An expanded over current rating of the voltage source converter
can be one main factor for obtaining LVRT capability with
a STATCOM of reasonable stationary ratings. This over current
rating is determined by the thermal design of the converter, and
especially by the thermal limits of the semiconductor devices.
Possibilities for expanding the transient over current rating lies
on achieving a larger temperature gap (from temperature at rated
operation) that will allow an increased over current rating before
the thermal limit is reached. Better heat dissipation in all
the components and well coordinated utilization of thermal time
constants could be one way of achieving this.Aconverter design
that allows for a transient rating of more than 200%–300% of
the stationary current limits could lead to a STATCOM solution
capable of performing stationary voltage regulation and voltage
quality improvement within a reasonably low rating, and at the
same time give a larger contribution to LVRT capability than a
SVC with much higher rating. Therefore, design of STATCOM
converter systems intended for LVRT purposes needs to be more
carefully approached taking into consideration the design factors
that will affect the LVRT capability and that will at the same
time enable an economical solution.