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Augmented block-diagram model for investigating primary-control performance at low inertia

: Schittek, W.
: Mende, D.

Kassel, 2017, 169 pp.
Kassel, Univ., Master Thesis, 2017
Master Thesis
Fraunhofer IWES ()
Bilanzmodell; Schwungmasse; Frequenz; Primärregelung

The continuously rising capacities of renewable feed-in have the potential to supersede the utilization of fossil fuels. However, the rotational inertia of synchronous generators is essential for the sound interrelation between active power and frequency in the interconnected system. Low inertia poses challenges for the primary frequency control. From the viewpoint of the optimized use of resources, the approach of innovative power-electronic contributions to primary control aiming at lowering the indispensable minimum amount of inertial generation might have some potential. Aggregated block-diagram models are well suited for rough investigations on the behaviour of primary control in one or more control areas or even a whole synchronous area. This work investigates how to augment a block-diagram model in order to conduct simulations of low-inertia situations on the interconnected-system level. Representing in a model a whole synchronous area like the ENTSO-E Continental-European system by the inertia of only one or a few aggregated generators would fail to reproduce the two-dimensional propagation of frequency variations, which becomes crucial at low inertia. However, a one-dimensional projection of a two-dimensional system can preserve the crucial information necessary to obtain valid simulation results. By reproducing in a model the different placements of control areas as well as their different interconnections, it becomes obvious that low inertia changes the character of frequency variations. Modelling the rotor-angle variations of generators becomes crucial for achieving valid results. Exemplary load-step simulations, directed towards the farther future and the worst case, show that low inertia imposes additional requirements on primary control. These depend on the load-step location (e.g. off-centre) and the strength (e.g. weak) of the interconnecting tie lines. The most helpful contributions to resolving severe regional power imbalances would be expected to intervene within a fraction of a second and to be able to deploy a multiple of the proportionate regular primary control reserve during the first second after the load step. The main result of the work is a Simulink model covering the interconnected-system level and suitable for rough investigations on primary-control performance at low inertia. The work is complemented by a compilation of characteristics that help assess the quality of the results. Sensitivity considerations for a lot of parameters give an impression about the many degrees of freedom of the model and constitute a guideline for worst-case parameterization. The intended scope of the model are first investigations on innovative ideas for power-electronic contributions to primary control, including the first steps to finding the most promising control strategies.