  This paper discusses some aspects of control schemes for voltage source
converters under abnormal conditions. The control schemes are developed
specifically for the situations when one or more system parameters vary
significantly to the extent that the system becomes unstable with a
conventional controller. The paper will present some recent works on control of
grid-interactive converters for parameter variations in weak grid. The paper
will also discuss some methods under abnormal dc-bus variation. Finally, the
paper focuses on the control schemes suitable for ac-dc converters that
addresses input frequency, voltage and load variation. All the discussed
control schemes have shown robust performance under abnormal conditions.
