The growing need for a highly reliable electrical supply for critical applications has spawned the development of local distribution systems with the extended use of power electronics. The advantages are numerous. The downside is that the higher interactions within the system can easily lead the system to operate outside its limits due to instabilities. For three-phase systems, system stability can be evaluated by generalized nyquist criterion using the impedance at the system interface. To identify the required impedance for real hardware system, an algorithm and its implementation is proposed for online measure in the real power system.