This study demonstrates the capability and performance of the developed state‐of‐the‐art hardware‐in‐the‐loop (HIL) testing platform based on the real‐time digital simulator (RTDS). The evaluations of (i) the effects of thyristor‐controlled series compensation (TCSC) on the feeder protection first main (unit protection) and (ii) the performance of feeder protection second main (distance protection) of multi‐ended transmission circuits with adjacent TCSC are carried out. Realistic parameters of transmission lines and TCSC are used for system modelling, and practical physical relays are used for the testing. The potential impacts of TCSC on both unit protection and distance protection are analysed based on the simulation results and relay measurements from the developed RTDS‐based HIL testing platform.