MotorLab Research Projects

Control of Cascaded Multilevel Inverters




The multilevel power inverter has become extremely popular in recent years considering advantages over traditional inverters. Advantages include lower switching losses, higher voltage capability, higher power quality, and better electromagnetic compatibility. Amongst the available multilevel topologies, the cascaded multilevel has distinct advantages due to a compounding effect of the voltage levels. According to this effect, when two multilevel inverters are cascaded through the load connection, the result is an operation with an equivalent number of levels that is the product of the two inverters. This results in exceptional power quality.

Typically, each inverter requires an isolated power source. This leads to difficulties in applications where high power density is required. Under this research project, a novel control was developed for the cascaded inverter system whereby only one dc source is required. The control was developed for the cascade-3/3 inverter (where two three-level diode-clamped inverters were involved in the cascade connection). According to this control methodology, two voltage levels are sacrificed and the redundant switching states are utilized so that one of the inverters can be supplied from a capacitor source. It was theoretically shown that the resulting control operates with seven-level performance and can regulate the capacitor voltage under any load conditions. Measurements were carried out on a 20-kW system. The resulting phase voltages exhibited 9% voltage THD without filtering.

An extension to this research involved operation of the cascade-3/3 inverter in the over-distention mode. In this mode of operation, two isolated dc sources are required. However, the inverter can operate effectively as an eleven-level inverter for low modulation indices and as a pseudo eleven-level inverter when the modulation index is above 91.7% of it's maximum. Laboratory measurements were carried out to demonstrate the over-distention control.


K.A. Corzine, M.W. Wielebski, F.Z. Peng, and J. Wang, "Control of Cascaded Multi-Level Inverters," Proceedings of the IEEE International Electric Machines and Drives Conference, Madison WI, June 2003. Accepted for publication in IEEE Transactions on Power Electronics.

X. Kou, K.A. Corzine, and M.W. Wielebski, "Over-Distention Operation of Cascaded Multilevel Inverters," Proceedings of the IEEE International Electric Machines and Drives Conference, Madison WI, June 2003.


A Unique Fault-Tolerant Design for Flying Capacitor Multilevel Inverter

Survivability is a key component for future power electronic systems. This project addresses the concept of "micro-survivability" which is survivability at the component level with the theory that a system constructed of survivable components will have overall improved reliability. In this case, a motor drive was constructed to survive an open- or short-circuit fault on any device within the inverter. The flying capacitor multilevel inverter was selected for this study since it contains per-phase switching redundancy which can be used along with joint-phase switching redundancy to provide post-fault operation. It was shown, through suitable control algorithms, that four-level performance could be maintained if one switch is faulted in any phase. The main principle of operation was to utilize the healthy phases to maintain capacitor voltages in the faulty phase. Further advancements in the control algorithm demonstrated that the inverter could operate with multiple faults if the modulation index was lowered. Laboratory measurements demonstrated the expected operation for a single phase fault.


X. Kou, K.A. Corzine, and Y.L. Familiant, "A Unique Fault-Tolerant Design for Flying Capacitor Multilevel Inverters," Proceedings of the IEEE International Electric Machines and Drives Conference, Madison WI, June 2003.  Accepted for publication in IEEE Transactions on Power Electronics.


A Cascaded Multi-Level H-Bridge Inverter Utilizing Capacitor Voltages Sources


The cascaded H-bridge inverter has been adopted by industry as a viable solution for medium voltage drive systems. With this topology, each phase voltage is developed by a series connection of H-bridge power cells. Each cell is made from low-voltage devices and provides small voltage steps. The resulting connection yields good power quality at the motor terminals.

One recent development in cascaded H-bridge inverters is the use of different dc voltages on the power cells which results in more voltage levels. Another advancement is the use of multilevel power cells which further increases the number of voltage levels. However, isolated dc sources are still required for each power cell in each phase. In this research project, a control method was developed which allows operation of the cascaded H-bridge inverter with only one isolated dc source per phase. Simulation examples were developed where up to three three-level power cells per phase were used (the cascade-3/3/3H inverter). Another system involving a five-level and a three-level cell (the cascaded-5/3H inverter) was simulated and also constructed in the laboratory. This inverter effectively operates as a nine-level inverter requiring only one isolated dc source per phase for the five-level cells. The laboratory measurements demonstrated control operation and high power quality.


K.A. Corzine, F.A. Hardrick, and Y.L. Familiant, "A Cascaded Multi-Level H-Bridge Inverter Utilizing Capacitor Voltages Sources," Proceedings of the IASTAD Power Electronics Technology and Applications Conference, Palm Springs CA, February 2003.


Four-Level Back-to-Back System


Future power systems will incorporate a greater degree of power electronic converters.  The nature of these applications demands high-voltage high-power operation suitable for multi-level power conversion.

The main research focus has been on a four-level back-to-back rectifier / inverter system. The primary benefits of this system are high power quality, high-voltage capability, low electromagnetic compatibility concerns, and unity power factor rectifier operation. Laboratory work has focused primarily on pulse-width modulation control and high-voltage operation. Presently, 2-kVdc operation has been demonstrated using 1.2-kV IGBT's. This suggests that currently available 3.3-kV IGBT's could be used in the same four-level structure to meet the IPS 6-kV bus voltage. Modeling efforts have included development of both detailed and average-value models of the back-to-back system which include the effects of capacitor voltage balancing. The laboratory setup has also been used to validate novel four-level dc/dc power converters. Future research plans involve advanced multi-level control development.


K.A. Corzine and J.R. Baker, "Multi-Level Voltage-Source Duty-Cycle Modulation: Analysis and Implementation," IEEE Transactions on Industrial Electronics, volume 49, number 5, pages 1009-1016. October 2002.

K.A. Corzine and J.R. Baker, "Reduced Parts-Count Multi-Level Rectifiers," IEEE Transactions on Industrial Electronics, volume 49, number 4, pages 766-774, August 2002.

K.A. Corzine, J. Yuen, and J.R. Baker, "Analysis of a Four-Level DC/DC Buck Converter," IEEE Transactions on Industrial Electronics, volume 49, number 4, pages 746-751, August 2002.

K.A. Corzine and S.K. Majeethia, "Analysis of a Novel Four-Level DC/DC Boost Converter," IEEE Transactions on Industry Applications, volume 36, number 5, pages 1342-1350, September/October 2000.



Development and Hardware Construction of a Three-Phase 400Hz Controlled Rectifier


The increasing number of high-dynamic power electronic loads in aircraft power systems necessitates higher performance ground based power supplies for electronic service. The present diesel cart power source used for this application is based on an uncontrolled generator / rectifier system and does not meet the transient requirements. The purpose of this project was to develop a controlled rectifier capable of supplying power electronic loads with suitable transient recovery. A detailed simulation of the system was created for evaluation of various control designs. A fully digital control was developed and the system was successfully tested at 70-kW.


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