Simulations and field tests indicate that unacceptable motor-terminal overvoltages and waveform shapes can occur in the normal operating range with high-output-voltage multilevel drive systems. These waveforms (if unattended) can result in premature motor insulation failures. A case study of an 11-kV multilevel system is presented. Simulations and calculations confirm the theory of resonance overvoltages. Different solution possibilities are analyzed. Further simulations and investigations are performed to determine the optimal carrier frequency. Test results confirm that the proposed modification has the desired effect, with waveforms well within limits stipulated by international standards. Simplified equations and recommendations are provided to determine suitable application solutions. Further simulated resonance case studies are presented, considering the effect of the system configurations, motor size, and cable length. Suitable carrier-frequency selection methods are presented to solve the problem.

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multilevel inverters. The application configuration of the Abstract-Simulations and high voltage field tests indicate that system is described in [3]. A synchronisation, deunacceptable and unexpected motor terminal overvoltages and synchronisation and process control scheme is applied to waveform shapes can occur in the normal operating range with * * * -multilevel drive systems. These waveforms (if unattended) would iperven p oes a bilit (to sroie the aodj.st an have resulted in premature motor insulation failures. A case Xspee driv mtofo certin di tr cotons) A study of an 11 kV multilevel system is discussed. Simulations and also to facilitate soft starting of other motors. A calculations confirm the theory of resonance overvoltages. synchronisation reactor is applied to limit the current during Different solution possibilities are analyzed. Further simulations synchronization. System modelling to simulate the and investigations are performed to determine the optimal carrier phenomenon is described. Simplified equations are developed.frequency. Test results confirm that the proposed modification has the desired effect; with waveforms well within limits soti to address the problem are outlined and finally on stipulated by international standards. Simplified equations and site measurements prove the effectiveness of the proposed recommendations are provided to determine suitable application solution without any additional hardware.solutions. II. INITIAL TESTS Index Terms-AC motor drives, equivalent circuits, harmonicTests were done to determine the maximum voltage, analysis, load modeling, overvoltage protection, resonance, waveform shape and maximum du/dt values at the motor rotating machine testing.terminals. Similar tests were also performed before and after

New Voltage Source Inverter technology now exists with high output voltage (≥11kV) suitable for high power synchronous motor applications. The technology with an advanced control system can be used as a multiple motor soft starter and adjustable speed drive for selected motors with significant benefits. Design, commissioning and operation experience is described for a first of its kind installation with three synchronous motors rated 11kV 17MW. The overall control and communication system is described with specific attention to safety, redundancy and process availability. New synchronization and desynchronization scheme experience is described. Unique synchronous motor protection and excitation control schemes for both adjustable speed and line operation are outlined. Site recordings prove system ride-through capability in terms of voltage dips and bypass of internal power cells. Conclusions are given which support new voltage source technology as a viable alternative to conventional load commutated inverter technology for certain large synchronous motor applications. Suggestions are made for additions to present medium voltage adjustable speed drive standards.Index Terms -adjustable speed drive, synchronous motors, voltage source inverter, medium voltage, load commutated inverter

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