NPC three level inverter with dual DC bus for independent distributed generators. Neutral-point voltage balancing under the input power imbalance

This paper concerns the application of three-level neutral point clamped (NPC) voltage source inverter (VSI) to electrical energy conversion of the photovoltaic (PV) power plant. The grid-connected 3-phase NPC inverter is powered via the split DC-link composed of the capacitive divider (Fig. 1). It enables the usage of two DC-buses independently which can be supplied by the separated boost-type DC-DC converters. They are used to generate the maximal power value (MPP) from each string of PV modules. The main objective of this paper is the analysis of a DC-link voltage unbalance effect and development of dedicated balancing control algorithm based on the 7-segment space vector modulation method (SVM). The goal of proposed solution is proper operation of the three-phase NPC inverter under permanent input PV power unbalance (|ΔP
_dc
| > 0). In the presented paper the maximal permissible level of input power unbalance was estimated which can be compensated by the use of developed NPC inverter modulation method as a function of instantaneous operation point (e.g. amplitude modulation index m
_aSVM
). The listed issues were verified via simulation research performed in Maltab\Simulink software as well as experimentally via the laboratory tests.