Advanced Techniques for Maximum Power Extraction from Photovoltaic Power Systems Under Partial Shading Conditions.

Photovoltaic (PV) cells or modules are mostly connected in series to achieve the higher voltages required by the applications, but they often experience severe power decrease when there is mismatch in the PV cells’ electrical characteristics due to partial shading. This thesis focuses on developing new techniques and control methods which enable PV arrays to achieve high efficiency and power output, under partial shading conditions (PSCs) with low cost.
The main contribution of the research lies in creating a novel technique, which combines three different schemes in a PV array for minimising the partial shading effects. These schemes include the Magic Square-Enhanced Configuration (MS-EC) algorithm, the Irradiance Equalisation (IE) method and the Series-Parallel Differential Power Processing (SP-DPPs) converter scheme. Acting on a PV array initially in a Tied-Cross-Ties (TCT) interconnection, the first MS-EC, a physical relocation algorithm, reconfigures the array in order to make the shading effects more evenly dispersed over the entire array surface, hence reducing the voltage and current drop due to the bypass diode conduction. The second method uses switches and relays to obtain equalisation of the irradiation levels over the rows of a set of TCT sub-arrays. The third technique employs power electronic converters to connect the PV arrays in both series and parallel configurations, to give flexible control for the maximum power generation. Extensive investigations on each of these techniques have revealed their respective advantages, but none can achieve high power generation under all possible shading conditions. The combined approach makes use of the advantages of each of these methods. Simulation studies have demonstrated that it outperforms anyone of the three schemes and can increase the maximum power generation of between 22 and 48% under a wide variety of shading patterns.
The second contribution is in the novel Series-Parallel Differential Power Processing converters scheme (SP-DPPs) for obtaining maximum power generation under non-uniform solar illumination conditions. In place of bypass diodes, the proposed system embeds Bidirectional Ćuk DC-DC Converters (BCC) within the serially connected PV modules for adjusting the module currents. In contrast, the DPP scheme based on an Inverted-Buck Converter (IBC) topology replaces the blocking diodes connected across the parallel strings. The mathematical models have been derived for these converters based on their transfer functions assuming Continuous Conduction Mode (CCM). The derived transfer functions are applied to develop novel design criteria for selecting the circuit parameters of the SP-DPPs system, leading to a well-behaved transient response and reduction of the effect of a non-minimum phase response. An experimental investigation validates the BCC model under different irradiation conditions. A model-based control scheme using P+I controllers has been developed to adjust the duty ratios of both inner BCC and outer DPP converters, enabling all PV modules to operate at their MPPs corresponding to their individual lighting conditions. The simulation results of such a system show significant improvements of about 26% in the total power of the SP-DPP system compared to that of the conventional PV array protected only using bypass and blocking diodes under PSCs.
A detailed analysis is made of the structure and operation of the parallel DPP scheme when using four different converter topologies, namely the inverted-buck converter, forward converter, flyback converter and SEPIC converter. To explore and compare their dynamic features, their transfer functions are derived. Based on these, their transient responses are simulated, and results are compared under different irradiation conditions. Power losses are studied and compared, based on practical characteristics of the switching devices, in a DPP system controlling two parallel PV modules. This leads to the clear conclusion that the inverted-buck converter has the best performance in terms of transient behaviour and total efficiency between 86 and 96% under partial shading conditions.