The closed loop controller gain characterization for enhanced current quality in solar inverters coupled with weak grid

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.