This paper presents an out-of-step protection algorithm based on angle derivatives, which makes use of wide-area measurements and can be applied on arbitrary tie-lines in electrical power systems. The developed algorithm uses PMU measurements that are taken at both ends of a transmission line. Based on the changes of the electrical quantities in the power system, the algorithm detects unstable system conditions. Thus, the developed solution is settingless and can be easily applied where an out-of-step condition is expected. The concept is deployed by using an industrial controller and tested by conducting numerous hardware-in-the-loop simulations. Additionally, recorded data from actual out-of-step events in the Icelandic power system are used to validate the developed algorithm. The performance of the implemented method is compared against the traditional impedance-based out-of-step protection methods. The results confirm that the proposed algorithm detects out-of-step conditions more reliably and faster than the traditional impedance-based solutions.