System Analyzer

A power system analyzer is nothing more than an instrument with both an AC voltmeter and ammeter used in a coordinated fashion. I presume you have an understanding of at least one of the many methods an instrument measures a time varying voltage across two nodes in a circuit.

The nominal technique to measure the current is to actually convert the current of at least one path into a differential voltage. One technique is to place a precision shunt (resistance) in series with that path and measure the time varying voltage drop across the shunt. This approach is the easiest to grasp what is happening but has some potentially significant drawbacks. The circuit must be taken off line first and altered to include the shunt in the current path. The accuracy and temperature variance of the shunt impedance will limit the precision of any measurement. With the exception of measuring the neutral current, both nodes of the shunt will be at a much larger voltage than their difference in voltage. So isolation circuitry must be included in some fashion.

The more common method of converting the current to voltage is to use a clamp on current transformer. The time varying voltage across the secondary windings is proportional to the time varying current. The advantages of this approach is that isolation is immediately achieved. The change in primary circuit loading is negligible. This approach has its own collection of possible errors. The core material of the transformer can cause waveform altering changes that are easy to miss without performing a harmonic analysis. The bandwidth and hysteresis of the core material along with eddy current effects can also change the precision of measuring current in this fashion.

The instrument must now obtain from these two time varying signals a time differential measurement. The two approaches to this are to either measure the time where each of these signals reach their peak value or to measure the rising or falling zero crossing time. A quick review of in the geometry of a sine wave tells me that the zero crossing approach will have less uncertainty.

The rest is just a number crunch.