Assessment of frequency defense mechanisms of an isolated power system
Department of Electrical Power Engineering and Environmental
Department of Electrical Power Engineering and Environmental Engineering, University of West Bohemia
Under frequency load shedding (UFLS) is a mechanism that helps the TSO to preserve frequency as much close to 50 Hz during negative frequency changes greater than 1 Hz (resp. 1 - 1.9 Hz, 4 levels) by disconnection of 50% of the connected load in all.
Unfortunately there are cases such as transition in to a stable island operation of a part of the electrical power system where the immediate disconnection of the connected load (via UFLS) during the primary regulation process might bring useless frequency collapse and black-start needs. This situation is presented on an example system where the generated electrical power in the area of electrical island is average but slightly lower than currently consumed electrical power in the moment of transition in to island operation. This state causes a transient process with frequency drop under the limit of first level of UFLS for few seconds resulting in the acceptable frequency range of 49 – 50 Hz.
Following paper describes generally situations that are threatened by the current settings of UFLS (given by the national grid code) based on real data from exemplary power system causing needless instability of the system and loss of operation primary thanks to sudden dangerous frequency jump over 50 Hz.
In the next part of the paper other approaches to isolated system frequency defense mechanisms are assessed (e.g. rate of change of frequency - RoCoF) in the view of safe transition in to the island operation.
In the end examples of use in north parts of United Kingdom, Ireland or Nigeria are presented as possible approach to the question of frequency defense in states such as island operation of the part of the Czech power system. Influence of these mechanisms is modelled as well as the UFLS.