Abstract
In order to reduce switching loss of neutral-point clamped (NPC) three-level inverter (TLI), a switching loss minimized discontinuous PWM (SLMDPWM) strategy is proposed firstly. There are DC offset and AC ripple on the neutral-point (NP) voltage when SLMDPWM is used, which causes an appearance of low frequency harmonics (LFHs) in the output voltage. Aiming at this drawback, reduced LFHs (RLFHs) method is then proposed. While using RLFHs, the self-equilibrium ability of NP voltage is thus lost. So, active neutral-point voltage control (ANPVC) is also proposed. The performances of NPC TLI with the proposed DPWM and other DPWMs are compared by simulation and experimental results. The results show that switching loss, LFHs can be reduced significantly and the NP voltage can be controlled within an allowable range. Therefore, the multiple control objectives of reduced switching loss, no LFHs and allowable NP voltage are achieved.
Highlights
Neutral-point (NP) clamped (NPC) three-level inverters (TLI) are more and more widely used because of some advantages over the conventional two-level inverter, such as reduced total harmonic distortion (THD), reduced voltage stress across switching devices, and higher inversion efficiency
Several PWM strategies have been reported to solve the NP voltage unbalance problem of neutral-point clamped (NPC) TLI, which are generally classified into space vector PWM (SVPWM) and carrier based PWM (CBPWM) schemes
The switching losses can be reduced about 37% in worst condition and about 50% in best condition under switching loss minimized discontinuous PWM (SLMDPWM)
Summary
Neutral-point (NP) clamped (NPC) three-level inverters (TLI) are more and more widely used because of some advantages over the conventional two-level inverter, such as reduced total harmonic distortion (THD), reduced voltage stress across switching devices, and higher inversion efficiency. A new DPWM, i.e. SLMDPWM is proposed, in which the relationship between phase voltage and current is considered with the variation of m and φ to minimize switching loss. (1) In order to eliminate LFHs in the output line-to-line voltage, which are caused by DC offsets and AC ripples on the NP voltage, the improved PWM strategy must be studied;. (2) While even-order LFHs are removed from the output line-to-line voltage by the improved PWM strategy, the self-equilibrium ability of the NP voltage is weakened. V. RLFHs FOR DPWM UNDER UNBALANCED NP VOLTAGE From analysis given, when the NP voltage is shifted, there are odd-order and even-order LFHs in the output line-to-line voltage if the duty ratios of every phase are not compensated. After modifying duty ratios, the LFHs caused by unbalanced NP voltage can be eliminated in theory
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