GE G30 Instruction Manual page 602

TESTING CHAPTER 9: COMMISSIONING
Figure 9-1: Typical underfrequency element test timing
The static accuracy of the frequency threshold is determined by slowly adjusting the frequency of the injected signal about
the set pickup. If the G30 frequency metering feature is used to determine the injected frequency, verify the metering
accuracy by checking it against a known standard (for example, the power system).
To accurately measure the time delay of a frequency element, a test emulating realistic power system dynamics is
required. Let the injected frequency ramp smoothly through the set threshold, with the ramp starting frequency sufficiently
outside the threshold so that the relay becomes conditioned to the trend before operation. For typical interconnected
power systems, the recommended testing ramp rate is 0.20 Hz/s.
The required delay time is the interval from the point the frequency crosses the set threshold to the point that the element
operates. Some test sets can measure only the time from the ramp start to element operation, necessitating the
subtraction of the 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 the threshold and subtract 1 second from the test set time reading of ramp start to relay operation.
Note that the G30 event records only show the "pickup delay" component, a definite time timer. This is exclusive of the time
taken by the frequency responding component to pickup.
G30 oscillography can be used to measure the time between the calculated source frequency crossing the threshold and
element operation; however, this method omits the delay in the calculated source frequency. The security features of the
source frequency measurement algorithm result in the calculated frequency being delayed by two to four cycles (or longer
with noise on the input). In addition, oscillography resolution is 0.004 Hz, which at 0.20 Hz/s corresponds to a delay of 20
ms. Do not use the tracking frequency in timing measurements, as its algorithm involves phase locking, which purposely
sets its frequency high or low to allow the G30 sample clock to catch-up or wait as necessary to reach synchronism with
the power system.
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9-2 G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL