Mission Profile-Oriented Control for Reliability and Lifetime of Photovoltaic Inverters

With the aim to increase the competitiveness of solar energy, the high reliability of photovoltaic (PV) inverters is demanded. In PV applications, the inverter reliability and lifetime are strongly affected by the operating condition that is referred to as the mission profile (i.e., solar irradiance and ambient temperature). Since the mission profile of PV systems is location-dependent, the inverter reliability performance and lifetime can vary considerably in practice, that is, from the reliability perspective, PV inverters with the same design metrics (e.g., component selection) may become over or underdesigned under different mission profiles. This will increase the overall system cost, e.g., initial cost for overdesigned cases and maintenance cost for underdesigned cases, which should be avoided. This article, thus, explores the possibility to adapt the control strategies of PV inverters to the corresponding mission profiles. With this, similar reliability targets (e.g., component lifetime) can be achieved even under different mission profiles. Case studies have been carried out on PV systems installed in Denmark and Arizona, where the lifetime and the energy yield are evaluated. The results reveal that the inverter reliability can be improved by selecting a proper control strategy according to the mission profile.</p

Active Temperature Control Algorithms for Improved Reliability and Lifetime of Photovoltaic Power Converters

Power converters are exposed to environmental stresses that affect reliability. Among these, ambient temperature and solar irradiance are the ones that affect photovoltaic (PV) systems the most. For solar irradiance this is especially true for days with dynamic irradiance like those with variable cloud conditions. The power conversion is not perfect and has power losses that heat up the components. Among the most fragile components in power converters are power semiconductors and capacitors, which are greatly affected by thermal variations. This thesis presents temperature control algorithms to enhance reliability and lifetime of PV converters. The algorithms are focused on thermal control through duty cycle regulation. The first one is called temperature-controlled (TC) maximum power point tracking (MPPT) algorithm and is used for lifetime improvement of PV converters under dynamic irradiance conditions during cloudy days. The second is called maximum-temperature-limited (MTL) MPPT algorithm and is used to improve the lifetime of PV converters by limiting the output power during days with high irradiance and high ambient temperature levels. The effectiveness of the algorithms is verified using extensive simulations, evaluating the lifetime and comparing it with the energy generated. For the TC MPPT, the evaluation is done under daily irradiance profiles for different cloud conditions, while for the MTL MPPT a maximum temperature is set to test during a yearly mission profile. The results show that the TC MPPT algorithm managed to reduce life consumption by 4.68% with 0.08% of energy reduction for very variable cloud condition days. Moreover, the MTL MPPT algorithm reduced the yearly life consumption by 28.36% with a small energy generation cost of 3.97%. Based on these results it is possible to validate the effectiveness of thermal management strategies, which apart from enhancing energy production with no extra hardware cost can improve reliability at the same time