Advances in Grid-tied PV Inverters: Current Control, PID Suppression and DC-bus Balancing

The renewable energy industry has grown dramatically in recent years as a result of global green missions. PV energy is considered the most cost-effective and reliable renewable energy source. Power electronics, an important interface between PV renewables and the grid, has advanced rapidly in recent years. As the percentage of PV renewables integration increases, new challenges emerge, such as high-quality grid current controlling, potential induced degradation suppression, DC-bus voltage balancing, and so on. These new issues create new challenges for PV power electronic inverter. As a result, in-depth discussions on these issues are required in order to provide technical support for the global energy transition. It is simple to implement conventional current control with a PI controller. However, system stability and dynamic performance are not perfect, particularly when operating under unfavorable conditions. In Chapter 2, an improved control method is proposed by introducing a compensation unit. The compensation unit can effectively compensate the system's phase around the crossover frequency, greatly enhancing the system's phase margin and stability. It is also capable of handling weak-grid conditions. Researchers have discovered that there are various degradation mechanisms affecting PV module. One of the degradation mechanisms is the PID, which is regarded as one of the leading causes of PV module degradation. In Chapter 3, a PID suppression method is proposed, in which a PID suppression unit is added between DC negative bus and ground. The idea is to regulate the output voltage of the added power supply, and then correspondingly raise the voltage of the PV module to the ground to suppress the PID effect. Chapter 4 investigates the dc-bus voltage balancing for 3-level DC/DC converters. Based on duty cycle and operating modes, four different cases are illustrated, and the output voltages under four cases are detailed. The dc-bus voltage balancing situation becomes worse when the system is in the condition of D>0.5 and DCM. Therefore, three methods are proposed to solve the issue raised.