What Are Multilevel Converters?

Multilevel converters are power electronic circuits that synthesize an output voltage waveform by combining multiple smaller dc voltage levels into a staircase approximation of a sinusoid, rather than switching abruptly between two voltage rails as in a conventional two-level converter. Each additional level in the output waveform reduces the harmonic content and the voltage stress on the switching devices, enabling operation at medium and high voltages with commercial semiconductor switches rated for a fraction of the total bus voltage. The increased output quality also reduces the size and cost of passive filters and transformers, making multilevel topologies attractive wherever power quality and efficiency are important.

The concept emerged from research in the 1970s and was systematically formalized in the 1990s. Three classical topologies, the neutral-point-clamped (NPC) converter, the flying-capacitor (FC) converter, and the cascaded H-bridge (CHB) converter, have dominated the literature and commercial deployment for industrial and utility-scale applications.

Core Topologies

The neutral-point-clamped converter, introduced by Nabae, Takahashi, and Akagi in 1981, uses clamping diodes to connect the midpoints of series-connected switch pairs to a neutral bus, producing a three-level phase voltage with reduced dv/dt. The flying-capacitor topology, developed by Meynard and Foch in 1992, replaces the clamping diodes with pre-charged capacitors, allowing the circuit to produce additional voltage levels with the same number of switches and distributing the voltage stress more evenly. The cascaded H-bridge topology assembles independently powered full-bridge cells in series; each cell contributes positive, zero, or negative output levels, and adding more cells increases the level count and the voltage rating proportionally. A survey of multilevel converter operation and comparison of these three topologies shows that each topology presents distinct trade-offs in terms of capacitor balancing difficulty, switch count, and control complexity.

Modulation and Control

The modulation strategy governs how the switching instants are chosen to track the desired output waveform and minimize harmonic distortion. Multicarrier PWM methods use N-1 triangular carriers offset in phase or level to control an N-level converter, with each carrier pair driving one switching cell. Space-vector modulation extends naturally to multilevel converters by expanding the two-dimensional hexagonal diagram of a two-level inverter into a finer lattice with more switching states to choose from. Selective harmonic elimination (SHE) pre-computes switching angles offline to cancel specific harmonic orders, an approach valued in high-power drives where switching frequency must remain low to limit device losses. Capacitor voltage balancing is a persistent challenge in NPC and FC topologies and requires closed-loop monitoring of individual capacitor voltages with active redistribution of switching duty.

Modular Multilevel Converters

The modular multilevel converter (MMC), proposed by Rainer Marquardt around 2003, extended the cascaded cell concept by connecting series strings of half-bridge or full-bridge submodules directly across the dc bus without a separate transformer. This structure scales from tens of kilowatts to over one gigawatt by adding submodules, making it well suited for high-voltage direct-current (HVDC) transmission and large static synchronous compensators. Each submodule contains a floating capacitor that must be kept within a voltage tolerance band; arm current circulation between the upper and lower legs is used for balancing. The IEEE Transactions on Industrial Electronics survey on multilevel converters documents how submodule design variations address DC fault current interruption, an important requirement for offshore HVDC grids.

Applications

Multilevel converters have applications across a wide range of power systems, including:

• Medium-voltage variable-speed drives for pumps, fans, and compressors
• HVDC transmission links connecting offshore wind farms to onshore grids
• Grid-connected photovoltaic and energy storage inverters
• Static VAR compensators and STATCOMs for reactive power management
• Electric traction systems in rail and marine propulsion