| Compared with two-level inverters,neutral-point clamped(NPC)three-level inverters can reduce voltage/current harmonic distortion,have lower electromagnetic interference and high efficiency,it has ability to meet the increasing demand of power quality and are suitable for medium and high voltage industrial application.Considering the advantages,the NPC three-level inverters have aroused great research interest in universities and research institutes at our country and abroad.It can even be applied in new energy electric vehicles and locomotive drive of high-speed railway.In NPC three-level VSI inverters,capacitance is one of the components with high failure rate,and its low reliability seriously restricts the safety performance of the inverters.In this paper,the reliability of DC-Link capacitor and neutral point potential balance of NPC three level inverter are analyzed.First the effect of ripple current flowing through DC-Link capacitors on reliability is analyzed.Then the expression of ripple voltage is derived from the RMS of ripple current.Finally,two novel modulation modes are adopted for the balanced of neutral point potential,and the effects of the two modulation modes on the neutral point potential are compared and analyzed.The main works are listed as follows:1.The working principle of NPC three-level inverters is analyzed,and introduce the realization of carrier-based PWM(CBPWM),space vector PWM(SVPWM)and selected harmonic elimination PWM(SHEPWM)modulation strategy;2.The loss of DC-Link electrolytic capacitors of NPC three-level inverters is studied,and the reliability of aluminum electrolytic capacitor is emphatically studied and analyzed;3.The ripple current and ripple voltage flowing through the capacitor of NPC three-level inverter are analyzed,and the factors affecting the ripple current and ripple voltage are studied and verified;4.The factors affecting the neutral point potential imbalance of NPC three-level inverters are analyzed.Two novel modulation modes(single modulation wave CBPWM and double modulation wave CBPWM)are used to suppress the neutral point potential imbalance.Both modulation modes can suppress the neutral point potential fluctuation and improve the reliability of capacitors.Moreover the balancing method of neutral point potential based on double modulation wave CBPWM can eliminate the low frequency fluctuation of neutral point potential,which has positive significance on reducing the number of parallel capacitors of DC-Link and improving the reliability of capacitors;5.The hardware experiment platform of NPC three-level inverter is built,including the design and debugging of power supply circuit,driving circuit,main circuit,and signal sampling circuit.The control system based on DSP28335 is designed and programming in software Code Compose Studio 6.0.The designed experimental platform is tested. |
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