MULTILEVEL INVERTERS


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Main feature

Ability to reduce the voltage stress on each power device due to the utilization of multiple levels on the DC bus
Important when a high DC side voltage is imposed by an application (e.g. traction systems)
Even at low switching frequencies, smaller distortion in the multilevel inverter AC side waveform can be achieved (with stepped modulation technique)
Diode-clamped multilevel inverter (DCMI)
Extension of NPC
Based on concept of using diodes to limit power devices voltage stress
Structure and basic operating principle
Consists of series connected capacitors that divide DC bus voltage into a set of capacitor voltages
A DCMI with nl number of levels typically comprises (nl-1) capacitors on the DC bus
Voltage across each capacitor is VDC/(nl-1)
( nl nodes on DC bus, nl levels of output phase voltage , (2nl-1) levels of output line voltage)

Modular structured multilevel inverter (MSMI)

Referred to as cascaded-inverters with Separate DC Sources (SDCs) or series connected H-bridge inverters
Structure and basic operating principle.