Coherency among Power System Devices
Ignacio Ponce, Rodrigo Bernal, Federico Milano
TL;DR
Addresses the need for a general coherency notion among heterogeneous power-system devices beyond synchronous machines. Introduces a model-agnostic, instantaneous coherency definition based on the difference between the complex-frequency (CF) of current injections, $\epsilon = \eta_x-\eta_y$, leveraging Clarke vectors. Derives device-specific coherence conditions for SMs, ZIP loads, and IBRs and presents a systematic procedure to verify coherence for any device model. Case studies on a two-machine system and the IEEE 39-bus system illustrate the approach, recover classical coherent groups, and demonstrate coherence across technologies for aggregation and control purposes.
Abstract
The paper proposes a novel general definition of coherency among power system devices of any type. The proposed approach is thus not limited to synchronous machines. With this aim, the paper shows that coherency can be formally based on the difference in the complex frequency of the current injections of any two devices electrically connected to the same grid. The proposed definition is model-agnostic, making it general and suitable for modern power systems composed of a heterogeneous mix of technologies. The paper also provides a systematic analytical procedure to study the properties that specific device models must satisfy to be coherent. Time-domain simulations are conducted in three case studies whose results illustrate the ability of our definition to evaluate coherency among any type of device.