Instrumentation & Measurement Magazine 26-4 - 44

Low-Power Instrument
Transformers and Energy Meters:
Opportunities and Obstacles
Alessandro Mingotti, Christian Betti, Lorenzo Peretto, and Roberto Tinarelli
L
ow-Power Instrument Transformers (LPITs) are becoming
the preferred measurement device in the
medium voltage (MV) distribution network (DN).
They have several benefits compared to legacy solutions. However,
the adoption of LPITs results in the need for adapting the
grid and its assets to accept them. One practical example is using
LPITs as the current and voltage source for energy meters
(EMs), which are also used for billing purposes. The resulting
measurement chain introduces several metrological challenges
that must be studied and investigated. Therefore, in
this work, the scenarios of LPITs and energy meters are introduced
along with the latest relevant international standards.
Afterwards, the opportunities and obstacles due to the implementation
of the LPIT plus energy meter measurement chain
are discussed. The discussion focuses on metrological requirements,
accuracy evaluation, target uncertainty, and influence
quantities affecting the performance of the devices.
Low-Power Instrument Transformers
The in-field measurements scenario, at any voltage level, has
been evolving since the introduction of the new generation
of LPITs. They are also referred to as non-conventional instrument
transformers (NCITs). There are several reasons for
the massive adoption of LPITs, either the current or voltage
version (LPCT and LPVT, respectively). The main reasons
are that they are cheaper than a legacy inductive instrument
transformer (IT); hence, their spread in the distribution network
(DN) is facilitated. The DN is characterized by a huge
number of nodes to be monitored, compared to the transmission
network (TN). They are compacter than other solutions;
hence, a system operator (SO) may easily install LPITs in
small electrical cabinets full of other equipment. LPITs, as
their name recalls, feature a low-power output. Hence, a few
milliampere currents and voltages below 1 V are perfectly
suitable for being connected to intelligent electronic devices
(IEDs), which introduce several new applications and functionalities.
Furthermore, a low-power output guarantees
higher levels of safety for the measurement environment and
the SOs.
44
All of the benefits from the introduction of LPITs triggered
the research and innovative findings. The literature, for example,
provides works dealing with LPITs' accuracy evaluation
[1], [2]. Other stressed topics are their modelling [3], [4] and
their characterization [5], [6].
Important results from the studies highlighted the main
drawback of LPITs. Each type of LPIT, hence each technology,
suffers from the presence of one or more influence quantities.
Consequently, the accuracy of the devices is altered, and typically
lowered, by such quantities. Some examples of influence
quantities are temperature, humidity, electromagnetic fields
[7], pressure, frequency, and positioning [8]. For instance, a
current LPIT, like Rogowski coils, suffers from its positioning
with respect to the conductor and from temperature variations.
Shunt resistors, instead, may suffer from temperature
and frequency, which also affect capacitive dividers. Fig. 1 illustrates
some typical LPITs.
These weaknesses of the LPITs are crucial and must be
considered and assessed whenever: accurate measurements
must be performed-of utmost importance in case of billing
purposes; and when LPITs are used together with other instruments,
and the overall accuracy of the measurement chain
needs to be computed.
To summarize, the main LPVTs and LPCTs technologies
plus the typical influence quantities are listed in Fig. 2.
Fig. 1. Pictures of LPITs.
IEEE Instrumentation & Measurement Magazine
1094-6969/23/$25.00©2023IEEE
June 2023
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