Phasor Measurement Units are one of the main building blocks of the Synchronized Measurement Technology, which is technologically enabler of the Wide Area Monitoring, Protection and Control (WAMPAC) system. The existing IEEE Std C37.118-2005 standard, determines how phasors' information should be forwarded to Data Concentrators, in where data obtained from PMUs are visualized, analyzed and used for specific WAMPAC applications. This standard defines steady-state performance, sampling frequency and frame format requirements for PMUs. However, this standard does not define the transient performance of PMUs. It is expected, that WAMPAC system should support the system operation particularly during transient conditions, so that the understanding of transient performance of PMUs is critically important. In this paper, a new concept for synchronized testing of PMUs and WAMPAC applications during steady state and transient conditions will be presented. An attempt to define standardised voltage and current signals will be given, as well. Furthermore, an example how to improve the quality of existing simulation tools for testing applications based on synchronized sampling will be described. A methodology for accessing the accuracy of time synchronization will be demonstrated using real testing devices and GPS synchronizing clock.