White Paper SSTI - Consideration of possible effects of the operation of HVDC systems in the network on the shafts of turbo sets in power plants

The number of power-electronic actuators in the power grids in Europe and especially in Germany will continue to increase significantly in the next few years in order to ensure the integration volatile, renewable energies and the security of supply.

The transport of power via high voltage direct current transmission (HVDC) will become increasingly important with the growing feed from both offshore and of onshore wind turbines.

Existing plants, e.g. thermal power plants and their control systems are interacting with the control of the HVDC systems. The interaction of torsional vibrations of the shaft train of a turbo set of power plants and the HVDC systems connected via the transmission network is designated as Sub Synchronous Torsional Interaction (SSTI). If the design and parameterization of the control systems in operation does not take into account the characteristics of the other components, inadmissible stress could occur on the shaft train in the power plants.

The White Paper was jointly developed by the German Transmission System Operators (TSO) and power plant operators and by involvement of universities and producers.

As an introduction, the current network modifications in the German transmission system are presented in an overview. Further, the processes and procedures for the integration of HVDC systems through exchange of information and cooperation between TSOs, HVDC operators and power plant operators are explained. Necessary analyzes to secure the design of the HVDC systems are described and specified. The White Paper concludes with an overview of the requirements for HVDC control and protection systems.

Thereby the basic contexts of the subject Sub Synchronous Torsional Interaction are described and joint recommendations to the procedure of avoidance of inadmissible SSTI stresses are shown.