Multilevel Inverters

Multilevel inverters synthesize a high voltage AC waveform from several isolated DC voltage levels. They are the standard choice for medium voltage grid integrations and high power industrial drives.

Core Concept

The diode clamped topology uses clamping diodes to limit the voltage stress on individual power devices. A three level inverter produces three distinct phase voltage levels. This reduces the harmonic distortion significantly.

Flying capacitor topologies use floating capacitors to clamp the voltage. The redundant switching states allow active balancing of the capacitor voltages. This topology requires a large number of bulky capacitors.

Cascaded H bridge inverters connect multiple single phase inverters in series. Each bridge requires an isolated DC source. This topology is highly modular and easily scales to higher voltage levels.

Key Formulas

Number of phase voltage levels m = 2N + 1 for cascaded H bridge with N sources. Line voltage levels = 2m - 1. Peak output voltage = N * Vdc.

Given: Cascaded H bridge with N = 4 isolated DC sources per phase.
Formula: Phase voltage levels = 2N + 1. Line voltage levels = 2(2N + 1) - 1.
Steps:
1. Phase levels = 2 * 4 + 1 = 9.
2. Line levels = 2 * 9 - 1 = 17.
Final Answer: Phase levels = 9, Line levels = 17.

Exam Tip: Memorize the level calculation formulas for different topologies. For a cascaded H bridge, N cells per phase yield 2N + 1 phase voltage levels.

Performance Parameters

• Drastically reduced voltage stress on motor insulation.
• Total harmonic distortion decreases as levels increase.
• Switching frequency can be kept very low.
• Removes the need for bulky step up transformers.
• Component count increases rapidly with the number of levels.

Quick Revision

• Excellent harmonic profile without heavy filtering.
• Diode clamped limits device voltage stress.
• Flying capacitor requires active voltage balancing.
• Cascaded H bridge requires multiple isolated sources.
• Used in medium voltage multi megawatt systems.
• Exam trap: Confusing the number of phase voltage levels with line voltage levels.