A frequency-stabilizing scheme for integrating photovoltaics into a small island grid

The use of photovoltaics (PV) has increased in some small islands of Okinawa Prefecture, Japan in response to various renewable energy demonstration projects. Since this increased usage could deteriorate the power quality, including frequency, of an isolated grid in a small island, a frequency-stabilizing system using lithium-ion capacitors (LiC) has been integrated into the grid. Because a frequency-stabilizing scheme is the heart of the frequency-stabilizing system, a scheme which employs deviation of PV output (ΔP) and system frequency (ΔF) as inputs was developed. The parameters of the ΔP input part were derived for mitigating stepwise changes of system frequency caused by a sudden fluctuation in PV output. The parameters of the ΔF part were set so as to alleviate the high-frequency components of the system frequency deviation caused by load fluctuation. The developed scheme was applied to the stabilizing system using LiC in the island grid and was found to greatly alleviate frequency fluctuation.