MULTI LEVEL INVERTERS (MLI)

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MLI (2)


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)


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)


Overcoming capacitor voltage balancing problem

Line-to-line voltage redundancies (phase voltage redundancies not available due to structure)

Carefully designed modulation strategies

Replace capacitors with controlled constant DC voltage source such as PWM voltage regulators or batteries

Interconnection of two DCMIs back-to-back with a DC capacitor page link (suitable for specific applications only – UPFC, frequency changer, phase shifter)