Effect of Communication Delay on Consensus-Based Secondary Controllers in DC Microgrid

In a dc microgrid, proportional current sharing among the sources is recommended while maintaining the system voltage within the limits. To achieve this, secondary control is often employed along with the primary droop controllers. As secondary controllers rely on communicated values, a larger delay in the communication channel can make the microgrid system unstable. Some specific studies on stability issues related to communication delay are reported in the literature using simulation studies, polynomial approximation of delay term, or by tracing the eigenvalues. However, these methods are either impractical or will give erroneous results. Considering increasing deployment of dc microgrids globally using different communication technologies and associated delays, a generic analysis is required in this area. In this article, existing secondary controllers are categorized into four classes. A comparison of these four classes based on the maximum delay before becoming unstable, termed as delay margin, is performed. Further, an expression for sensitivity of delay margin to system parameters at steady-state operation is derived. This analysis would help in determining the suitability of different system parameters for dc microgrid applications. The analytically obtained delay margin values are verified with the help of simulation studies as well as experiments.