Research On NPC Half-bridge Grid-tied Inverters

Solar energy is one of the renewable energy which is widely applied. Neutral point clamped(NPC) three-level half-bridge topologies are generally used in the high-voltage and large-capacityphotovoltaic gird-tied inverter system. This paper mainly studies technologies of single phase NPCthree-level half-bridge inverters, including the loss distribution of switch devices, voltage balancecontrol strategies of dc terminal capacitors and photovoltaic grid-tied technologies, respectively.The paper analyzes three derived NPC three-level half-bridge topologies (3L-ANPC,3L-SNPC,3L-ASNPC) and characteristics of their freewheeling pathways. Then four kinds of SPWM strategiesare summarized. On this basis, the effect of each modulation strategy on the loss distribution of NPCtopologies’ switch devices is studied. Compared to traditional3L-NPC, the changes of derived NPCtopologies’ structures and their SPWM strategies would balance the switch devices’ loss distributionin better, and the total loss of switch devices are essentially invariant. Based on this, it shows that3L-NPC is suitable for small and medium power applications,3L-ANPC is not easy to adjust the lossof switch devices,3L-SNPC is applicable in high power applications, and3L-ASNPC has no obviousadvantages because of its complex structure, respectively.The natural capacitor voltage balance of3L-NPC topology is analyzed in frequency domain andtime domain. Based on the nature balance, two kinds of balancing capacitor voltage control strategiesare summarized. One of the strategies is that use closed loop control of capacitors’ difference voltageto eliminate the factors causing the capacitor voltage unbalance. The other one is that use appropriateregulator or control structure to ensure the nature balance of3L-NPC.A unified prototype of5kW NPC three-level half-bridge inverter has been developed. Four kindsof SPWM strategies and new voltage balance control strategies are verified through simulations bysaber and experiments on the unified prototype. The results of comparing the grid-tied efficiency ofeach NPC topology verifies that the changes of NPC topologies’ structure and SPWM strategies haveno effect on the total system efficiency, which is consistent with the theoretical analysis.