GE T60 Instruction Manual page 613

CHAPTER 8: COMMISSIONING
3. Phase currents
4. Ground current
For example, if auxiliary voltage and phase currents are configured, the source uses the auxiliary voltage, not the phase
voltages or any of the currents.
When phase voltages or phase currents are used, the source applies a filter that rejects the zero-sequence component. As
such, the same signal must not be injected to all three phases, or the injected signal is completely filtered out. For an
underfrequency element using phase quantities, the phase A signal must be above the
Therefore, either inject into phase A only, or inject a balanced three-phase signal.
The static accuracy of the frequency threshold is determined by slowly adjusting the frequency of the injected signal about
the set pickup. If the T60 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 T60 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.
T60 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
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 8-6: Typical underfrequency element test timing
FREQUENCY ELEMENT TESTS
setting value.
MIN VOLT/AMP
8
8-15