A modified control strategy for centralized PV - grid systems for assisting dynamic stability to overcome penetration issues

Interfacing inertia-less large PV sources to the power system raises complex PV penetration issues such as bus voltage fluctuation, active power variation, reactive power flow and sharing, system instability etc. This paper proposes a new control technique for large capacity centralized PV fed grid connected systems which enhances the system stability by damping out the low frequency power swing. The presented work addresses a major PV penetration issue - that of system instability, by proposing a modified control scheme of the power conditioning unit of a PV source. The proposed scheme improves the small signal stability and dynamic security of the system without introducing any additional devices (e.g. PSS). The developed analytical approach for the control law is based on the structure preserving energy function (SPEF) model. In order to evaluate the proposed controller and validate the control law, a conventional model of the two machine system with double line transmission is interfaced with a large PV source of rating comparable to that of conventional generator capacities. A notable feature of this work is that the derived control law uses only the locally measurable signals and is independent of the location of the PV source in the system. Further, it does not affect the MPPT control of the PV system in any manner.