Capacity Determination of the DC-side Battery for Hybrid Batteries in PV Generation System

Lots of studies are published to mitigate the photovoltaic (PV) power output fluctuation by using batteries. Especially, under the islanding operation of the microgrid, the PV output fluctuation causes the instability of the system. Although batteries are installed to mitigate the instability, a control delay of the batteries affects the frequency and voltage of the micro-grid extensively. Generally, a control delay inevitably exists since the PV output is measured at every designated discrete time period and the control signal is sent to the batteries after calculating the difference between load and PV power output. Therefore, so as to reduce this time delay influences, the authors have proposed so called "PV system with hybrid batteries" which has two types of the batteries (AC side battery and DC side battery). In this system, since the DC side battery absorbs only the power fluctuation caused by the control delay of the AC side battery, the size of the DC side battery seems to be very small. The purpose of this paper is to develop a method to estimate the size of the DC side battery. Through the qualitative discussions, the maximum necessary capacity can be estimated from the amount of charge or discharge of the DC side battery when the PV output fluctuation cycle is the same as the one of the PV output measurement. The size of the DC side battery calculated by the developed method is verified through the real system applications.