an increased transient rating in both the
capacitive- and the nductive-operating regions.
maximum attainable transient overcurrent in the capacitive region is determined
by the maximum current turn-off capability of the converter switches.
Ø In the
inductive region, the converter switches are naturally commutated; therefore,
the transient-current rating of the STATCOM is limited by the maximum allowable
junction temperature of the converter switches.
practice, the semiconductor switches of the converter are not lossless, so the
energy stored in the dc capacitor is eventually used to meet the internal
losses of the converter, and the dc capacitor voltage diminishes.
when the STATCOM is used for reactive-power generation, the converter itself
can keep the capacitor charged to the required voltage level. This task is
accomplished by making the output voltages of the converter lag behind the
ac-system voltages by a small angle (usually in the 0.18–0.28 range).
Ø In this
way, the converter absorbs a small amount of real power from the ac system to
meet its internal losses and keep the capacitor voltage at the desired level.
Ø The same
mechanism can be used to increase or decrease the capacitor voltage and thus,
the amplitude of the converter-output voltage to control the var generation or
reactive- and real-power exchange between the STATCOM and the ac system can be
controlled independently of each other.
combination of realpower generation or absorption with var generation or
absorption is achievable if the STATCOM is equipped with an energy-storage
device of suitable capacity, as depicted in Fig.With this capability, extremely
effective control strategies for the modulation of reactive- and real-output
power can be devised to improve the transient- and dynamic-system-stability