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

Development of novel non-isolated unidirectional DCDC multistage power converter configurations for renewable energy applications- hardware implementation and investigation studies

Abstract: In the last decades, there is a rapid development towards new energy sources due to the increasing demand of energy and cost of the fossil fuels. Renewable energy sources getting more popular day by day due to government support and carbon dioxide (CO2) emission reduction policy to reduce greenhouse gas emissions. Photovoltaic energy generation is the excellent example of energy generation through various serious parallel arrangement of a small voltage generating cells or modules. There are directly use of synchronous generators to transfer power to grid from hydro energy plant, geothermal energy plant, bio-fuel energy plants. However, the photovoltaic energy generation systems requires the power electronic converters system to satisfy the demand of realtime application or electric grid. Therefore, for real-time applications or before feeding energy to the grid via inverter, photovoltaic systems linked with DC-DC converters, which have high-voltage conversion ratio capability. Thus, DC-DC power converter is the paramount constituent in the photovoltaic power conversion stage. This research work carried out in focusing on hardware implementation and investigation studies of novel non-isolated unidirectional DC-DC multistage power converter configurations for renewable energy application. The comprehensive review of various unidirectional non-isolated DC-DC multistage power converters are presented and it is found that not all of them have the capability to convert low voltage into high voltage, thus not suitable for photovoltaic energy applications. It is investigated that there is a scope to design new DC-DC multistage power converter topologies configurations with high voltage conversion ratio by employing a new arrangement of reactive elements and semiconductor devices. A new breed of DC-DC multistage power converters called “X-Y converter family” proposed for photovoltaic application by utilizing the switchedinductor, the switched capacitor, the voltage lift switch capacitor and modified voltage lift switched capacitor, voltage doubler and multiplier boosting techniques. The derivation of voltage conversion ratio, advantage of each converter of X-Y family and hierarchy of X-Y family is discussed. The research work also proposed a new DC-DC multistage power converter without a magnetic component for photovoltaic application by utilizing the concept of switched capacitors. An original Transformer and Switched Capacitor (T-SC) based multistage power converter proposed for high-voltage/lowcurrent photovoltaic applications by combining the feature of the boost converter, transformer and switched capacitor. New Nx IMBC (Nx Interleaved Multilevel Boost Converter) or Cockcroft Walton (CW) Voltage Multiplier based Multistage/Multilevel Power Converter (CW-VM-MPC) converter topologies are presented to achieve maximum voltage conversion ratio by utilizing the feature of Cockcroft Walton (CW) voltage multiplier. Moreover, the proposed multistage power converter compared with each other as well as recently proposed multistage power converters in term of voltage conversion ratio, number of devices and costs.D.Eng. (Electrical and Electronic Engineering