A new approach for new challenges
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.
Improve grid performance with flexible HVDC technologyTransmission system operators need to handle increasingly complex grids and meet new demands. Space-saving, innovative HVDC PLUS systems help live up to challenges such as the reliable power supply of metropolitan areas, the improvement of grid stability, and grid access for offshore wind farms.
The smart answer to new challenges in complex transmission systems
- Full four-quadrant operation for independent, flexible active and reactive power control
- Flexible AC voltage control to support weak AC networks
- Robustness against AC network disturbances: Can be operated under asymmetrical AC network voltage conditions and can help compensate unbalanced loads
- Ability to contribute to short circuit current and to black-start shut-down networks
- HVDC PLUS technology establishes a firewall against cascading system disturbances
- Ideal support for AC grids with extremely low short-circuit levels and for the supply of fully passive systems
- Suitable for back-to-back operation as well as for long-distance cable and OHL HVDC transmission
- Can be integrated into multi-terminal HVDC systems and future HVDC grids
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.
Half-bridge topology (HB)
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
- the Trans Bay Cable between San Francisco and Pittsburg in the U.S., the world’s first HVDC system based on Siemens’ trendsetting MMC topology
- the INELFE electricity interconnection between France and Spain, the world’s biggest MMC-based HVDC system with two symmetrical monopoles and a capacity of 1.000 MW each
- various grid connections of offshore wind farms in Germany
Full-bridge topology (FB)
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.