Resistance Control MPPT for Smart Converter PV System
DC nano-grid system shows promising prospect and enjoys some advantages over AC micro-grid system. It enables easier integration of multiple renewable energy sources with multiple loads. Photovoltaic (PV) is essentially a typical renewable source that serves as main power source in DC nano-grid system. Traditional PV system includes centralized PV system, string PV system and micro-converter PV system. More recently, smart converter PV system has been introduced and shown great improvement in aspects of power generation achieved by distributed Maximum Power Point Tracking (MPPT). It is also advantageous over micro-converter PV system due to lower cost and flexibility.
Detailed case study demonstrates that power generation efficiency can be easily compromised because of mismatch between different panels in centralized and string PV systems. In smart converter PV system, this problem can be solved due to distributed MPPT for each individual panel. The smart converter system has a very wide voltage range within which all panels can generate maximum power. The location and the width of this range are subject to change under different mismatch conditions. A second stage converter is needed to locate the array MPPT range. However, there is instability problem when doing second stage MPPT with traditional methods.
Modified methods based on conductance control and resistance control are analyzed and compared. Both methods can solve the MPPT instability problem. However, in terms of steady state performance, resistance control MPPT is more promising in terms of higher utilization ratio and faster tracking speed. It is because both methods are of inherited variable operating point step size with constant conductance or resistance perturbation step size. However, the operating point change decreases with resistance perturbation but increases with conductance perturbation otherwise. Therefore, resistance control MPPT is chosen as a good candidate. Both simulation and experimental results verifies the concept.