Stability Analysis of Two Parallel Converters With Voltage–Current Droop Control

Voltage-current (V-I) droop control has been proposed for real and reactive power sharing. Compared with the conventional droop control using real power/frequency and reactive power/voltage, it enhances stability since adding a V-I droop is equivalent to providing a resistor. However, for a system with parallel converters with V-I droops, oscillations are observed when small droop coefficients are adopted. In this paper, a multi-input multioutput (MIMO) model of the system is derived in the frequency domain. This model is based on the dq-reference frame and consists of converter control and the power network model, which is represented by an admittance matrix. Linear system analysis is carried out to identify the root causes of oscillations in both grid-connected mode and autonomous mode. Analysis results are validated by simulation results of detailed model-based systems built in Matlab/Simpowersystems.