Data collection and processing for digital AC power system monitor/analyzer

A digital monitor/analyzer (1) for an ac electrical power system (3) samples the current and voltage waveforms at a slow rate for performing metering functions, and at a high rate for analyzing harmonic content. Sampling is performed in sampling frames (35) comprising a plurality of repetitions (371-374) of sampling for a selected number of cycles followed by a delay ( delta ) of a fraction of a cycle. This effects equivalent sampling at a higher rate for slow speed sampling over a frame (35). High speed sampling is performed over only one repetition (373) of the selected number of cycles in a frame (35) so that it can be carried out sychronously as required for the harmonic analysis. The high speed sampling rate is an integer multiple of the slow speed rate so that slow speed data can be extracted from the high speed samples for continuous metering. Fourier analysis of the harmonic content of the waveforms is performed (61) on the odd interrupts while all of the other tasks are apportioned out (59) over the even interrupts of a sampling frame. Both the odd and even interrupts initiate sampling.