Instrumentation & Measurement Magazine 26-4 - 47

increase the costs (extra equipment and test duration). Note
also that keeping additional devices under metrological confirmation
is quite expensive.
Target uncertainty: is the starting point of every measurement
installation. It answers the question " with which accuracy
am I expecting to perform my measurement? " or " what is
the maximum allowed accuracy/uncertainty for that specific
measurement? "
It is consequently fundamental to run the aforementioned
uncertainty evaluation to obtain precious information: the
minimum accuracy value for each component of the measurement
chain. In fact, from the implementation of uncertainty
evaluation techniques, it is possible to obtain overall measurement
accuracy, starting from the single contributions.
For example, an LPCT and an LPVT sharing the same accuracy
class such as 0.5, provide the voltage and current
measures with an accuracy within the interval ±0.5%. The
worst case is that both LPITs have the maximum allowed uncertainty,
resulting in the minimum accuracy associated with
the energy measurement.
This approach may lead to the conclusion that the adopted
devices feature an accuracy class that is inadequate for the target
uncertainty. Consequently, if the evaluation is performed
with already installed equipment, this may result in significant
replacement costs. Note, that the same consideration can
be extended to the actual scenario that involves legacy ITs and
not LPITs.
Influence quantities: as anticipated, are a significant issue for
any kind of device. Even assuming that the LPIT is characterized
before the installation, the in-field conditions may change
its behavior. In detail, the characterization before the installation
should (i) test all the influence quantities affecting the
device (temperature, positioning, etc.); (ii) replicate the in-field
conditions to ensure that it has been performed realistically;
(iii) provide reliable coefficients for a real-time adjustment of
the EM measurements. If even one of the three conditions is not
met, it would be difficult to fully trust the measurements performed
by an EM.
Of course, dealing with all the potential influences on
EM accuracy is a complicated task. However, when billing
purposes must be fulfilled, a huge effort is expected from operators
and authorities to avoid unfair treatment of the involved
parties.
Periodical calibration: has always been a topic of discussion
among experts. In fact, since the first introduction of legacy
ITs, defining how long the accuracy of the device will be kept
has been difficult. The same can be extended to LPITs and
treated like a sort of ageing that the device is experiencing. It
is not expected that an LPIT or an EM will maintain its performance,
considering the harsh environment in which they
operate. Therefore, a simple, cheap, and efficient testing procedure
should be designed to ensure the measurement chain
accuracy over time.
June 2023
Alternative: solutions should be discussed and potentially implemented.
One option consists of the manufacturer providing
the complete transformers plus EM measurement chain. In this
way, it would be the responsibility of the manufacturer to test
its devices in all possible conditions. Afterwards, he would
provide ready-to-use coefficients for the final user. However,
this solution has also drawbacks, for example, it's limiting the
free market and the retro compatibility of the devices. Millions
of LPITs are being and have been installed worldwide, and
they will not be compatible with the new solution.
Another option would be to exclude LPITs from being the
source of billing EMs. A simple choice with a huge impact on
innovation enhancement should not be considered at all.
A third option is a well-structured and organized standard.
This could be the preferred solution which has only one drawback:
time. Preparing such a complicated document requires
time and effort from experts and manufacturers. Furthermore,
they shall agree on all kinds of issues (which include all the
aforementioned) finding a tradeoff between industry and scientific
requirements. To the authors' knowledge, there is no
ongoing standard writing, which means that for many years
this aspect will be uncovered. In addition to those years, many
more are needed to wait for the manufacturers' alignment with
the standard specifications. Consequently, one open question
remains, until a choice will be taken, what will happen with
the uncertainty related to EMs? And who is going to pay for
that uncertainty?
References
[1] M. Faifer et al., " An innovative approach to express uncertainty
introduced by voltage transformers, " IEEE Trans. Instrum. Meas.,
vol. 69, no. 9, 2020.
[2] A. Mingotti, L. Peretto, and R. Tinarelli, " Uncertainty
analysis of an equivalent synchronization method for phasor
measurements, " IEEE Trans. Instrum. Meas., vol. 67, no. 10, pp.
2444-2452, 2018.
[3] A. Mingotti, L. Peretto, and R. Tinarelli, " A smart frequency
domain-based modeling procedure of Rogowski coil for power
systems applications, " IEEE Trans. Instrum. Meas., vol. 69, no. 9,
pp. 6748-6755, 2020.
[4] E. So et al., " A high-precision current transformer for loss
measurements of EHV shunt reactors, " IEEE Trans. Instrum.
Meas., vol. 68, no. 6, pp. 1680-1687, 2019.
[5] G. Crotti et al., " Traceable characterization of low power voltage
instrument transformers for PQ and PMU applications, " in Proc.
Conf. Precision Electromagnetic Meas., Aug. 2020.
[6] W. Olszewski, G. Charot, N. Presser, M. Kuschel, and Z.
Qastalane, " Customer process for technical qualification of nonconventional
instrument transformer products for high-voltage
GIS applications, " in Proc. CIGRE, Aug. 2016.
[7] A. Marinescu, S. Coatu, and D. Rucinschi, " About the EMC of nonconventional
electronic instrument transformer case study, " in
Proc. 2012 Int. Conf. Applied and Theoretical Electricity (ICATE), 2012.
[8] A. Mingotti, L. Peretto, and R. Tinarelli, " Effects of multiple
influence quantities on rogowski-coil-type current transformers, "
IEEE Trans. Instrum. Meas., vol. 69, no. 7, pp. 4827-4834, 2020.
IEEE Instrumentation & Measurement Magazine
47
Instrumentation & Measurement Magazine 26-4

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 26-4
