This paper presents a distributed secondary control scheme for accurate reactive power sharing in an islanded multi-bus radial microgrid. The scheme employs consensus control to adaptively tune the virtual output impedance (VOI) into achieving reactive power correction. The adaptive VOI-based control structure is essentially non-linear. However, this work shows that the approximate range of stable coupling gain can be established by linearizing the problem about every probable operating point of the distributed energy resources (DERs). On the basis of islanding mode, it is also shown that only the dynamic virtual output impedance component is needed while the static component, which has been used extensively to date, can be nullified. It will also be shown that under the established gain tuning guideline, the virtual reactance typically results in quicker correction dynamics as compared to the virtual resistance. The proposed control scheme can realize an accurate power sharing among the DERs regardless of the microgrid topology, load condition, and communication delay (within the allowable limits defined by the consensus theorem). The study is carried out in conjunction with an islanded microgrid model modified from the IEEE 34 Node Test Feeder.

A transformer-less Buck-Boost direct current–direct current (DC–DC) converter in use for the fast charge of electric vehicles, based on powerful high-voltage isolated gate bipolar transistor (IGBT) modules is analyzed, designed and experimentally verified. The main advantages of this topology are: simple structure on the converter’s power stage; a wide range of the output voltage, capable of supporting contemporary vehicles’ on-board battery packs; efficiency; and power density accepted to be high enough for such a class of hard-switched converters. A precise estimation of the loss, dissipated in the converter’s basic modes of operation Buck, Boost, and Buck-Boost is presented. The analysis shows an approach of loss minimization, based on switching frequency reduction during the Buck-Boost operation mode. Such a technique guarantees stable thermal characteristics during the entire operation, i.e., battery charge cycle. As the Buck-Boost mode takes place when Buck and Boost modes cannot support the output voltage, operating as a combination of them, it can be considered as critically dependent on the characteristics of the semiconductors. With this, the necessary duty cycle and voltage range, determined with respect to the input-output voltages and power losses, require an additional study to be conducted. Additionally, the tolerance of the applied switching frequencies for the most versatile silicon-based powerful IGBT modules is analyzed and experimentally verified. Finally, several important characteristics, such as transients during switch-on and switch-off, IGBTs’ voltage tails, critical duty cycles, etc., are depicted experimentally with oscillograms, obtained by an experimental model.