The Modular Multilevel Converter, introduced for HVDC by Siemens Energy more than a decade ago, is the well-established standard for high voltage, high power VSC applications today.
In Siemens Energy HVDC PLUS systems, one modular multilevel converter comprises three Single-phase inverter. One converter comprises three identical phase units with two converter arms, and each converter arm contains a number of sub modules supporting the full DC voltage. Each sub modul contributes only a small voltage step and is controlled individually. Practically speaking, each module within an MMC is a discrete voltage source with a local capacitor to define its voltage step without creating ripple voltage distortion across the converter’s other phases. This way it is possible to achieve the required sinusoidal AC and smooth DC side output voltage waveforms without excessive harmonic distortion and HF noise.
The insulated-gate bipolar transistors (IGBTs) at the heart of the sub modules are fully controllable. This enables modular multilevel converters to absorb and generate reactive power independently from active power up to the converter rating. The output currents can be varied over the complete operating range in a smooth, linear way. This enables independent and very flexible control of active and reactive power, which supports the connected AC grid.