Hierarchical control strategy for a three-phase 4-wire microgrid under unbalanced and nonlinear load conditions

Area of Science:

• Electrical Engineering
• Power Systems
• Control Theory

Background:

• Microgrids require robust control strategies for stable operation under diverse load conditions.
• Unbalanced and nonlinear loads pose significant challenges to microgrid voltage and power quality.
• Existing control schemes often struggle with accurate power sharing and harmonic mitigation.

Purpose of the Study:

• To propose an improved hierarchical control strategy for three-phase, four-wire microgrids.
• To enhance power sharing accuracy and voltage/current harmonic distortion reduction.
• To validate the control strategy's effectiveness in both islanded and grid-connected modes.

Main Methods:

• A two-layer hierarchical control: primary (multi-loop, harmonic compensator, droop) and secondary (reactive power compensator, frequency restoration).
• Utilized DIgSILENT Power Factory software for simulations.
• Conducted laboratory testing for practical performance validation.

Main Results:

• Achieved zero reactive power sharing error and zero frequency steady-state error.
• Demonstrated superior voltage control with fast transient response and low voltage total harmonic distortion (THD).
• Reduced voltage THD from >5.1% to <2.7% and current THD from >21% to <2.4% under nonlinear loads.

Conclusions:

• The proposed hierarchical control strategy effectively manages unbalanced and nonlinear loads in microgrids.
• The strategy significantly improves power quality, transient response, and steady-state errors.
• Outperforms conventional filter-based control schemes in microgrid stability and performance.