In my previous blogs, I gave a brief introduction to Voltage Source Converter (VSC) HVDC as well as a comparison between HVDC classic and VSC HVDC. Today, I will discuss the aspects that make VSC HVDC a key technology for smart transmission grids.

Key issues with future smart transmission grid include: the connection of large scale remote renewables—such as solar thermal plants and off shore wind farms, the increased need for power flow steering as (renewable) generation, and the fact that (active controlled) loads fluctuate faster and more often. A more precise and fast reactive power control keeps the voltages within the specified band and increased stability of the power system together with the availability of the power delivery.

VSC HVDC technology is perfectly suitable to fulfill the above mentioned needs, as it is a technology for the meshed network as we have today. It permits continuous and independent control of real and reactive power. Reactive power, controlled independently at each terminal, can be used for dynamic voltage regulation to support the interconnecting AC system following contingencies and, thus, increasing system stability. A VSC HVDC system has a low contribution to the short circuit (current) level and can be connected to a weak AC network, or to a network where no generation is available. The forced commutation with VSC technology even permits black start; the converter can be used to create a balanced three phase AC system similar as a synchronous machine.

Because of the highly controllable VSC converter, the contribution to the smart transmission grid can be summarised as:
• Precise control of the power that flows on prescribed transmission routes
• Secure loading of transmission lines to levels close to their thermal limits
• Greater ability to transfer power between controlled areas, allowing generation reserve margin to be reduced
• Damping of power system oscillations and consequently increasing stability of the grid
• Prevention of cascading outages by limiting the effects of faults and equipment failure, due to its ability to act as a “firewall” between two grid areas.

Due to the advantages, with respect to dynamic voltage control and black start capability, the VSC HVDC transmission is capable to supply remote locations (islands) using submarine cables, without any need of expensive local generation. The ability to control reactive power, as well as the AC voltage, makes VSC HVDC very suitable for transmitting power from off shore wind farms to the mainland AC grid. The use of HVDC cable systems is not limited by distance, as in the case of AC cable systems. Moreover, the losses are reduced when an HVDC cable system is used. Mainly due to Right of Way (RoW) and land use constraints, the compact VSC HVDC technology represents a feasible solution to city power infeed and combines increased capacity and voltage support.

VSC HVDC will certainly be a key technology in future smart transmission grids. It is a mature technology and already has a proven track record for power ratings less than or equal to 1,000 MW. If we compare VSC HVDC to High Temperature Superconducting (HTS) cables (see my blog from January 17th: Is there a future role for superconducting cables in Smart Grids?) it becomes clear that both technologies are not competing, but rather complement each other.

By: Peter Vaessen

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