Summary
The rapid adoption of renewable energy sources has accelerated DC microgrid deployment as efficient alternatives to traditional AC systems. However, power electronic converters introduce significant stability challenges through active impedance characteristics exhibited by constant power loads. This thesis addresses stability challenges in multi-converter DC microgrids through passivity-based design principles and impedance measurement techniques.
Problem Statement
DC microgrids rely on power electronic converters to interface renewable sources, energy storage, and loads. These converters can exhibit active incremental input impedance, violating traditional stability assumptions and causing oscillatory instability when interacting with grid resonances. The challenge intensifies in multi-converter systems where multiple feedback loops interact through the common DC bus, creating complex dynamics that traditional design approaches struggle to manage.
Research Contributions
This research makes four primary contributions:
Passivity-Based Stability Analysis: A theoretical framework was developed linking converter impedance characteristics to system stability. The analysis establishes that requiring positive real impedance above a threshold frequency enables stable interconnection regardless of system complexity.
Active Damping Control Strategy: A digital control approach was investigated that modifies converter impedance characteristics above a design frequency while maintaining low-frequency regulation performance.
Frequency Threshold Investigation: Through analysis of typical component values in practical systems, a frequency threshold range of 200–400 Hz was identified for stability considerations.
Impedance Measurement Platform: A broadband impedance characterization system was developed using pseudo-random binary sequence excitation as an alternative to traditional frequency response analysis methods.
Validation and Findings
The research employed theoretical analysis, simulations, and experimental verification. Simulation studies demonstrated significant improvements in oscillation reduction and settling time compared to uncompensated systems.
Analysis established that enforcing positive real impedance conditions above the threshold frequency prevents destabilizing interactions between converters and grid resonances. The control strategy successfully modifies impedance characteristics while preserving dynamic performance.
The measurement system demonstrated capability to characterize both power supply and device impedance from single measurements, though accuracy limitations indicate need for enhanced calibration approaches.
Impact
This work provides analytical frameworks for designing stable DC microgrids accommodating high renewable energy penetration. The passivity-based approach offers scalable stability analysis methods that remain valid as system complexity increases, addressing barriers to widespread DC microgrid deployment.
The research contributes to emerging standardization efforts and provides guidance for both converter manufacturers and system integrators in ensuring stable operation.