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3. Supervision and Diagnosis of Photovoltaic Systems  235




                     Normalized thermal losses give the amount of power losses due to the rise of

                  temperature above 25 C. The thermal losses, L T , can be then determined by the
                  following expression:
                                                      E dc         E dc

                          L T ¼ Y a ðG; 25 CÞ  Y a ðG; T c Þ¼  ðG; 25 CÞ   ðG; T c Þ  (7.7)
                                                      P o           P o

                  where Y a (G,25 C) is the normalized energy yield at real working irradiance and STC
                  temperature.


                  3. SUPERVISION AND DIAGNOSIS OF PHOTOVOLTAIC
                     SYSTEMS
                  Power generation from PV systems is variable in nature due to changes in weather
                  conditions such as solar irradiance, wind, temperature, and other factors. Thus moni-
                  toring or forecast of climatic data is essential in the supervision of PV plants.
                     In PV applications, the supervision systems can be divided into two groups:
                  distributed and centralized supervision systems. In the distributed systems, the su-
                  pervision system is installed at the PV plant location, whereas centralized supervi-
                  sion systems are based on a remote supervision of the PV system.
                     Distributed supervision systems include complex monitoring systems and
                  software tools. In this case the cost is not a limitation because these systems
                  are to be applied in big PV generation plants or industrial applications. However,
                  most inverter manufacturers include several monitoring, communications, and
                  specific supervision capabilities that can be used in commercial or residential ap-
                  plications at relatively low costs. Normally, in this kind of low-cost supervision
                  systems, the supervision is just based on the analysis of the energy output or
                  the evaluation of the yields. Therefore, these modest monitoring systems are
                  not able to make a good diagnosis and evaluation of possible failures in the
                  photovoltaic system but can detect when the system is working below its opti-
                  mum performance.
                     On the other hand, centralized supervision systems are developed for residential
                  and building-integrated PV (BIPV) systems or commercial applications. These su-
                  pervision systems minimize the monitoring system at the PV site to reduce the costs.
                  Moreover, the hardware and software needed for the supervision tools are central-
                  ized and shared by several PV systems. The communication between the central su-
                  pervision site and the remote PV systems can be based on the Internet, Ethernet, or
                  Wi-Fi communications, among others. At the supervision site, the PV system perfor-
                  mance is evaluated and a basic diagnostic is applied to generate malfunction alarms
                  to be sent to the PV system owners.
                     The PV energy generation prediction can be based on the forecasting of the
                  global horizontal irradiance and ambient temperature as well as the PV system
                  configuration. Therefore, on-site monitoring is not required and additional cost
                  reduction can be achieved while maintaining a good degree of monitoring accuracy.
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