Abstract:
Majority of solar installations are coming up in
remote locations where the stiffness of the grid is very weak,
enforces to connect solar inverters to weak grid. In weak grid
scenario, solar inverter would inject harmonic current into the
grid that affect the power quality of the system. The solar
inverters are designed to operate with stiff grid in which the
closed (feedback and feed forward) loop control is planned
by considering absolute decoupling between the direct and
quadrature grid injected current. With the conventional design
specifications in weak grid scenario, the absolute decoupling
between the direct and quadrature current cannot be achieved
due to grid inductance. The improper decoupling would create
the oscillations in the grid injected current that result in increased
harmonic distortion. To compensate the current oscillations, the
current controller feedback loop gain need to be designed by
accessing the degree of coupling. Since the grid injected current
sensed at the point of common coupling and transformed to DC
quantities (direct current id and quadrature current iq) using
synchronous angle, the same current oscillations appear in daxis and q-axis current control loop. In this work, the d-axis
and q-axis loop oscillations with respect to grid inductance are
analyzed analytically. Based on the analysis the d-axis and q-axis
PI controllers are designed independently unlike conventional PI
controller to ensure not only faster dynamic response of the
system but also improved power quality.
