This paper studies stability analysis of Constant Voltage Constant Frequency(CVCF) control for the Battery Energy Storage System(BESS) system, and seamless Maximum Power Point Tracking(MPPT) mode transfer method to prevent the discontinuity of photovoltaic(PV) power generation system. The stand-alone microgrid''s BESS stores energy in a large battery connected in parallel to the DC-Link and provides high-quality power to the load through control algorithms that produce the stable voltage and fixed frequencies by using CVCF control. In this paper, compares and analyze the characteristics of the proportional?integral(PI) voltage controller and the proportional?resonance(PR) voltage controller for CVCF control according to the presence or absence of feedforward. The stability of the controller is mathematically equivalent to the circuit system, and the stability analysis is performed by applying the parameters of the real system to the equivalent model. Simulation and experiments using a 5kW class BESS show the same results as the stability analysis, demonstrating the superiority of the PR voltage controller and the need for feedforward.
In addition, MPPT algorithm is designed to be robust to variations in output characteristics due to temperature changes of solar panels in PV system among renewable energy sources capable of parallel operation in BESS system, and can be generated between two MPPT algorithms to prevent the discontinuity, seamless MPPT mode transfer method is proposed. If PV voltage is below the 650V, the converter MPPT operation mode follows the maximum power point by increase or decrease the duty. If PV voltage exceeds the 650V, the inverter is transfer to the MPPT operation mode, the DC-DC boost converter stops. And the power is supplied to the load by following the increase or decrease of the DC-Link voltage reference of the inverter. In this transfer process, a seamless transfer method can be applied without the discontinuity that may occur depending on the on / off status of the DC-DC boost converter and the change of the DC-Link voltage controller. The validity of the proposed method is verified through a 10kW photovoltaic power generation system.