Effects of Partial Shading Conditions on Maximum Power Points and Mismatch Losses in Silicon-Based Photovoltaic Power Generators

Abstract

Photovoltaic (PV) power generators can be used for converting the energy of solar radiation directly into electrical energy without any moving parts. The operation of the generators is highly affected by operating conditions, most importantly irradiances and temperatures of PV cells. PV power generators are prone to electrical losses if the operating conditions are non-uniform such as in a case where part of the modules of a generator are shaded while the rest are receiving the global solar radiation. These conditions are called partial shading conditions and they have been recognized as a major cause of energy losses in PV power generators.

In this thesis, the operation of silicon-based PV power generators under partial shading conditions is studied using Matlab Simulink simulation model. The operation of the model has been verified by measurements of electrical characteristics of a PV module under several different operating conditions and also under partial shading conditions. A systematic approach to study the effects of partial shading conditions has been developed and used. In addition to the systematic approach, a vast amount of data measured from the Tampere University of Technology (TUT) Solar Photovoltaic Power Station Research Plant are analyzed and used as input for the simulation model to study operation of PV power generators under actual operating conditions.

Partial shading conditions have severe effects on the electrical characteristics of PV power generators and can cause multiple maximum power points (MPPs) to the power-voltage curve of the generators. In most cases, partial shading conditions lead to the occurrence of multiple MPPs, but also only one MPP can be present despite of partial shading. Reasons for this phenomenon are presented and analyzed in this thesis. Because of multiple MPPs, a considerable amount of available electrical energy may be lost when the generator is operating at a local MPP with low power instead of the global MPP. In order to optimize the operation of PV power generators under partial shading conditions it is crucial to be familiar with the operation of the generators under these conditions. Results of a systematic study of the effects partial shading conditions on MPP characteristics are shown and a method to differentiate between local and global MPPs will be presented in this thesis.

Partial shading conditions cause also mismatch losses when the individual PV cells are not operating at their own MPPs although the generator would operate at its own MPP. The amount of mismatch losses depends on the partial shading conditions but also on the electrical configuration of the PV power generator. In this thesis, different configurations are based on different inverter concepts such as central inverter, string inverter and multistring inverter. The mismatch losses under partial shading conditions of these different PV power generator configurations are studied. It is shown that long series connections of PV modules are most severely affected by partial shading conditions.