Current and Voltage Transducers

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.

Areas of Expertise

DJA provides a broad range of engineering services from technical consulting to full design/testing of AC power measurement systems operating at voltages from 120V to 35KV. We help get your power metrology job done cost-effectively with a wealth of knowledge in the following areas:
We are experts using modern digital sampling techniques with both time and frequency domain data analysis methods for the full range of home/commercial/professional applications. [learn more]
Current/voltage transducers are a necessary evil and we understand how to identify the best ones for every job. We also have particular expertise in Rogowski coil current sensors. [learn more]
Extracting what one needs to understand from standards is rarely easy and some important ones are downright daunting. We have a broad understanding of the applicable standards and can help navigate the best path through the maze. [learn more]
Safety must be designed into every power metrology product and assurance is a big deal, especially for high voltage electric utility equipment. We are also experts at making best use of expensive external labs. [learn more]
Neutral, third party evaluation of the methods used and data being delivered by newer SmartGrid instruments can streamline procurements, expedite trials and maybe even avoid legal bills. [learn more]
We have deep experience with both narrow and broad band PLC technology can make it work for you when that makes sense or make sure you avoid it when it doesn’t. [learn more]
We have a collection of reference instruments and intimate knowledge of the tricks of the trade. We can work with you in depth or provide just the specific answers you need for your project. [learn more]