Novel Carrier-Based PWM Strategy of a Three-Level NPC Voltage Source Converter without Low-Frequency Voltage Oscillation in the Neutral Point

Abstract

A novel carrier-based PWM (CBPWM) strategy of a three-level NPC converter is proposed in this paper. The novel strategy can eliminate the low-frequency neutral point (NP) voltage oscillation under the entire modulation index and full power factor. The basic principle of the novel strategy is introduced. The internal modulation wave relationship between the novel CBPWM strategy and traditional SPWM strategy is also studied. All 64 modulation wave solutions of the CBPWM strategy are derived. Furthermore, the proposed CBPWM strategy is compared with traditional SPWM strategy regarding the output phase voltage THD characteristics, DC voltage utilization ratio, and device switching losses. Comparison results show that the proposed strategy does not cause NP voltage oscillation. As a result, no low-frequency harmonics occur on output line-to-line voltage and phase current. The novel strategy also has higher DC voltage utilization ratio (15.47% higher than that of SPWM strategy), whereas it causes larger device switching losses (4/3 times of SPWM strategy). The effectiveness of the proposed modulation strategy is verified by simulation and experiment results.

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“…The dipolar DCBPWM has been implemented in three‐phase three‐level converters, where the four switches in each leg of the bridge may be in one of ‘three configurations’: −1, 0 and 1 over a switching cycle [18, 19]. However, the overall assessment of the dipolar modulation for the three‐level NPC converters, specifically its high‐order harmonics, has seldom been studied.…”

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“…The modulation methods and control strategies [1]- [8] of three-level converters have long been a research issue. Many scholars have exerted fruitful efforts to reduce grid-side harmonics significantly.…”