Designing a PV inverter system
The two primary sub-components in a PV inverter systems typically are the controller used to implement system management tasks and control algorithms, and the AC-to-DC conversion circuit.
The controller is used for tasks including grid and system monitoring, system synchronization with utility power for grid-connected systems, and output power quality monitoring. The controller also performs protective functions for safety and compliance with various standards and regulations. Other key functions include data logging, firmware updates, and communications with the system operator, as well as battery charging control for standalone systems, and smart metering used for grid-connected PV systems. One other important controller responsibility is the execution of control and energy management algorithms. In addition to being very computationally demanding, these tasks can also impact power efficiency.
The DC-to-AC conversion circuit also plays an important role, handling all the tasks related to converting raw DC power from the panels into clean AC power that is consistent with the utility grid’s voltage and power quality requirements. To accomplish this, the circuit uses a set of switching power devices such as metal oxide semiconductor field effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs). The inverter circuit also includes active filtering circuitry to reduce the distortion caused by harmonics resulting from high-frequency switching.
Choosing the right configuration
Designers have a choice of several possible PV conversion circuit configurations. The choice depends on a number of factors, including the number of power processing stages, the type of power decoupling, the types of intra-stage interconnections, and the type of grid interface.
There are also important considerations related to power levels. Micro inverters are typically integrated in the PV module for power levels up to 400 watts (W), whereas string inverters are used for power levels up to 10 killowatts (Kw). For power levels between 5 and 50 kW, multi-string inverters are generally the best choice. For higher power levels, central inverters should be used.