An Embedded Impedance Measurement for DC Microgrids Based on a Lock-In Amplifier

The advancements in DC microgrid architectures show a trend towards the use of multiple converters from distributed sources (photovoltaic, wind, battery) connected to different loads (passive and active) coexisting in a weak network. Since these topologies lack the large inertia of big AC generators, the stability of the microgrid can be compromised by the presence of tightly regulated active loads that can behave as constant power loads (CPLs). These loads present a characteristic negative incremental resistance, in contrast to the positive resistance of the passive loads. Detecting the nature of the loads as seen by the source converters, as well as the magnitude, can lead to improvements in the stability of the system, as indicated by the Middlebrook criterion. In this work, a novel incremental impedance measurement technique based on a Lock-In Amplifier (LIA) is presented. The LIA uses a perturbation of a known frequency to extract the equivalent incremental impedance of the load circuit very accurately. The characteristics of this method enable embedded implementation in real-time in the power converter, providing an extra tool to improve the stability of the converters in the microgrid. The proposed embedded instrument allows the incremental impedance of the network to be accurately measured, as seen by the source converter, with reduced complexity and sensor requirements. Simulations of the proposed measurement technique are presented in order to illustrate its behavior. Experimental results for different kinds of loads and transients are presented to validate the proposed strategy.