Frequency Monitoring

Time and Frequency Monitoring

One of the oldest and most widespread uses of accurate substation time is monitoring the average system frequency. While the grid itself, and the machines connected to it, generally have a reasonable tolerance so far as small frequency offsets is concerned, many customers use the line frequency as a sort of time standard. Consider how many clocks in your own home require resetting when the power fails, even briefly.

All of these devices use the line frequency as their time standard. The short-term frequency is less important for this sort of timekeeping than the frequency averaged over a period of hours or days. This allows the utility to control frequency during times of peak load, as one of many variables to direct power flows and ensure orderly operation of the grid. Then, when load is down (generally in the early morning hours), the frequency can be adjusted to “zero out” the accumulated time offset. System time and frequency monitors provide the information required to do this, by measuring and comparing system time (from the grid frequency) to precise time.

Accuracy

Time offsets have no effect on frequency measurements, provided that the time offset is constant. This is because frequency is the rate of change of phase (time); so any fixed phase or time error will cause zero frequency error.

Time offsets will cause an error in the system time measurement, which again is an estimate of the time indicated by a synchronous-motor clock attached to the power line. Allowable error is therefore a direct function of the accuracy required in measuring system time. Since the application relates to timekeeping with mechanical clocks for human activities, an error of a fraction of a second is likely to be acceptable. If we consider also using the power line as a time base for automated timekeeping, the error requirements tighten to a few milliseconds.