MISMATCH LOSSES IN PHOTOVOLTAIC POWER GENERATORS DUE TO PARTIAL SHADING CAUSED BY MOVING CLOUDS Anssi Mäki and Seppo Valkealahti Tampere University of Technology, Department of Electrical Energy Engineering P.O. Box 692, FI-33101 Tampere, Finland Tel.: +358 3 311 511, Fax: +358 3 3115 3015 e-mail: anssi.maki@tut.fi, seppo.valkealahti@tut.fi ABSTRACT: In this paper, we have investigated the effect of partial shading due to moving clouds on mismatch losses of five different PV power generator configurations. These configurations are a string of six series-connected PV modules, two strings of three series-connected PV modules, which are either separately controlled or connected in parallel, and three strings of two series-connected PV modules, which are also either separately controlled or connected in parallel. The work has been conducted by using Matlab Simulink simulation models of the PV power generators with actual measured irradiance and temperature data as an input for the models. The results show that the mismatch losses due to partial shading caused by clouds increase with increasing length of the series connections. In order to minimize these mismatch losses, the length of the strings should be kept as short as possible. Keywords: Mismatch losses, cloud shading, partial shading, photovoltaic power generators. 1 INTRODUCTION Energy of solar radiation can be utilized by using photovoltaic (PV) cells. PV cells convert the energy of solar radiation directly into electrical energy without any moving parts [1]. Unfortunately, the voltage of a single silicon-based PV cell is too low to be conveniently used for large scale energy production. Therefore, PV modules are built by connecting tens of PV cells in series to increase the output voltage and power. PV modules are then typically connected in series in order to get sufficient voltage level and to increase the nominal power of the PV power generator. Series connection of PV cells is, however, prone to losses if the electrical characteristics of the PV cells are not similar or the cells don’t operate under uniform conditions. The PV cell with the lowest short-circuit (maximum) current limits the current of the whole series connection [1]. Short-circuit current of a PV cell is directly proportional to incident irradiance and, therefore, it varies due to shading by clouds, trees, building etc. Under partial shading conditions, the shaded cells become reverse biased due to the other cells in the series connection and act as load dissipating part of the power generated by the other cells leading to power losses. In the worst case this can lead to hot-spots in the shaded cells, which can be damaged [2]. PV cells in PV modules are typically prevented from being damaged due to hot-spots by connecting bypass diodes in anti-parallel with the cells [3]. When operating under partial shading conditions, the shaded cells become reverse biased due to the other cells in the PV module and the bypass diodes start to bypass the current exceeding the short-circuit current of the shaded cells and limit the power dissipated in the shaded cells. Typically one bypass diode is connected in anti-parallel with approximately 20 series-connected PV cells. The effects of moving clouds on the operation of PV power generators has been studied earlier, e.g., in [4]-[7]. The main objective in these studies has been to understand the effects of clouds on energy production of PV generators from the electrical grid perspective. The changes in power production due to clouds have been noticed to be large and fast. Therefore, other types of power generation are required to quickly respond to the decrease or increase in the power fed to the grid by the PV power generator. It has also been noticed that changes in power output of PV generators due to clouds are smaller for systems that are built in wide areas. The effects of partial shading have also been extensively studied in several papers, e.g. in [8]-[13]. In many of these papers the focus is on the modelling of the operation of partially shaded PV power generator or on the verification of the operation of maximum power point tracking algorithm under partial shading conditions. In [14]-[16] more systematic and comprehensive studies on the effects of partial shading has been presented, but the main focus has still been on the shading caused by static objects such as buildings. In this paper, the mismatch losses due to partial shading conditions caused by moving clouds are investigated by using Matlab Simulink simulation models with actual measured PV module irradiances and temperatures as an input for the models. The data has been recorded once per second for every PV module during two consecutive summers. Mismatch losses due to moving clouds in different PV power generator configurations are then studied. Typically the configurations of PV power generators have been named after the interfacing device controlling the operation of the generator [8],[9]. In this paper, five different configurations are studied. Two of the five configurations are based on the multi-string inverter concept in which every PV module string is individually controlled by an interfacing converter. Another two configurations are based on central inverter concept in which PV module strings are connected in parallel. Fifth configuration is based on the string inverter concept in which a large number of PV modules are connected in series. All five PV generator configurations studied in this paper were composed of the same amount of PV modules. 2 SIMULATION MODEL AND STUDY 2.1 Simulation model Simulation model of the operation of a PV cell