A double-machine equivalent method for evaluating the plant- and unit-level stability of hybrid grid-connected renewable power plants

Equivalent models are essential for the time-domain simulation and harmonic stability evaluation of renewable power plants. Such harmonic instability may co-occur at the plant-level and the unit-level, or it may only occur at the unit-level, manifesting as local resonances. However, the conventional equivalent methods can only simulate the plant-level instability, but cannot consider such unit-level instability. To address this problem, the main contributions of this paper are as follows: 1) Based on the impedance matrix and the eigenvalue analysis, the reason why the conventional equivalent method cannot accurately simulate the unit-level stability is studied. 2) A double-machine equivalent method is proposed. The model used in this method is the minimum-scale equivalent model capable of simulating the plant-level and unit-level stability simultaneously. It uses two single-machine models to represent a generation cluster containing n generation units. A single-machine model M 1 represents the n — 1 generation units for simplification, while another single-machine model m 2 represents the remaining one generation unit for simulating the unit-level stability. 3) Furthermore, a practical improvement approach of the double-machine equivalent method is proposed for taking into account the operating point differences and complex connection conditions. Finally, the proposed method is verified by simulations and experiments.