9-2 N60 Network Stability and Synchrophasor Measurement System GE Multilin9.1 TESTING 9 COMMISSIONING9The desired delay time is the interval from the point the frequency crosses the set threshold to the point the element oper-ates. Some test sets can measure only the time from the ramp start to element operation, necessitating the subtraction ofthe pre-threshold ramp time from the reading. For example, with a ramp rate of 0.20 Hz/s, start the ramp 0.20 Hz before thethreshold and subtract 1 second from test set time reading of ramp start to relay operation.Note that the N60 event records only show the “pickup delay” component, a definite time timer. This is exclusive of the timetaken by the frequency responding component to pickup.The N60 oscillography can be used to measure the time between the calculated source frequency crossing the thresholdand element operation; however, this method omits the delay in the calculated source frequency. The security features ofthe source frequency measurement algorithm result in the calculated frequency being delayed by 2 to 4 cycles (or longerwith noise on the input). In addition, oscillography resolution is 0.004 Hz, which at 0.20 Hz/s corresponds to a delay of20 ms. The tracking frequency should not be used in timing measurements, as its algorithm involves phase locking, whichpurposely sets its frequency high or low to allow the N60 sample clock to catch-up or wait as necessary to reach synchro-nism with the power system.
