Equivalent models are essential for time-domain simulation and harmonic stability evaluation of renewable power plants. Such harmonic instability may co-occur at the plant-level and unit-level

or it may only occur at the unit-level

manifesting as local resonances. However

conventional equivalent methods can only simulate 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 conventional equivalent methods cannot accurately simulate unit-level stability is studied. 2) A double-machine equivalent method is proposed. The model used in this method is a minimum-scale equivalent model capable of simulating 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

$N-1$

generation units for simplification

while another single-machine model

$M_2$

represents the remaining one gener

ation unit for simulating unit-level stability. 3) Furthermore

a practical improvement approach of the double-machine equivalent method is proposed for taking into account operating point differences and complex connection conditions. Finally

the proposed method is verified by simulations and experiments.