Improving DC Microgrid Dynamic Performance Using a Fast State-Plane-Based Source-End Controller

Abstract

DC microgrids interconnect load-end converters and distributed renewable energy sources within efficient and reliable networks that can operate independently from the main grid. When load-side converters tightly regulate their output voltages, they behave as constant power loads (CPLs) from the standpoint of the source-end converters. CPLs can cause instability within the network, including large voltage drops or oscillations in the dc bus during transients, which can lead to the collapse of the dc bus. Traditionally, the stability of CPL-loaded dc microgrids relies on the addition of passive elements, usually leading to increase in dc-bus capacitance. In these scenarios, source-end converter controllers are usually linear dual-loop proportional-integral compensators, which exhibit a limited dynamic response. State-plane-based controllers have been proposed to improve the dynamic response of stand-alone power converters loaded by CPLs. However, the operation of these converters in the context of a microgrid, where they interact with other converters of slower response, has not been studied thoroughly. This work proposes the use of a fast state-plane controller to replace one of the system's source-side controllers in order to improve three aspects of the microgrid operation: resiliency under CPL's changes, load transient voltage regulation, and voltage transient recovery time. Since the converter is operating within a microgrid, the controller incorporates a traditional droop rule to enable current sharing with the rest of the converters of the network. The system performance improvement is analyzed mathematically for a linear model, and a parametric analysis is performed for a more detailed model. Simulations and experimental results of a microgrid with three converters feeding a CPL are provided for different transients.