V-dp/di droop control technique with d²p/di² to improve dynamic performance for multiple PV modules in islanded DC microgrid

Abstract

This thesis proposes the V-dp/di droop control algorithm of an islanded dc microgrid (MG) system with multiple photovoltaic (PV) arrays. A comprehensive literature review on three main coordinated control strategies is carried out, namely centralized, distributed and decentralized control. Decentralized control has some advantages over centralized and distributed control, including plug-and-play capability, easy to implement, and absence of communication agent. The conventional control method implemented in the simulation model is V-dp/dv droop control, which can be used to investigate the dynamic performance of the PV-based dc MG. The simulation results show that the accuracy issue of dp/dv leads to doubling settling time and ±5% power oscillations. Therefore, it is important for a PV generator to perform voltage regulation in a PV standalone mode, in order to continue stabilising the bus voltage of a dc MG. The stability analysis of the proposed V-dp/di control and conventional V-dp/dv method is undertaken based on a PV-based dc MG. The mathematical results show 50% improvements in voltage fluctuations and power ripple during steady state. Compared to conventional V-dp/dv control, the inner loop of the proposed dp/di controller can independently be stabilised without the implementation of outer voltage loop. A comparison between conventional and proposed control schemes is carried out to validate the feasibility and robustness of proposed V-dp/di method, and to present the working principle of the control variable dv/di. This is referred to as incremental-resistance (INR) maximum power point tracking (MPPT) algorithm. The characteristic of a searching MPPT algorithm leads to huge oscillation of the common dc bus voltage, which is the drawback of traditional MPPT schemes. Another contribution of this thesis is the introduction to second differential term of current approach d²p/di². The 2nd derivative is used to minimise the fluctuations of dp/di during steady state. The application of second derivative of power to current yields an improved steady-state performance in a PV-based dc MG. The simulation results present a 30% further improvement in PV power ripple with the help of second differential term.