30-03-2011, 03:08 PM
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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)
Output phase voltage can assume any voltage level by selecting any of the nodes
DCMI is considered as a type of multiplexer that attaches the output to one of the available nodes
Consists of main power devices in series with their respective main diodes connected in parallel and clamping diodes
Main diodes conduct only when most upper or lower node is selected
Although main diodes have same voltage rating as main power devices, much lower current rating is allowable
In each phase leg, the forward voltage across each main power device is clamped by the connection of diodes between the main power devices and the nodes
General features
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
Clamping diodes block reverse voltage (Dc1, Dc2, Dc3 block VDC/4, 2VDC/4 and 3VDC/4 respectively)
Unequal conduction duty of the power devices
DCMI with step modulation strategy have problems stabilizing/balancing capacitor voltages
Average current flowing into corresponding inner nodes not equal to zero over one cycle
Not significant in SVC applications involving pure reactive power transfer