Improving dynamic performance in dc microgrids using trajectory control

Direct-current (dc) microgrids interconnect dc loads, distributed renewable energy sources, and energy storage elements within networks that can operate independently from the main grid. Due to their high efficiency, increasing technological viability and resilience to natural disturbances, they are set to gain popularity. When load-side converters in a microgrid tightly regulate their output voltages, they are seen 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 load transients, which can lead to dc bus voltage collapse. Traditionally, the stability of CPL-loaded dc microgrids relies on the addition of passive elements, usually leading to dc-bus capacitance increase. In this scenarios, source-end converters controllers are usually linear dual proportional-integral (PI) compensators. The limited dynamic response of these controllers exacerbates the CPL behavior, which leads to the use of larger passive elements. Recent contributions focus on implementing control modifications on the source-end converter in order to improve the system performance under CPLs. Particularly, the use of state-plane based controllers has been studied for the case of a single dc-dc power converter loaded by a CPL, showing fast and robust transient performance. However, the microgrid problem, where these faster converters interface with others of a 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-end converters controllers in order to improve three aspects of the microgrid operation: resiliency under CPL's steps, 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 small-signal stability improvement of the whole system obtained by the addition of a single faster controller is analyzed for a linear model, and a parametric analysis demonstrates the improvements in a detailed model. Simulations and experimental results of a microgrid with three converters feeding a CPL prove the effectiveness of the technique for large-signal transients.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat