Distribution system operation is becoming increasingly complex with installation of active components such as distributed solar photovoltaic (PV) generation, battery storage, and inverters. So far, specialized tools have been developed to capture a portion of the overall impact of distributed energy resources (DERs) to the distribution system. However, Distribution System Operators (DSOs) still lack visibility into the overall system conditions and capabilities. Furthermore, the impacts of increasing amount of DERs is not be limited at the distribution level but also influence the transmission grid. In this paper, we propose a co-simulation platform that enables simulation modules to interact in a plug-and-play fashion regardless of the platform they were developed in. Cyber-Physical Co-simulation Platform for Distributed Energy Resources in Smart Grids (CyDER) is a modular and scalable tool for power system planning and operation, focused on high penetration of DERs. The CyDER platform is based on the Functional Mock-up Interface (FMI) standard and provides several applications for power systems planning and analysis. A case study with a large amount of PV and electric vehicles loads on feeders is shown to demonstrate a successful interlink between the transmission and distribution system simulations.