This paper presents a distributed parameter line model based on the steady-state transmission line equations, which is com- pared to the lumped parameter model. The proposed model is integrated into the distance relay scheme, directly indicating whether a fault is within or outside the protection zone. Exper- imental tests are carried out on an RTDS (real time dynamic system),whichrealisticallysimulatesa645-km,1000-kVUHV transmissionlinethat iscurrently beingconstructedinChina. It is shown that the lumped parameter model causes up to 17% error regarding fault distance estimation while the error asso- ciated with the proposed line model is only about 3%, well below the 5% limit set in the corresponding Chinese regula- tion [13]. The distance relay developed using the described line model,hasthereforebeenadoptedasoneofthemainprotection schemes to be included in the practical protection strategy.