A Loop Gain-Based Technique for Online Bus Impedance Estimation and Damping in DC Microgrids

In modern dc microgrids, several feedback-controlled power electronic converters are connected to the common dc bus. Although the control loops of each individual converter are designed with good stability margins, the interconnection of multiple source and load converters can cause stability and performance concerns, due to potential interactions. Therefore, in order to ensure the desired dynamic performance of the interconnected power converter system, an interesting approach is to perform online stability monitoring of the dc bus, and to properly damp the dc bus impedance, which has been demonstrated to ensure system-level stability and performance. In order to accomplish that, this article first derives a representation of the dc bus impedance in terms of voltage (or droop) loop gain of the source-side converter. Second, under certain simplifying assumptions it provides an estimate for the peak value of bus impedance - an indicator of system dynamic behavior - based on the phase margin. Third, it proposes to continuously monitor the peak value of bus impedance by only injecting a single sinusoid in the voltage (or droop) loop. The monitored value can then be used to autotune the voltage regulator, in order to keep the bus impedance in a well-known allowable impedance region. The proposed monitoring and stabilization technique eliminates the need for the time consuming and memory intensive impedance measurement tasks. This technique is verified by simulation and experimental results on a laboratory prototype system of interconnected power converters.