Instrumentation & Measurement Magazine 25-3 - 30

A New Technique for High Current
Calibrations Using Low Voltage
Reference Standards
Omar M. Aladdin, Hala M. Abdel Mageed, Rasha S. M. Ali, and Marwa A. Elmenyawi
I
n the electrical power industry, high current measurements
have a fundamental importance. This necessitates
the use of highly accurate, precise, and traceable techniques
to calibrate measuring instruments in order to preserve
the output quality. High current measuring techniques are
being developed continuously to improve measurement accuracy
and reduce uncertainty [1]. [2]. It is crucial to develop
traceable strategies for calibrating high current sourcing instruments
such that high current measurements can be traced
to SI units, hence ensuring that the specifications and calibration
procedures are in agreement with quality standards [3].
Calibrators combined with current coils are commonly used
in high current measurements in industrial applications and
laboratory measurements. Measuring high currents involves
current clamps in both laboratory and industrial measurements
as they provide safety due to their high insulation level
[4], [5]. Clamp meters and clamp transducers that are based on
Hall-effect current sensor are capable of measuring dc and ac
currents [6]. Hall-effect sensors sense the magnetic field due to
the flow of current, in terms of low voltage corresponding to
the high current induced in the current coil [7]. Thus, the low
voltage detected will facilitate the traceability of high current
measurements to the SI units.
Traceability is established by means of measurements
linked up with higher national or international standards
through an unbroken chain of comparisons [8]. Traceability of
dc voltage measurements to the SI units is achieved directly
to the international primary standard of dc voltage, the Josephson
Voltage Standards (JVS), which provides a reference
voltage based on the fundamental constants [9], [10]. Additionally,
traceability of accurate ac measurements is nationally
achieved in terms of reference dc values by ac-dc thermal converters
[11]. The key component of thermal converters is the
thermo-element that compares the power dissipation caused
by applying an ac signal to the power dissipation caused by
applying the corresponding standard dc signal. Temperature
rise that is proportional to the power dissipation is sensed
by the thermocouple of the thermo-element. The relative difference
in response between the applied ac and dc signals is
30
called ac-dc transfer difference [12], [13]. In this article, Josephson
Voltage Standard System (JVS) and a 1 V reference ac-dc
thin film Multi Junction Thermal Voltage Converter (MJTVC)
are used to realize the traceability to the SI units for dc and ac
high current measurements, respectively. A multifunction calibrator
associated with its 50-turn current coil has been chosen
as a High Current Source (HCS) under calibration for dc and
ac high current calibration. A traceable clamp transducer with
calibrated ratio of 1 mV/A is used to detect the high current induced
in the coil of the HCS. In dc high current calibration, the
output of the clamp transducer is connected to a reference digital
voltmeter (DVM) that was calibrated directly by the JVS.
The ac high current calibration is based on using 1V-MJTVC
as the reference standard for ac measurements. The MJTVC is
connected to the output of the clamp; thus, ac-dc differences
are measured using the traceable DVM. All of the measurements
were performed automatically using a specially
designed LabVIEW graphical language program. Traceability
has been achieved for dc high current up to 1000 A and up
to 350 A for ac current at 50 Hz and 100 Hz. Root mean square
setting is applied. The combined uncertainty calculations are
considered (Type A and Type B) for all measurements. The expanded
uncertainty is obtained by multiplying the combined
uncertainty by the coverage factor k=2 giving confidence level
of approximately 95% according to ISO GUM [14].
Digital Voltmeter Calibration
In the introduced technique of calibrating high current, all of
the devices included are calibrated before being utilized in
both dc and ac calibration setups to achieve traceability directly
to the SI units in terms of low voltage. A FLUKE 8508A
8½ digit reference multimeter is the DVM used for this technique.
The calibration of the DVM in dc voltages has been
carried out from 50 mV up to 1 V, corresponding to high
current range from 50 A up to 1000 A and according to the
calibrated clamp ratio 1 mV/A. It has been connected to the
JVS electronics unit directly. Twenty one data points have
been automatically recorded by the JVS software for each dc
voltage range of the DVM. Each point is the average of ten
IEEE Instrumentation & Measurement Magazine
1094-6969/22/$25.00©2022IEEE
May 2022
Instrumentation & Measurement Magazine 25-3

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