Utilization of supercapacitors in adaptive protection applications for resiliency against communication failures: A size and cost optimization case study

Microgrids' adaptive protection techniques rely on communication signals from the point of common coupling to adjust the corresponding relays' settings for either grid-connected or islanded modes of operation. However, during communication outages or in the event of a cyberattack, the relays' settings are not changed. Thus adaptive protection schemes are rendered unsuccessful. Due to their fast response, supercapacitors, which are present in the microgrid to feed pulse loads, could also be utilized to enhance the resiliency of adaptive protection schemes against communication outages. Proper sizing of the supercapacitors is therefore important in order to maintain a stable system operation and the cost of the protection scheme. This paper presents a two-level optimization scheme for minimizing the supercapacitor size along with optimizing its controllers' parameters. The latter will lead to a reduction of the supercapacitor fault current contribution and an increase in that of other AC resources in the microgrid in the extreme case of having a fault occurring simultaneously with a pulse load. It was also shown that the size of the supercapacitor can be reduced if the pulse load is temporarily disconnected during the transient fault period. Simulations, which were also experimentally verified, showed that the resulting supercapacitor size and the optimized controller parameters from the proposed two-level optimization scheme were feeding enough fault currents for different types of faults and minimizing the cost of the protection scheme.