Abstract
In this paper, double-line frequency (2ω) ripple suppression and SMC with time-invariant (fixed) switching frequency methods are proposed for single-phase grid-connected three-level neutral-pointclamped quasi-Z-source inverters. The 2ω ripple suppression method is based on the 180° phase difference existing between the 2ω ripple components of the capacitor and inductor voltages in the dc-side. Hence, when these components are added in the closed-loop, a phase cancellation occurs so that the inductor current reference can be generated without 2ω ripple component. In this case, the actual inductor current, which is forced to track its reference, has no 2ω ripple component. In addition, the grid current control is achieved via sliding mode control (SMC). Unlike the existing SMC methods, the proposed SMC achieves fixed switching frequency which is made possible by eliminating the discontinuities in the sliding surface function using a boundary layer. The proposed ripple suppression method together with the SMC method offers many advantages such as fast dynamic response, zero grid current error, simple implementation, robustness to parameter variations and fixed switching frequency. The effectiveness of the proposed control method is verified experimentally under steady-state and transient conditions.
Highlights
Voltage source inverters are widely utilized in grid-connected systems for injecting power obtained from renewable energy sources into the grid
The effort of the researchers and industry has led to rapid development of different multilevel inverter topologies such as cascaded multilevel [3], neutral point clamped (NPC) [4], flying capacitor (FC) [5] and Packed-U-Cell (PUC) [6] inverters
Unlike the sliding mode control (SMC) methods presented in [24]–[26], the SMC method proposed in this study offers fixed switching frequency
Summary
Voltage source inverters are widely utilized in grid-connected systems for injecting power obtained from renewable energy sources (e.g. fuel cells, solar energy, wind energy, etc.) into the grid. The NPC-qZSI topology offers continuous input current, and provides wide input voltage operation range, shoot-through immunity, twice lower semiconductor voltage stress, good performance at reasonably low switching frequency and ability to work with one dc input voltage source In spite of these attractive features, the singlephase inverter topologies suffer from inherent double-line frequency (2ω) ripple existing on the ZS network capacitor voltages and inductor currents which have adverse effects on the lifetime and efficiency of the photovoltaic (PV) array utilized to supply input voltage to the system. A new closed-loop based 2ω ripple suppression method is proposed for a single-phase grid-connected three-level (3L) NPC-qZSI. In the NST state, where the NPC inverter is represented as a current source, the inductor voltages and capacitor currents can be written from Fig. 2(a) as follows. These control strategies are described in the following sub-sections
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