Power electronics applications in high-voltage electric power systems date from the early 1970s when converters for HVDC systems bagan using thyristors to replace mercury arc valves. Since the mid-1970s all new HVDC systems have employed thyristor valves. Advances in power electronics have extended the application of fast switching currents to AC systems as well, resulting in the technology commonly known as FACTS, Flexible AC Transmission Systems. This document is intended to provide guideliens into the kinds of interactions that can occur and the means of analysis and design that are available to assure that a coordinated response is achieved and adverse interactions are avoided.