Instrumentation & Measurement Magazine 24-3 - 14

Fig. 3. NRC digital sampling system for calibrations under nonsinusoidal conditions [2].

A few two- and three-stage step-down current transformers with electronically enhanced cores covering the ratios 1:2,
1:5, 1:10, 1:20, 1:50, 1:100 are used to keep the current in the ac
shunt sufficiently below its rated value.
An ac standard shunt of 0.1 Ω for a maximum current of
10 A and a 10 kHz frequency range, which has highly stable resistance value and small phase error [13], is used for
converting current into voltage. The shunt is built of 100 lowinductance metal-foil resistors connected in parallel.
Revenue meters with harmonic-measurement capability,
digital multifunction relays with functions for power quality
monitoring, or power analyzers can be a DUT. Their functionality, as well as available inputs and outputs, vary.
An active low-turn three-stage voltage transformer, T, is
used to transfer the voltage drop across the shunt to the buffer-amplifier B2 input and to make it referenced to ground
potential.
Two wideband high-input-impedance buffer-amplifiers,
B1 and B2, are used to isolate the voltage divider and ac shunt
voltage outputs from the loading caused by the input impedances of digital samplers. To minimize the amplitude and
phase errors of B1 and B2, a two-stage input driven power supply has been implemented.
Several options are available for the selection of voltage dividers. It is possible to use inductive voltage dividers (IVDs),
voltage transformers (VTs) and resistive voltage dividers
(RVDs).
The advantages of using IVDs are their high accuracy (on
the order of 1 μV/V with respect to the input voltage) and absence of heating related issues. The disadvantages are their
large size and weight, cost, difficulty in building a high-accuracy IVD for input voltages from 120 V to 600 V and operating
frequencies from 50 Hz up to 6 kHz (100th harmonic for the
14

60 Hz fundamental), or higher, which are the ranges of interest for this calibration. The calibration of IVDs is demanding,
using a reference IVD built for similar voltage and frequency
ranges, or using a bootstrapping method involving a high accuracy calibrating transformer, a voltage comparator, a special
switch, injection transformers and several auxiliary coaxial IVDs also built for similar voltage and frequency ranges.
Moreover, the frequency range of the best three-stage IVDs
built for 1 kV are limited to 50 Hz-1 kHz. The basic reason
for this is that input voltages from 120 V to 600 V require a
large number of turns of IVD windings at the low end of the
frequency range, which leads to large stray inter-turn, interwinding and winding-to-ground capacitances, which result in
large errors at high frequencies.
The advantages and disadvantages of using precision twostage or three-stage VTs are similar to those of using IVDs. An
advantage of a VT over an IVD is that the VT provides galvanic
isolation between primary and secondary windings, whereas
a disadvantage is that its accuracy is somewhat lower.
The advantages of using RVDs are the wide frequency
range they can provide, their smaller size and weight, lower
cost, and a relatively simpler construction with respect to multistage IVDs and VTs. The disadvantages are their voltage
dependence, self-heating, temperature stability, long term stability, and demanding calibrations at voltages 120 V to 600 V
and at the high end of the frequency range, especially of phasewise calibrations. Overall, obtaining high performance of
RVDs at relatively high input voltages and in a relatively wide
frequency range is not trivial. Due to the above mentioned advantages they have long been the subject of active research.
For this application, a set of shielded RVDs for voltages
up to 600 V have been designed and built. Their shields are
intended to reduce stray capacitance of the resistors in the

IEEE Instrumentation & Measurement Magazine

May 2021

Instrumentation & Measurement Magazine 24-3

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