23-05-2012, 12:20 PM
MULTILEVEL INVERTERS
MLI.ppt (Size: 334.5 KB / Downloads: 174)
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)
MLI (6)
For three-phase DCMI, the capacitors need to filter only the high-order harmonics of the clamping diodes currents , low-order components intrinsically cancel each other
For DCMI employing step modulation strategy, if nl is sufficiently high, filters may not be required at all due to the significantly low harmonic content
If each clamping diode has same voltage rating as power devices, for nl-level DCMI,
number of clamping diodes/phase = (nl-1) x (nl-2)
Each power device block only a capacitor voltage