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Synchronous optimal pulsewidth modulation for marine electric propulsion drives

Abstract:

In typical marine electric propulsion drives, classical two-level inverters or multilevel inverters (MLIs) are used to convert the dc output of the front-end converter into a variable-voltage-variable-frequency source for speed control of the propulsion motor. In general, higher device switching frequency (s) leads to motor currents with lower distortion. However, in high power marine applications, where propulsion motors are in the multi-megawatt range, lower inverter switching frequency is preferred for reducing switching losses. Synchronous optimal pulsewidth modulation (SOP) is an emerging low switching frequency technique which enables the reduction of switching frequency without compromising total current harmonic distortion (THDi). In this paper, the principle of SOP for three-level (3L) waveforms is explained before implementation results are shown for a 3L neutral-point-clamped (NPC) MLI. The performance of SOP is compared with conventional space vector pulsewidth modulation (SVPWM) for driving propulsion motors onboard a diving support vessel (DSV). Results obtained show that SOP (s = 200 Hz) has similar THDi performance as SVPWM (s = 1 kHz), even with a reduction of 80% in switching frequency. Based on the actual load profiles of the DSV, the thermal losses of SOP (s = 200 Hz) and SVPWM (s = 1 kHz) are compared. Results show that inverter losses with SOP is lower than SVPWM for any power level. Moreover, the reduction in losses with SOP becomes more significant at higher power levels. In the case of a DSV, the highest percentage reduction in inverter losses (almost 16%) can be realized with SOP when the vessel is operating in dynamic positioning (DP) mode.