High penetration of variable renewable energy resources to distribution networks brings new challenges to frequency control in transmission systems; notwithstanding, coordinated exploitation of some widely distributed resources in distribution networks could support overall system frequency control. We developed a centralized control scheme to utilize existing solid oxide fuel cell systems in distribution networks to mitigate frequency deviation in the transmission system. This novel control scheme minimizes curtailing renewable resources. It appropriately prioritizes adjusting active or reactive power of medium voltage connected fuel cell systems to efficiently provides frequency support, while keeping the renewable sources connected. A co-simulation platform was also developed to allow for analyzing the interactions between transmission and distribution networks. It enhances simulation capability through parallel distributed real-time simulations. This platform enables ex-ante analysis of the impact of control strategies of distribution system on transmission system. A set of experimental results on European transmission and distribution network benchmarks demonstrated the performance of the developed primary frequency control scheme using solid oxide fuel cell systems.