Editing Power electronics (section)

=== Multilevel inverters ===

[[File:Three-Phase Voltage Source Inverter Circuit Schematic.jpg|thumb|left|'''FIGURE 10''': Three-level neutral-clamped inverter]]

A relatively new class called multilevel inverters has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters because the power switches connect to either the positive or the negative DC bus.<ref name=Trzynadlowski /> If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave.<ref name=Rashid3 /> For this reason multilevel inverters, although more complex and costly, offer higher performance.<ref name=Trzynadlowski /> A three-level neutral-clamped inverter is shown in Figure 10.

Control methods for a three-level inverter only allow two switches of the four switches in each leg to simultaneously change conduction states. This allows smooth commutation and avoids shoot through by only selecting valid states.<ref name=Trzynadlowski /> It may also be noted that since the DC bus voltage is shared by at least two power valves, their voltage ratings can be less than a two-level counterpart.

Carrier-based and space-vector modulation techniques are used for multilevel topologies. The methods for these techniques follow those of classic inverters, but with added complexity. Space-vector modulation offers a greater number of fixed voltage vectors to be used in approximating the modulation signal, and therefore allows more effective space vector PWM strategies to be accomplished at the cost of more elaborate algorithms. Due to added complexity and the number of semiconductor devices, multilevel inverters are currently more suitable for high-power high-voltage applications.<ref name=Trzynadlowski />
This technology reduces the harmonics hence improves overall efficiency of the scheme.