Modelling and control of integrated PV-converter modules under partial shading conditions

It has been well-recognized that non-uniform solar irradiation of photovoltaic (PV) panels causes electrical mismatching of cells and may result in reduced output
power and cell thermal breakdown. Bypass diodes are commonly used, but challenges exist into obtaining the maximum power point tracking in these partially
shaded PV panels for each weather condition. This is due to that there are multiple peak power points present in their Power-Voltage characteristic curves which makes difficult to locate the global maximum power point. The work presented in this thesis studies in detail the converter topologies and control methods which can be used in the PV power generation systems to overcome effectively the
shortcomings caused by partial shading.
The proposed topology is an integrated bi-directional Cuk converter and PV-panel module. The particular example investigated includes two PV panels connected across two terminals of the Cuk converter. The features of this system in power harness are studied under partial shading conditions, its superior performance in power generation is demonstrated through simulation and practical tests. The
generated power is 30% higher than that from a two PV panel system using only bypass diodes.
To develop the control schemes for the above system a detailed study was performed leading to the derivation of the transfer function model describing the dynamic responses of voltages across the two PV panels corresponding to the variations of converter switch duty ratio. Experimental verification of this confirms that the model is sufficiently accurate for the application of controller design and tests.
A novel maximum power point tracking scheme is developed. This consists of a switching selection scheme and a model based on an optimal control algorithm. The former determines which switch-diode pair in the bidirectional Cuk converter to be active according to measured light levels on each PV panel and the ability to predict the optimal voltage values across the individual PV panels under any
practical irradiance and temperature levels. The performance of the controller is tested in simulation as well as in practice under various modes of partial shading,
all giving desired results in achieving the maximum power generation.
The final contribution lies in the design and construction of an experimental prototype consisting of an inner bidirectional Cuk converter across two PV panels and a terminal boost converter, controlled by DSP-based microcontroller. This setup enables further development and verification of the control schemes for this integrated converter and PV-panel system.
Keywords: Photovoltaic Systems, Partial Shading, Cuk Converter, DC-DC Power
Converters, Solar Power Generation, Maximum Power Point Tracking, Bypass Diode.