The main objective of this paper is to reintroduce some physical knowledge into the mathematical representation
of the external part while keeping its size controlled. Hence, we propose to develop a dynamic equivalent based on the
concept of scale-invariance with the aim of coping with the complexity of smart grids. As many complex systems,
smart grids are expected to be scale invariant because of the self-similarities of their hierarchic structure [27,28]. Scale
invariance means that some patterns are repeated in the hierarchical arrangement of the different levels of the smart
grid. Scale invariance shows itself by power-law relations which appear between different levels of observation of the
system. A crucial parameter to characterize the scale invariance is the power-law exponent [3]. Power-law functions
have already been used for power systems in [4] and for ICT infrastructure in [7]. Usually, power-law functions
are used for characterization purposes and the value of their exponent helps to sort out the topological properties of
networks. In the present paper, scale invariance will be introduced into the dynamic equivalent to control its size while
representing the dynamic couplings inside the smart grid.
This paper presents an unconventional and innovative approach to obtain equivalent dynamic model of smart grids.
Of course, the authors do not pretend to be able today to solve the complete problem. So, the aim of this paper is
first to give the basis of our approach and shows its relevancy. For that reason, the presented works will focus on high
frequency equivalents of power systems only.
These equivalents are generally obtained by the identification of an approximation of the frequency response of the
Norton admittance of the external network seen from its point of connection with the study zone [18]. The paper will
aim at introducing scale-invariant properties into this approximation.
As previously mentioned, this paper will focus on the power systems only. Even if our work is expected to be
extended in the future to ICT networks, the preliminary developments have been based, for simplicity purpose, on the
continuous equations describing the electrical grids.
Then, the first section of the paper refers to the characterization of the scale invariance of power systems. The
second part details the scale invariance properties of fractal synthetic grids and realistic power systems. The third and fourth parts propose a new modeling approach based on the scale invariance and suitable for the approximation of the
frequency response of electrical grids. Results will be demonstrated for an IEEE 118-bus network [11].