Tailwind for the integration of offshore wind power

VDE FNN presents test criteria for HVDC transmission systems and offshore wind turbines, thus creating an important basis for the grid stability of inverter-based systems.

Offshore wind is in expansion - the German government recently raised its target of 15 gigawatts (GW) of installed capacity to 20 GW by 2030. The federal government, the coastal states and the transmission system operators (TSOs) have adopted a joint roadmap for the expansion of offshore wind power. Key for this development is an efficient transport of the electricity generated to the customers. High-voltage direct current (HVDC) transmission systems have proven to be particularly effective for this purpose thanks to their low transmission losses over long distances. VDE FNN presents test criteria for these systems in order to evaluate their grid-stabilizing behaviour even before they are installed.

"More and more large power plants are expected to be going offline any, due to the coal and nuclear power phase-out. So far, they have made a decisive contribution to grid stability," explains Heike Kerber, Managing Director of the Network Technology & Network Operation Forum in the VDE (VDE FNN). "We must fill this gap. If the HVDC transmission systems meet certain criteria, such as grid-forming behaviour, they can also have a significant contribution to grid stability. This is an important basis for evolving the current power system to integrate a higher percentage of renewable energies".

Pioneering work for high-voltage direct current (HVDC) transmission

As for other areas in life: one thing is to set requirements and another is to have a way to verify them. VDE FNN supports manufacturers and operators of HVDC transmission systems as well as network operators by providing a concrete implementation guideline to obtain the necessary certification in accordance with the connection conditions for HVDC transmission systems (VDE-AR-N 4131). This includes, for example, an exemplary test sequence, exemplary test networks and scenarios as well as a visualized reference behaviour of systems for grid-forming control methods specified by network operators. This allows the behaviour of plants in the system to be simulated in advance.
Heike Kerber: "With these criteria, VDE FNN has done pioneering work for high-voltage direct current transmission. The properties of the HVDC transmission systems can be verified based on simulations and tests". The tests can be used to demonstrate the grid-forming behaviour of HVDC transmission systems, which has a positive impact on grid stability.

The methods and verification procedures for grid-forming behaviour described in the FNN Guideline are in principle applicable to other power electronic generation systems and are not limited to HVDC transmission systems and offshore wind turbines alone. The implementation guideline is available as FNN Guideline "Grid forming behaviour of HVDC systems and DC-connected PPMs" in the VDE Shop in German and English.

Technical knowledge: stress-inducing behaviour

In high-voltage direct current transmission, direct current is converted into alternating current at the end of the transmission and fed into the existing network. This task is performed in the HVDC transmission systems by inverters using power electronics, which can be controlled in two ways:

Grid-following: here the output current is the reference variable, i.e. the output current is controlled. The system usually behaves like a current source. From the point of view of grid stability, this behaviour cannot provide decisive functions which are necessary for the operation of the grid.

Grid-forming: The inverters control their voltage and frequency; the reference variable is the output voltage. Generation plants with this type of inverters have grid-forming properties and can replace conventional power plants with these properties.