Reliability-Oriented Design of NPC Inverter in PV Systems With Focus on Lifetimes of DC-Link Capacitor Bank and Power Devices

Reliability has become one of the important factors in designing photovoltaic (PV) inverters. Accordingly, interest in the reliability of reliability-critical components such as power devices and capacitors has been increasing. The DC-link capacitor is generally used in forms connected in parallel or in series called the capacitor bank to fulfill the requirements in voltage rating and capacitance. However, since the capacitor is one of the bulkiest and most expensive components and also one of the vulnerable components, it is required to optimize the configuration of the DC-link capacitor bank. Furthermore, the cooling system capacity should properly be chosen because it plays an important role not only in the power density and volume of the inverter but also in the reliability of the power devices. This article presents a reliability-oriented design procedure of the neutral point clamped (NPC) inverter in a 30-kW PV system by considering different factors such as mission profile, DC-to-AC ratio of the PV system, and pulsewidth modulation (PWM) methods of the PV inverter, focusing on designing the DC-link capacitor bank with optimization in terms of lifetime, cost, volume, and cross-sectional area, and on optimizing cooling system capacity for the reliability of power devices to achieve the target lifetime of the NPC inverter.