Virtual Impedances Optimization to Enhance Microgrid Small-Signal Stability and Reactive Power Sharing

Abstract

Microgrid instability poses critical issues to the power delivery following a load change or a tripping event. In island operating mode lack of grid intensifies this challenge. This study aims at controlling several converter-based distributed generations (DG) sharing the power in an island microgrid (MG). At first, the microgrid model including virtual impedances and phase-locked loop (PLL) is introduced. Afterwards a novel small-signal stability analysis for island microgrids is proposed. Finally, an optimization algorithm based on particle swarm optimization (PSO) is proposed to design the virtual impedances. The optimization algorithm analyzes all possible operating points and aims at maximizing the microgrid stability index while keeping the reactive power mismatches at minimum level. The fractional objective function facilitates reaching at these objectives simultaneously. The proposed optimization algorithm is implemented in two separate case-studies and the corresponding virtual impedances are drawn in any microgrid. On the other hand, The voltage drops are checked as a condition in the optimization process. The results drawn from two separate case-studies verify that the proposed algorithm effectively maximizes the microgrid stability index and minimizes the reactive power mismatches.

Enhancing Microgrid Small-Signal Stability and Reactive Power Sharing Using ANFIS-Tuned Virtual Inductances

“…The virtual impedance optimization in [19] has minimized the reactive power mismatches among converters and enhanced the microgrid small-signal stability by an off-line PSO algorithm. Following the analysis of microgrid stability domain in [20], it has been suggested that the proportionalderivative reactive power controller enhances the microgrid stability margin.…”

“…• The permissible range and optimal values for virtual impedance are affected by the microgrid load levels, this concept has not been considered in the recent researches such as [18] and [19].…”

“…i loadD1 , i lineD1 ) are presented in global reference frame. The reference frame of converter 1 is chosen as global reference frame and the transformation between local and global frames are explained in [7] and [19].…”

“…The idea is that the ANFIS network is trained to perform optimally in all operating conditions. The training data is produced by PSO optimization in different load changing scenarios [19]. When the training data is collected, ANFIS networks are trained for all converters in microgrid.…”

Microgrid Stability Analysis Considering Current State-Feedback

“…The microgrid small-signal stability analyzed in [2] was a milestone for microgrid stability analysis. The advanced researches such as [3] optimize the virtual impedances based on small-signal stability analysis. However, the phase locked loop (PLL) as a necessary unit to control and coordinate several DGs in the MG must be taken into account, either comparing the units phases to the phase of one local DG or have a central phase reference [4].…”