DC-link voltages balance method for single-phase NPC inverter operating with reactive power compensation

Neutral point clamped (NPC) are one of the most used topologies for inverters in photovoltaic (PV) applications due to several operative advantages. The same hardware used for DC/AC conversion can be exploited to add reactive power compensation capabilities. In absence of PV generation (i.e. night hours) the DC link voltage have to be controlled by injecting or extracting active power to or from the DC link capacitors. While this technique is widely used in active power filters (APF) based on H-bridge inverters, the use of NPC inverters imposes a restriction due to potential imbalances in the voltages of DC-link capacitors, produced by DC components in the current of nonlinear loads being compensated. In this paper an analysis of DC currents effect is presented and an enabling method to allow reactive power compensation with single-phase NPC inverters is proposed and tested.