Abstract
In response to the unbalanced DC-port fault of three-level neutral point clamped cascaded converter (3LNPC-CC), a sequence pulse modulation (SPM) voltage-balancing strategy is proposed in this paper to balance DC-link voltage, not only within the module but also among modules. With the steps of carrier cascaded calculation and sequence pulse generator, the voltage level of cascaded modules would take a smooth transition. Then the limitation of the SPM strategy is calculated according to the law of volt-second balance and the law of energy conservation. The proposed strategy has the advantage of simple calculation and control stability. Simulation and experimental results show the superiority of the proposed strategy.
Highlights
With the increase of voltage and capacity demand, various multilevel topologies are proposed, such as three-level neutral point clamped converter (3LNPC), cascaded H-bridge converter (CHB)and modular multilevel converter (MMC)
To decrease the total harmonic distortion (THD) of the Us, the current loop is added as a proportion feedback loop
In Figure (C), Module 1 is the module without input voltage, before adding the sequence pulse modulation (SPM) strategy, Uab exhibits a large number of switch state jump problems, which and the output voltage of 3LNPC-CC remains unchanged
Summary
With the increase of voltage and capacity demand, various multilevel topologies are proposed, such as three-level neutral point clamped converter (3LNPC), cascaded H-bridge converter (CHB). References [9,10,11,12] propose an auxiliary voltage-balancing circuit to solve the unbalanced problem of capacitor voltage inner the module. In Reference [4], a strategy is proposed by using redundant vectors to change the current flow direction This modulation changes switch states inner the module and maintains the output voltage constant, which has been widely used in 3LNPC convertors. In References [20,21,22], some power balance and fault-tolerant strategies are proposed for the three-phase cascaded inverter to balance the power of three phases, but the unbalance problem is not serious compared to the situation in 3LNPC-CC or DC-port fault. This paper is laid out as follows: an SPM-based voltage-balancing strategy to balance the voltage within the module and among modules for 3LNPC-CC is proposed when DC-ports are unbalanced.
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