9-14 T60 Transformer Protection System GE Multilin9.5 FREQUENCY ELEMENT TESTS 9 COMMISSIONING99.5FREQUENCY ELEMENT TESTS 9.5.1 TESTING UNDERFREQUENCY AND OVERFREQUENCY ELEMENTSUnderfrequency and overfrequency protection requires techniques with subtle testing implications. Whereas most protec-tion is designed to detect changes from normal to fault conditions that occur virtually instantaneously, power system inertiarequires frequency protection to pickup while the frequency is changing slowly. Frequency measurement is inherently sen-sitive to noise, making high precision in combination with high speed challenging for both relays and test equipment.Injection to a particular T60 frequency element must be to its configured source and to the channels the source uses for fre-quency measurement. For frequency measurement, a source will use the first quantity configured in the following order:1. Phase voltages.2. Auxiliary voltage.3. Phase currents.4. Ground current.For example, if only auxiliary voltage and phase currents are configured, the source will use the auxiliary voltage, not thephase 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. Assuch, the same signal must not be injected to all three phases, or the injected signal will be completely filtered out. For anunderfrequency element using phase quantities, the phase A signal must be above the MIN VOLT/AMP setting value. There-fore, either inject into phase A only, or inject a balanced three-phase signal.Figure 9–6: TYPICAL UNDERFREQUENCY ELEMENT TEST TIMINGThe static accuracy of the frequency threshold may be determined by slowly adjusting the frequency of the injected signalabout the set pickup. If the T60 frequency metering feature is used to determine the injected frequency, the metering accu-racy should be verified 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.The injected frequency should smoothly ramp through the set threshold, with the ramp starting frequency sufficiently out-side the threshold so the relay becomes conditioned to the trend before operation. For typical interconnected power sys-tems, the recommended testing ramp rate is 0.20 Hz/s.FrequencyPickupfrequencyRelay conditioning timeInjection frequencySource frequencyTracking frequencySource frequency calculation delayUnderfrequency element detection timeUnderfrequency element pickupUnderfrequency element operateset “pickup delay”Time831771A1.CDR
