Generalized Droop Control For Mixed Impedance Microgrid

Abstract

When the microgrid is isolated from the main grid, grid-forming units control the frequency and voltage to maintain the stability of the microgrid. Hierarchical control structure is utilized to provide these functionalities through three levels of control. Droop control is used at the primary level to sustain the frequency and voltage of the microgrid and to maintain the power sharing between units according to their power rating. Units generate active and reactive power which is appropriate for their rating by varying their terminal frequency and voltage. However, the microgrid is implemented in a distribution network where the voltage level is medium. In such voltage level, the X/R ratio approach unity hence, the frequency and voltage are related to the active and reactive power through line parameters. Therefore, to supply active power or reactive power, both frequency and voltage must be tuned accordingly. In this paper, a normalized dependency matrix is derived to identify the per unit relationship between the frequency and voltage with the active and reactive power. Then, droop control utilizes the dependency matrix to achieve an accurate relationship between frequency and voltage with active and reactive powers. To implement the dependency matrix the X/R ratio is required to calculate the line impedance phase angle. Phasor Measurement Unit (PMU) communicates the grid voltage and current through synchrophasor communication to calculate the active and reactive power consumed by of the line. Then, the line parameters are estimated and used to generate the normalized dependency matrix. The proposed droop controller is simulated in MATLAB/SIMULINK on inductive, mixed impedance, and resistive grids to verify its functionality. The results of the proposed controller are discussed in the simulation result section.