CHAPTER 7: COMMANDS AND TARGETS COMMANDS MENUL30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 7-57When enabled, the function continuously compares the present time with the pre-set PMU ONE-SHOT TIME. When the twotimes match, the function compares the present sequence number of the measured synchrophasors with the pre-set PMUONE-SHOT FRACTIONAL SEC. When the two numbers match, the function freezes the synchrophasor actual values and thecorresponding protocol data items for 30 seconds. This allows manual read-out of the synchrophasor values for the pre-set time and pre-set sequence number (via the faceplate display, supported communication protocols such as Modbus orDNP, and the EnerVista UR Setup software).When freezing the actual values, the function also asserts a PMU ONE-SHOT OP FlexLogic operand. This operand can beconfigured to drive an output contact and trigger an external measuring device such as a digital scope with the intent toverify the accuracy of the PMU under test.With reference to the following figure, the PMU one-shot function (when enabled) controls three FlexLogic operands:• The PMU ONE-SHOT EXPIRED operand indicates that the one-shot operation has been executed, and the present time isat least 30 seconds past the scheduled one-shot time• The PMU ONE-SHOT PENDING operand indicates that the one-shot operation is pending; that is, the present time isbefore the scheduled one-shot time• The PMU ONE-SHOT OP operand indicates the one-shot operation and remains asserted for 30 seconds afterwardsWhen the function is disabled, all three operands are de-asserted.The one-shot function applies to all logical PMUs of a given L30 relay.Figure 7-1: PMU one-shot FlexLogic operands7.1.5.1 Testing accuracy of the PMUThe one-shot feature is used to test accuracy of the synchrophasor measurement. GPS-synchronized tests sets perform asimilar function to PMUs; instead of measuring the phasor from physical signals with respect to the externally providedtime reference, they produce the physical signals with respect to the externally provided time reference, given the desiredphasor values. Therefore the GPS-synchronized test sets cannot be automatically assumed more accurate than the PMUsunder test. This calls for a method to verify both the measuring device (PMU) and the source of signal (test set).With reference to the following figure, the one-shot feature can be configured to trigger a high-accuracy scope to captureboth the time reference signal (rising edge of the 1 pps signal of the IRIG-B time reference) and the measured waveform.The high-accuracy high-sampling rate record of the two signals captured by the scope can be processed using digitaltools to verify the magnitude and phase angle with respect to the time reference signal. As both the time reference and themeasured signals are raw inputs to the PMU under test, their independently captured record, processed using third-partysoftware, is a good reference point for accuracy calculations. Such a record proves useful when discussing the test results,and can be retained as a part of the testing documentation.
