Abstract
In order to solve the neutral-point voltage unbalancing problem for neutral-point clamped (NPC) three-level inverters under non-ideal grid conditions, a novel zero-sequence component injection method for modulation signals was proposed in this paper. Based on the mathematical expressions of neutral-point voltage unbalancing, the fluctuation of the neutral-point voltage under different grid conditions was studied. The proposed method dynamically calculates the zero-sequence component for modulation signals by region selections based on sinusoidal pulse width modulation (DCR-SPWM). Under non-ideal grid conditions, this DCR-SPWM method can control the neutral-point voltage balance for NPC three-level inverters with different grid and output power conditions, and improves the grid current quality. The simulation results verified the correctness of the theoretical analysis in this paper, and the feasibility and effectiveness of this method were verified by the experiments in the experimental platform of the NPC three-level grid-connected inverter based on DSP-CPLD controllers.
Highlights
In the neutral-point clamped (NPC) three-level inverter circuit, two capacitors for the dc bus are used in series to clamp the neutral-point (NP) potential
The control principle diagram of the NPC three-level grid-connected inverter under non-ideal grid conditions is shown in Figure 5, where the DDSRF-PLL represents the decoupling double
It can be seen that with the traditional significantly deteriorates the pulse width modulation (SPWM) method, the ripple of vc1 increased much higher and the total harmonic distortion (THD) value was higher than 5% when λ varied from 0 to ripple of vc1 increased much higher and the THD value was higher than 5% when λ varied from 0 to
Summary
The neutral-point clamped (NPC) three-level inverter has been widely used in medium or high voltage and high power conversions, especially as a dc/ac converter for a photovoltaic (PV) power generation system [1,2], which influences the safety and stability of the PV system for grid connection. In the NPC three-level inverter circuit, two capacitors for the dc bus are used in series to clamp the neutral-point (NP) potential. The voltage of the two capacitors are both half of the dc bus voltage. [5], the NP voltage unbalancing problem appears, which will increase the voltage stress on switching devices [6,7], and lead low-order harmonic currents into the grid [8]. The NP voltage balance has been the research emphasis of three-level inverters
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