Experimental Comparison of Carrier and Space Vector PWM Control Methods for Three-Phase NPC Converters
Abstract.
Multilevel converters are powerful converters inthe medium-voltage high-power range. Inside this family ofconverters, the three-level neutral point clamped converter hasits own importance. The main control methods associated withthis converter are the carrier-based PWM with added DCcomponents and the space vector PWM. Two of the mostimportant features of the NPC converter are the neutral pointbalance and the output voltage spectrum. Both control methodsefficiently manage the two characteristics. This paperexperimentally compares these two issues and shows that thetwo methods produce almost the same results; the choicebetween one and the other should be made according to otherrequirements.
Keywords Neutral point voltage, Pulse width modulation, Spacevector modulation, Three-level converter, Voltagespectrum
1. Introduction
The smaller voltage steps created by a multilevelconverter lead to the production of higher power qualitywaveforms and also reduce the dv/dt stresses on the load,especially in AC motors, and reduce the electromagneticcompatibility problems [1].Some applications for these relatively new convertersinclude industrial drives, [2], Flexible AC TransmissionSystems (FACTS), [3], and vehicle propulsion. One areawhere multilevel converters are particularly suitable is inmedium-voltage drives. Although the main applicationsof multilevel converters are related with powerconversion from DC to AC voltages (inverter operation),they are now being also applied in rectifier operation,namely interfacing renewable energy sources with thegrid, [4], and interfacing high power drives in electrictraction [1].The most common multilevel converter topologies arethe diode-clamped, the flying capacitor, and the cascadedH-bridge converter, [5].The diode-clamped multilevel converter employsclamping diodes and series DC capacitors to produce ACvoltage waveforms with multiple levels, [1], [6]. Theconverter can be generally configured as a multileveltopology, but only the three-level converter, also referredas Neutral Point Clamped (NPC) converter, has foundwide application in medium-voltage high-powerapplications [1]-[2].The main features of the NPC converter include reduceddv/dt and Total Harmonic Distortion (THD) in its ACoutput voltages in comparison to the conventional twolevelconverter. As in any multilevel converter it can beused in the medium-voltage applications to reach acertain voltage level without series connection of powersemiconductors.The three-level NPC converter is the most widely usedtopology, which is offered by several drivemanufacturers, [5]. However, all of the three topologieshave specific advantages and disadvantages, [5].In this paper it is analyzed the main operatingcharacteristics of the NPC converter, schematicallyrepresented in Fig. 1, namely its operation under carrierbasedPWM control and space vector control withemphasis in the output voltage spectrum and the neutralpoint voltage control.
2. PWM Control of the Three-Phase NPCConverter
In a conventional two-level converter, carrier PWMtechniques control each phase leg of the converterseparately and the line-to-line voltage is a result of thephase legs voltage


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