Generalized Nyquist MIMO Stability of Frequency Regulation Services in Power Networks

Frequency regulation provision by local distributed generators and storage units is becoming a trending paradigm even in medium and low-voltage power networks. Nevertheless, the electrical characteristic of the connection point (expressed by the local input impedance at the grid interface) plays a major role in the provision of a stable regulation service, potentially jeopardizing a robust support from the unit and leading to the injection of spurious harmonics from the electronic converter. Thus, this paper analyses the stability conditions associated to a typical power-frequency regulation service at the light of the Multi Inputs - Multi Outputs (MIMO) stability theory, to define acceptable operating ranges for the parameters as a function of several aspects like: the equivalent network impedance, the converter synchronization loop and feedforward compensation. The goal is to show how the MIMO technique allows a more precise identification of the stability boundaries compared to a simpler Single Input - Single Output formulation.