Ensuring system stability with 100% renewable energies

Renewable power plants must replace large power generating units in the power system and adopt their vital role - not only in terms of delivering power but also through their technical capabilities and services for the system. VDE FNN ensures that renewable power plants are developed accordingly and in a forward-looking manner, by developing technical requirements. In this way, a system is designed that optimally supports the integration of renewables equipped with vital system-supporting capabilities.

Background

Under the term "operation along the characteristic", the current Technical Connection Rules (TCRs) define requirements for power plants that are not subject to any stability criteria. They ensure neither grid-forming nor system-supporting behaviour. The interconnected power system or separated subsystems can tolerate a certain penetration of power plants that are not of the grid-forming kind as long as the stability in total is ensured. The generation units without grid-forming behaviour are expected to at least include system-supporting capabilities. However, it is fundamental that controller structures and parameterisations are determined based on defined stability criteria. At the heart of this approach is the frequency control for active power balancing and the voltage control.

Further development of regulation

The basis for system stability is the separate network operability of power-generating and power-consuming power plants (storage units, PGCUs). Therefore, new requirements are currently being defined in the TCRs under the cooperation of the FNN committees. System-supporting capabilities for new plants play a vital role here. In future, these will be anchored as a minimum requirement in the TCRs. Since Germany plans to phase out conventional power plants until 2030, renewable power plants must be equipped with the capabilities of these power plants. This affects grid-forming capabilities in particular. Therefore, new requirements must be defined and implemented quickly to allow new power-generating and power-consuming power plants to provide these capabilities in the future. Such requirements are currently being developed jointly by all players in the VDE FNN and will be published in an FNN Guideline in 2023.

EU-aspects

VDE FNN contributes to an in-depth and technical discussion at the European level to define and address aspects potentially endangering system stability. System-supporting behaviour of power plants is critical for the safe operation of the interconnected grid and should be taken into account in the revision of the European Network Codes, in particular the NC RfG. The VDE FNN Infopaper "Future requirements for power system stability" was submitted to the ACER consultation as a proposed amendment to the NC RfG in November 2022.

Ensuring system stability: how generation plants behave in a system-supporting way

In a nutshell

The FNN Guideline "Grid-forming & system-supporting behaviour of power-generating modules" describes requirements and proofs for power plants and continuously controllable PGCUs to ensure separate network operability as well as fast and stable over-frequency regulation in case of severe disturbances. These capabilities are prerequisite for ensuring system stability. The required control engineering measures assume that each power plant and PGCU allows stable separate network 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 basic approach is to revive control characteristics that were still in use decades ago, even for primarily efficiency-optimised plants. The described control behaviour is only fully requested in the case of rare disturbance events. The power plant and PGCU are not additionally stressed during normal operation. In addition to the definition and detailed justification of the requirements, it is shown how proofs are to be designed within the certification process.

Context

The discussion results of the internal FNN expert workshop on the topic of "Active power adjustment in the event of over- and under-frequency and controller stability” (October 2018) have revealed a need for action concerning the further development of the Technical Connection Rules (VDE-AR-N 4105/4110/4120/4130). The requirements in the aforementioned TCRs for “operation along the characteristic“ and the corresponding verifications cannot exclude the possibility that unstable behaviour of generation plants may occur. In the case of the formation of a sub-network (e.g. as a result of a system split), which consists of a part of the transmission network and subordinate 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 “). In the VDE-AR-N 4130, the proof of separate network operability ensures this stability. In the VDE-AR-N-4105/4110/4120, corresponding requirements and verifications are currently not mandatory.

Target groups

Grid operators: can assess the importance of the requirements for power-generating and power-consuming power plants concerning sperate network operability and adequately approve them in grid connection situation.
Plant operators and project developers: can consider the requirements and certification processes for stable separate network operability into the planning and implementation concepts in time.
Manufacturers of power-generating and power-consuming power plants: are informed about the necessary developments of components for power-generating and power-consuming power plants with corresponding properties and requirements for the verifications.

Benefits

The FNN Guideline offers in particular manufacturers, operators and planners the opportunity to implement the expected amended requirements in the TCRs in time and to plan for these in connection with the certification process of the plants.