Novel technique for maintaining balanced internal DC link voltages in diode clamped five-level inverters

The paper considers the problem of providing four regulated DC sources for use in a diode clamped five-level inverter. The preferred approach is to provide a single DC link supply and subdivide this into five levels using four series connected capacitor banks which have equally balanced voltages. The paper first considers the use of modified output switching strategies to redistribute energy through the DC link capacitors and maintain correct voltage balance, but concludes that this technique is restricted to outputs with a low modulation. The paper then presents a new technique whereby the voltage across each of these capacitors is maintained constant using a switching circuit: the balancer circuit. The design and control of the balancer circuit is described and its operation is validated on an 11kW prototype vector controlled induction motor drive. Full four-quadrant operation is demonstrated. This arrangement eliminates the need for complex isolated supply arrangements typically found in multilevel drives. The balancer circuit takes the form of a fourth output leg with the addition of two inductors. This circuit is certainly viable for medium voltage drive applications.