Design and Analysis of Grid-Tied PV Panels with Cascaded H-Bridge Multilevel Inverters
Due to the shortage of fossil fuels and environmental problems caused by conventional power generation, renewable energy, particularly solar energy, has become very popular. Solar-electric-energy demand has grown consistently by 20%–25% per annum over the past 20 years, and the growth is mostly in grid-connected applications. With the extraordinary market growth in grid-connected photovoltaic (PV) systems, there are increasing interests in grid-connected PV configurations.
This paper presents cascaded H-bridge multilevel photovoltaic (PV) inverters for single-phase transformerless grid-connected applications. The cascaded multilevel inverter topology helps to improve the efficiency and flexibility of PV systems. To realize better utilization of PV modules and maximize the solar energy extraction, Maximum Power Point Tracking (MPPT) algorithm is implemented based on the inverter output power to assure optimal operation of the inverters. Grid-tied inverters are synchronized with power grid using Phase Locked Loop (PLL) for phase and frequency match. A Sinusoidal Pulse Width Modulation (SPWM) scheme is applied in this paper to be used with the solar panels that can account for voltage profile fluctuations among the panels during the day. Simulation results are shown for voltage and current during synchronization mode and power transferring mode to confirm the methodology for grid connection of PV panels.
In this study, a single-phase 11 level cascaded H bridge inverter has been implemented by 5 H-bridge modules. Each inverter is supplied by 5 identical 1.2kW powered PV arrays and transfers the output power (6kW) to the grid by %96 efficiency. PLL provides synchronization of inverter with grid in 50ms. The isolation level between inverter and grid can be compatible with standard limits using high efficient reliable inverter control (HERIC) and H-5 topologies. Inverter supplies current with %3 total harmonic distortion (THD), well below %5 compatible with standard limitations. To achieve power balancing of the solar arrays and improve MPPT performance, a code is written to sort solar arrays and define pulse durations of currents. Designed inverter can achieve reactive power sharing with grid, between +1000 / -1000 VAr, by adapted droop control method (DCM), if required. Simulation results are examined under different temperature and solar irradiance cases. The results are compared with literature.
Ömer Koca is working as a senior researcher at Tübitak-Sage, Ankara, Turkey. He received his BSc degree in Electrical Engineering and additionaly Electronics and Telecommunication Engineering (Double major) from Kocaeli University in 2012. He continues his master degree education in Electronics Engineering at Gazi University.