Under the term "operation along the characteristic", the Technical Connection Rules (TCR) define requirements for generation plants which are not subject to stability criteria and do not ensure the deployment of grid-forming or system-supporting capabilities. A certain penetration of generation plants without grid-forming properties into the Continental Synchronous Area or into separated subsystems is still tolerable as long as stability is ensured in its totality. However, generation units without grid-forming properties must at least have system-supporting properties. In any case, it is required that controller structures and parameterisation are determined based on defined stability criteria with a focus on frequency control (active power balancing).
The FNN Guideline "Grid-forming & system-supporting behaviour of power-generating modules" describes requirements and verification procedures for generation plants or power-generating modules (PGM) as well as continuously adjustable storage systems and consumption plants (PGCM) to ensure separate Network operability as well as fast and stable overfrequency regulation in the event of major grid disturbances. The necessary control measures are based on the assumption that each PGM and PGCM allows stable separate grid operation or island grid operation within a limited control range and that restrictions of the active power gradient are only applied outside this limited operating range. The rationale behind this is to revive control characteristics that were still in use decades ago, even for primarily plants optimized for efficiency. The described control behaviour is only fully requested in the case of rare disturbance events, which does not place additional stress on PGM and PGCM during normal operation. In addition to the detailed description of the requirements, the Guideline describes how verification procedures are to be designed within the certification process.
VDE FNN experts representing different relevant stakeholders have contributed with their expertise and experience to the development of the FNN Guideline. This FNN Guideline can provide early guidance for requirements which are yet to be specified in the TCR.
Considering the European system
This Guideline provides the basis for in-depth technical discussions at European level in order to define and address potential threats to system stability. The system-supporting behaviour of generation plants is critical for the secure operation of the European Synchronous Grid and should be taken into account in the revision of the European Network Codes, in particular the NC RfG. The document and its insights will be presented in EU expert circles to support the further development of the requirements for generation plant as well as continuously controllable storage systems and consumption plants.
The English translation of the FNN Guideline is available in the VDE Shop since end of February 2022.
A bit of history
Areas for further development of the Technical Connection Rules (VDE-AR-N 4105/4110/4120/4130). were identified during an FNN Workshop on the topic of "Active power adaptation in the event of over- and underfrequency and controller stability" (October 2018). The TCR requirements for "operation along the characteristic" and the corresponding verification procedures cannot exclude the possibility that generation plants may exhibit an unstable behaviour. In the case of the formation of a separate network (e.g. due to System Split) consisting of a part of the transmission network and lower voltage distribution networks, the plants in the distribution network must also remain stable in the case of a frequency-related power adjustment ("operation along the characteristic"). The proof of separate network operability ensures this stability in the TCR for EHV VDE-AR-N 4130, the proof of sub-grid operability ensures this stability. In the TCR for HV/MV/LV (VDE-AR-N 4105/4110/4120), corresponding requirements and verification procedures are currently not mandatory.
This Guideline provides important insights for the evolution of the power grid as a whole and is specially aimed to support stakeholders in Germany in the development of their processes and technology for the future grid.
- Network operators: can better assess the importance of the requirements for PGM and PGCM for separate network operability and adequately examine them during different grid events.
- Plant operators and project developers: can incorporate the requirements and certification processes for stable separate network operability into the planning and implementation concepts in advance.
- Manufacturers of PGM and PGCM: receive a timely kick-start for necessary development of PGM and PGCM components considering network-supporting properties and verification requirements.
Transfer to technical regulation
The findings of the FNN Guideline will be considered in the further development of the requirements for generation plants in the planned revision of the TCRs. This Guideline is only a first step: VDE FNN will continue to define technical requirements in consensus with the relevant stakeholders and thus make an important contribution to the system stability of the German and European power grid.