MEASURED TIME-DELAY AND RE-SYNCHRONIZATION TIME IN DIGITAL MICROWAVE RADIO SYSTEMS

The transition from analog to digital transmission systems has been slower for Electric Power Utilities than other industrial sectors because of concern that digital transmission system delay times could adversely impact the response time of transfer trip protective relaying equipment. Alcatel Network Systems has completed a comprehensive testing program to measure the absolute time delay and various re-synchronization times through analog and digital transmission systems. The general conclusion is that the worst case absolute time delay of 577 microseconds, VF to VF, through the digital microwave system is approximately one half the delay for a typical analog system. Absolute time delays of 500-600 microseconds should cause no problems to transfer trip protective relaying systems. The re-synchronization times encountered after any event that causes a break in the digital signal, such as the switching of hot standby transmitters on microwave systems, are of tremendous concern to Electric Utilities. If a digital microwave system is designed using hot standby transmitters and the assumption is made that one catastrophic event will cause simultaneous disruption of a power transmission line and the microwave transmission system, then the signal disruption caused by switching hot standby transmitters must be taken into account in the design of the protective relaying system. These re-synchronization times range from 11-19 milliseconds for a DS-1 interface to 32-45 milliseconds for a VF interface through the digital channel banks.