Abstract:
The continuous increase in power demands and need for a cleaner environment makes
decentralized renewable energy production, like solar and wind, more and more interesting.
Decentralized energy production using solar energy could be a solution for balancing the
continuously-increasing power demands. This continuously increasing consumption overloads
the distribution grids as well as the power stations, therefore having a negative impact on power
availability, security and quality. One of the solutions for overcoming this is the grid-connected
photovoltaic (PV) system.
With the extraordinary market growth in grid-connected PV systems, there is increasing interests
in grid-connected PV inverters. Focus has been placed on inexpensive, high-efficiency, and
innovative inverter solutions, leading to a high diversity within the inverters and new system
configurations. This report chooses cascaded multilevel inverter topologies for grid connected
PV systems to reduce the cost and improve the efficiency.
First, a single-phase cascaded H-bridge multilevel PV inverter is discussed. To maximize the
solar energy extraction of each PV string, an individual maximum power point tracking (MPPT)
control scheme is applied, which allows independent control of each dc-link voltage. A
generalized non active power theory is applied to generate the reactive current reference. Within
the inverter’s capability, the local consumption of reactive power is provided to realize power
factor correction. Then, the modular cascaded H-bridge multilevel inverter is connected to a
three-phase utility system and nine PV panels. Individual MPPT control is also applied to realize
better utilization of PV modules. Also, mismatches between PV panels may introduce
unbalanced power supplied to the three-phase grid-connected system. Thus, a modulation
compensation scheme is applied to balance the three-phase grid current by injecting a zero
sequence voltage.
A modular cascaded multilevel inverter prototype has been built and tested in both the single
phase and three-phase PV system. Simulation and experimental results are presented to validate
the proposed control schemes. The targets of reducing the cost and improving the overall
efficiency of the PV inverters can be achieved by applying the cascaded PV inverters and the
proposed control schemes.
