1. Grid-Tie Inverters :
A grid-tie inverter (GTI) is used to complement the generated solar power with grid power. In addition to regulating the voltage and current received from the solar panels, a GTI ensures that the power supplied to the distribution panel of the load will be in phase with the grid power. On the DC side, maximum power point tracking (MPPT) optimizes the power output by varying the closed loop system voltage. On the AC side, these inverters ensure that the sinusoidal output is synchronized to the grid frequency. In addition, the voltage of the inverter output needs to be variable and slightly higher the grid voltage to enable current to supply the loads or even have excess power flow out to the utility.
2. PV INVERTERS:
1- Centralized Inverters
Initially, the interface between Photovoltaic power supply and the grid rely on the centralized inverter technology, as shown on Fig. 1. Inverters are connected in into series, called strings, generating a sufficient high voltage to avoid amplification. All strings are then connected in parallel to support high power to output. Only one inverter is utilized to interface the grid. This technology suffers from disadvantageous issues, including high voltage DC cable from a big number of strings to the inverter and losses in string diodes. This structure is also limited from Maximum Power Point (MPP) Tracking and controlling mismatch between strings so individual PVs, resulting in low efficiency and reliability. The nonflexible design makes it less appealing in mass production. With all these issues, this technology is not used in new solar systems installation.
Fig-1- Centralized Inverters
2- String Inverters
This technology, shown in Fig. 2, illustrates effort to solve problems of the previous design. It has a string of inverters connected in series with an AC module. While still avoiding high voltage amplification, this structure has improved performance with no diode loss in series, separate MPP tracking for each string and lower cost with mass productions. The inverter can be implemented with high voltage MOSFET/IGBT. It is possible to have less PVs in string with voltage amplification by DC-DC converter or a line frequency transformer, which increases total area. However, in a common scenario of partial shading, MPP tracking may still not be sufficient to achieve a certain efficiency requirement.
The micro-inverter solution, also called AC module, shown in Fig. 3, is the integration of PV and inverter into one electrical device. With only one PV to control, there is no PV mismatch. MPP tracking can be done at individual PV level, maximizing possible efficiency. As it is modularized, the micro-inverter is good for mass production, which potentially leads to low manufacturing cost and low retail prices. This technology is also very...