A New Controller Design for Eliminating the Negative Impedance Increase Effect Caused by Constant Power Loads in DC Microgrids
DOI:
https://doi.org/10.13052/dgaej2156-3306.4158Keywords:
Constant power load, DC microgrid stability, Model Predictive Control, disturbance observer, negative incremental impedanceAbstract
The growing use of power electronic loads in DC microgrids has turned out to be a significant source of stability challenges, mainly due to the detrimental increased impedance behaviour of Constant Power Loads (CPLs) that can cause the danger of especially high and low DC bus voltage fluctuations and even voltage collapse. The proposed control strategy is based on a continuous-time Model Predictive Control (MPC) approach. A Disturbance Observer (DOB) is integrated to enhance robustness. This combination effectively mitigates the negative incremental impedance effect caused by constant power loads (CPLs) and improves DC bus voltage stability. A detailed nonlinear model of a solar Photovoltaic (PV)-battery-based DC microgrid providing power for a CPL is constructed, and the unstable condition is analytically expressed. Simulation results reveal that, during open-loop operation, the voltage at the DC bus drops from 400 V to around 265 V with a voltage deviation of nearly 135 V and a negative incremental impedance of approximately −27 Ω, which indicates a voltage collapse tendency. The DC bus voltage goes back to its reference value with virtually no steady-state error, smaller overshoot, and faster settling time, all while voltage oscillations are effectively suppressed when the MPC-DOB controller is in place. The comparative results also indicate better damping, smoother control current profiles, and improved robustness against sudden changes of the CPL power, which altogether prove that the proposed control strategy has a very positive impact on the stability of DC microgrid operation.
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