Transmission systems need to live up to new challenges in the wake of decarbonization, decentralization, and digitalization: A growing number of large-scale offshore wind farms needs to be connected to the grids, strained AC grids must be stabilized to ensure reliable operation, energy-hungry megacities require low-loss power supply solutions. This is where Siemens’ outstandingly compact HVDC PLUS technology comes into its own. Based on trendsetting modular multilevel voltage-sourced converters, it is blazing the trail for modern trends in HVDC transmission by making it possible to combine the advantages of HVDC transmission with extra benefits such as AC voltage control, black start capability, and other functions that improve the performance and security of any transmission system.
Invented by the Siemens HVDC experts a couple of years ago, the modular multilevel voltage-sourced converter for HVDC has become an industry standard today.
In Siemens HVDC PLUS systems, modular multilevel converters usually come in six-pulse bridge topology. One converter comprises three identical phase units with two valves, and each valve contains a number of power modules supporting the full DC voltage. Each power module 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 power 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.
The half-bridge topology is primarily used for cable connections and for back-to-back systems. The DC voltage of a half-bridge power module is always controlled and the power capacitor can be connected to the terminals in one polarity. Consequentially, the DC voltage is always higher than the AC voltage.
HVDC PLUS systems with half-bridge topology have proven their high efficiency and reliability in DC cable connection projects such as
In full-bridge topology, the DC voltage is independent of the AC voltage, because the power capacitors can be connected to the terminals in either polarity. This way, the DC voltage can be reduced to zero and even entirely reversed, while current control on the AC and DC sides is maintained even under short-circuit conditions.
The full-bridge topology is the ideal solution for HVDC systems that work with overhead lines. It makes it possible to clear DC line faults safely by reverting the voltage for a short time for current extinction and electric arc deionization. It also enables HVDC system operation at reduced DC voltage levels, which helps tackle problems caused by polluted line insulators. The break time and the rate-of-rise of the DC voltage during recovery can be variably adapted to the dielectric strength – a flexible strategy that significantly increases supply security.
Siemens has been successfully using the full-bridge circuit for Sitras® traction frequency converters as well as for numerous FACTS and industrial installations with SVC PLUS® since 2009.
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