- Model 933A Portable Power Sentinel™ with AS0077600 Remote GPS Receiver or equivalent IRIG-B source
All Arbiter precision time clocks may be used as a timing source in the use of synchrophasor measurements. Popular models include:
- Model 1088B GPS Satellite Clock - highest accuracy (40 ns typical), most flexible solution
- Model 1093A/B/C GPS Satellite Controlled Clock - Arbiter's most popular model (500 ns typical)
- Model 1094B GPS Substation Clock - with 250 ns typical accuracy, the most common features included in a sub-station application
Synchronized Power Measurement
Synchrophasor measurement of phase shift across portions of the grid can be used to estimate stress and predict system stability. A Phase Measurement Unit (PMU) is used to measure and analyze the phase shift characteristics
Synchrophasors and Grid Stability
Synchrophasors are “synchronized phasor measurements,” that is, measurements of ac sinusoidal quantities, synchronized in time, and expressed as phasors. With a fixed temporal reference frame, synchrophasor measurements may be used to determine useful information about operation of the grid.
For example, power flows may be monitored in real-time, and by measuring changes in the phase shift across parts of the grid, estimates of stress and future stability can be made. Postprocessing of synchrophasor data (which can also be done in real time) can extract information about low-frequency system modes, and by examining whether the amplitudes of these modes are changing, advance warning of impending instability can be given. Synchrophasor measurements are made practical by a widespread, economical source of accurate time.
Synchrophasor measurements, compliant with IEEE Standard C37.118, must not be in error by more than 1% ‘total vector error’ (TVE), where this error includes components due to time offsets in the PMU clock; phase errors or delays in the signal processing circuitry; and magnitude errors. 1% TVE corresponds to a phase angle error of 0.57 degrees, if no other errors are present. This is about 26 microseconds at 60 Hz (32 us at 50 Hz).
If we allow time synchronization error to be 20% of the error budget, this allows for a time error in the PMU of 5 to 6 us. This level of error is easily met with current technology. However, realistically, in a typical substation errors in the instrument transformers often contribute a few degrees of phase shift, especially with relaying CTs operating at nominal (non-fault) current levels. Under realistic conditions, therefore, even an error of several tens of microseconds is unlikely to have much effect on the measurement.