We propose a Convertible Static Transmission Controller (CSTC) concept that enables coordinated power flow control with emphasis on large penetration of renewable energy resources based transmission in a meshed network. CSTS can be connected across the substation power transformer and reconfigured for different modes of operation to perform as a versatile transmission controller with several functions including: power flow control for transmission of renewable resources, and as a transformer back-up for disaster management and/or life extension purposes. Different connecting configuration options, i.e. shunt-shunt, series-shunt, and series-series can be obtained. In this paper, we demonstrated the viability of the proposed concept using Typhoon HIL400 ultra-high fidelity Hardware-in-the-Loop (HIL) system in three different modes of operation. HIL simulations are used to verify the validity of the proposed control architecture for CSTC operation during both normal and unbalanced power system conditions for different connecting configurations.