Line Sensors from Piedmont-Bushings-and-Insulators
It is rarely convenient to use the actual currents and voltages for measurement. Instead all modern instruments use analog transducers to generate voltage proxy signals of the order of a volt or two for both channels. For the voltage side a simple resistive divider works well up to about 1,000 volts. Above that specialized high impedance dividers using combinations of resistance and capacitance or precision stepdown transformers (often called, “potential transformers”) are common.
The transducer on the current channel is usually more problematic than the one on the voltage side because of the larger dynamic range involved and the isolation required from the driving potential. The simplest (and arguably the optimal) transducer for current is a small, accurately-known series resistance inserted into the external current loop but this is rarely practical. Instead measuring the magnetic field surrounding the current-carrying conductor is usually the approach of choice. (This can be done at DC with Hall Effect devices.)
Current transformers (CTs) are very useful tools for AC but rank among the most poorly understood devices out there. These magnetic devices create a secondary current with a magnitude that is a precisely known integer ratio to the primary current but only if they are operated into a short circuit. Modern electronics works with voltage signals – not currents- so some clever techniques are required to use CTs effectively. If you scan any industrial catalog you will find a plethora of CTs with obscure specifications and huge differences in price. All of this is about compromises that come with operating CTs into something other than a perfect short circuit, i.e. allowing the CT to actually transfer some small amount of power from the primary to secondary side and still act as an accurate transducer.
There is another class of current sensor with a direct voltage output called a Rogowski coil (a subset of what are broadly called “di/dt sensors”). Rogowski coils contain no permeable magnetic material and therefore enjoy the benefit of having a very large dynamic range. The main issue with Rogowskis is that the voltage output signal tends to be small and also is the time derivative of the current input signal that must then be accurately integrated in real time by the instrument. Doing this real time integration can be a tall order depending on the dynamic range and the magnitude/phase accuracy required.
There are some very clever combinations of passive transducers for electric utility applications on the medium voltage and above. We especially like the line sensors (sometimes called “line post sensors”) from Piedmont Bushings and Insulators. These combine very high impedance voltage dividers with an open geometry, high ratio di/dt current sensor into one relatively inexpensive epoxy structure that replaces the post insulator on poles. The images show Piedmont’s sensor family (analogy to Baby, Mama and Papa Bear is unavoidable) and a beautifully done installation of the 15KV units by an Australian utility for a project we did a few years ago.
The bottom line is that DJA has extensive experience with commercial voltage and current transducers of all makes/types/specifications. We also offer particular expertise with Rogowski coils and digital integrators that provide the highest possible reconstructed signal fidelity. You will find a short paper that explores the physics behind the Rogowski in our articles.