Grid-Friendly High-Reliability Photovoltaic Systems

Photovoltaic (PV) systems have much potential to become a major renewable source for clean electricity and they are expected to provide a significant share in the electricity demand in the future. Accordingly, several efforts have been initiated to increase the penetration level of PV systems and introduce more renewables. In order to enable more PV installations, challenging issues in both technical and economic aspects should be properly addressed. Firstly, seen from the technical perspective, one major concern is related to the integration of the PV systems to the electricity grid. Since the PV power production is strongly dependent on the environmental conditions (e.g., solar irradiance and ambient temperature), the power injected from the PV systems can vary considerably during a day. In the case of wide-scale PV system installations, a considerable amount of fluctuating power will be delivered to the grid which may induce overloading during peak-power generation periods and voltage fluctuations, challenging the system operators. On the other hand, maintaining a high-level of power quality is always mandatory for the future installation of the PV systems. The PV systems have been reported as one source of harmonics including interharmonics delivered to the grid. Thus, solutions to the above challenges are demanded. Secondly, the cost reduction of PV energy is another important aspect to enable more PV installations. For the application with relatively long expected lifespan such as PV systems (e.g., 20-25 years of operation), the operation and maintenance costs play a major part in the overall cost of energy. According to the field experience, the PV inverter has been reported as one of the most critical components that cause failures in the PV systems. The consequence of the PV inverter failure will not only lead to extra financial and labor efforts to the inverter replacement but also the loss of revenue from the reduced energy yield during the inverter downtime. Therefore, ensuring a highly reliable operation is strongly demanded for the PV inverter, since it has high potential for reducing the cost of PV energy. iii To tackle those issues and thus enable more PV systems, this Ph.D. project discusses solutions to improve the control functionality and reliability of power electronics in PV systems. Throughout this project, a flexible power control strategy of PV systems has been developed. A power limiting control strategy has been proposed to limit the maximum feed-in power of the PV system to a certain level. A control solution to limit the ramp-rate of the PV output power is also proposed, namely, a power ramp-rate control strategy. Then, two solutions to realize the power reserve control have also been proposed: 1) Coordinating the control of different PV units with the Constant Power Generation (CPG) control and the Maximum Power Point Tracking (MPPT) operation and 2) Combining the CPG control and the MPPT operation to routinely estimate the available power using only one PV unit. The performance of the proposed control strategies has been validated experimentally under several operating conditions. Furthermore, the power quality issue of the PV systems, specifically interharmonics is also analyzed in this project. It is confirmed by the experiments that the perturbation of the DC-link voltage during the MPPT operation is one main cause of interharmonics in the grid current. To address this issue, a model to predict interharmonics according to the control parameters has been proposed. The effectiveness of the proposed interharmonic model has been validated. It has been demonstrated that the predicted interharmonics and the experimental measurements are in close agreement. The reliability of the PV inverter is also analyzed in this project, where the mission profiles of the PV systems are considered. From a reliability point of view, the PV array degradation and oversizing are the two main aspects that can strongly affect the reliability of PV inverters, whose impact has thus been investigated in this project. The results have shown that the PV array degradation can deviate the reliability performance of the PV inverters (i.e., B10 lifetime) by more than 50 % compared to the case without considering the degradation. The impact of PV array sizing on the PV inverter reliability has also been analyzed, which shows that the B10 lifetime of the PV inverter may decrease by more than 40 % compared to the case without oversizing PV arrays, especially, for the mission profile with low solar irradiance conditions. Moreover, the solution to the reliability enhancement of PV inverters with the integration of battery systems is also explored in this Ph.D. project. With the integrated battery system, the loading of the PV inverter can be modified in a way to benefit the reliability of the PV inverter. In that case, the damage of the PV inverter can be reduced by 50 % compared to the case without battery systems. Thus, integrating energy storage systems may be a promising solution to enhance the reliability of PV inverters.