NPC multilevel multistring topology for large scale grid connected photovoltaic systems

Large scale photovoltaic energy conversion systems are currently stepping into the multi-megawatt range. Therefore high power multilevel converters are becoming an attractive alternative for this application. In this work, the two-level voltage source converter based multistring topology concept is extended and explored for the three-level neutral point clamped (NPC) converter. The split dc-link of the NPC enables the use of two series connected dc-buses, naturally doubling the voltage level of the system. Hence, less voltage step-up effort is required to reach medium voltage operation by the dc-dc stage or by the line side transformer. In addition, the system allows to connect as many strings to both dc-buses as permitted by the total power rating of the NPC, which is capable of concentrating a large PV farm into a single converter, power control system, line filter and transformer, with lower losses, size and cost than several two-level converters needed to handle the same power. In addition, the inherent superior output voltage waveforms of the NPC enables operation with lower switching frequency improving the efficiency of the system, and provides better power quality that complies with the more demanding grid codes at higher power levels.