Submodule Differential Power Processing for PV Systems

Abstract

Photovoltaic (PV) energy is one of the most prominent renewable energy sources today. Traditionally, PV modules are connected in series to form a PV string, interfacing with PV inverter for grid connection. Since all PV modules are operating at the same current, the energy yield of the system is limited by the underperforming modules. PV optimizers, a concept which enhances maximum power point tracking (MPPT) of PV systems using power electronics, has been studied over the last few years. An emerging technique, submodule differential power processing (DPP) is proposed to improve the efficiency and enhance MPPT to a finer granularity. By diverting the differential current between submodules, DPP optimizers only process a fraction of the power the system produces, improving the overall efficiency.

This thesis aims to research PV-to-PV DPP systems and its PV-to-Serial-Port variant, including modelling, control techniques, and MPPT strategies. The inverting buck-boost converter is the preferred topology for optimizers, as it is one of the simplest topologies which supports bidirectional power flow and both step-up and step-down voltage conversions.

A small-signal model is derived as the basis of controller tuning involved in this thesis. A novel pairing between control inputs and outputs based on relative gain array analysis is proposed, mitigating coupling effects between optimizers, and simplifying the controller design. Furthermore, a double-loop outside-in exact MPPT strategy is presented, improving the tracking speed. Simultaneous voltage regulation of multiple submodules and exact MPPT strategy have been verified by experiments.

Voltage equalization MPPT techniques are investigated as they eliminate the communication requirements between modules. Voltage equalization is investigated for both MPPT and flexible power point tracking (FPPT) applications, leading to the discovery of an unstable operation mode caused by linearization with the differential resistance method. Experiments validate that, with proposed tuning techniques closed-loop equalization is stable in regions where the power is sufficiently low to perform FPPT. A comparison of MPPT performance between open-loop and closed-loop equalizations shows that closed-loop equalization has better energy yield under severe mismatch, as it vastly reduces steady-state errors.

Finally, a PV-to-Serial-Port variant, aiming to improve the practicality of DPP techniques is presented. With only one inter-module power connection and no communication requirements, this new architecture is more suitable for modular integration than its PV-to-PV counterpart. A novel topology of PV-to-Serial Port architecture which supports voltage equalization is proposed. A clear contribution that thoroughly analyses and compares the inductor sizing of practical PV-to-Serial-Port topologies considering arbitrary PV current mismatch is presented. It is validated by simulation that, with the combination of PV-to-Serial-Port and voltage equalization, MPPT can be performed at module-level autonomously and can handle sudden irradiance changes.

A high-level comparison between systems in three core chapters is given in the conclusion chapter, outlining the techniques utilized in each chapter and their limitations.

Experiments in this thesis are undertaken on two identical inverting buck-boost converter prototypes, rated at 100W, switching at 200kHz, and interfacing with a TI controlCARD. Control techniques and MPPT strategies are implemented digitally and can be applied to existing inverting buck-boost optimizers if power ratings and sensory requirements are met.

The research outcome of this thesis improves the practicality of DPP techniques by simplifying control methods and eliminating communication requirements. Furthermore, it establishes the FPPT capability of DPP systems for providing grid support. Lastly, it provides a modular-integrable topology, enhancing the scalability of DPP systems.