Power Swings are a major reason for many of the latest blackouts all around the world. Regardless of cause, once a system has a blackout it often needs several hours to re-establish the system. This has a huge economical impact and has to be prevented in a modern smart grid. This has a huge economical impact and has to be prevented in a modern smart grid. Any sudden load change in a power system, such as a fault, line switching etc. causes the system's generators to adjust to the new condition. As they do so a power swing may occur. The changing generation mix of today's power grid has complicated power swing detection. Traditional blinders and timers cannot be relied on to detect a swing and initiate appropriate actions because the impedance trajectories often do not follow the typical path of a two machine power swing. A case involving the transmission system of Peru is presented. A single phase fault on a transmission line was tripped single pole by the line differential protection. During the single pole open condition the transmission line was not able to transmit the needed power and a power swing started. The impedances entered the Zone 2 distance element and lead to an unselective three pole trip of the distance protection because the unsymmetrical power swing was not detected by the relay. In general power swings are analyzed as a symmetrical phenomenon. However this approach does not consider power swing and network faults occurring at the same time. Because faults and swings are related, it is important to detect power swing that arises at fault condition or during single pole open conditions. On the other hand it is important to detect any kind of fault that may occur during a power swing so that the relay can quickly and selective trip. This paper describes a zero-setting power-swing detection method that is independent of network parameters and based on the verification of the impedance trajectories. Power swings with up to 10Hz swing frequency, are detected, also during single pole open conditions. A blocking logic prevents unselective trips by the distance protection. It will be shown how the described algorithm is able to detect this unsymmetrical power swing and hence would have prevented the blackout in the case presented.