Combined measurement of power quantity and quality in DTSs (Distribution Transforming Stations).
Ing. Ladislav Pospíchal, CSc.
MEgA – Měřící Energetické Aparáty, a.s.
Several variables have been measured in Distribution Transforming Stations (DTS) since the very
beginning, in particular the phase and line voltages, phase currents, active electric power and in some
cases also the frequency. Initially, independent pointer (analog) instruments have been in use and later
the development has brought instruments combining the above mentioned measuring functions. Similar
development has been observed in the field of electricity meters. Recently, we have seen the tendency
of extension of metering functions towards the measurement of voltage quality parameters. However,
we should take several objective facts into account:
1.
Measurement of voltage quality, as a quite new subject, has been characterised by dynamic
development in recent times.
2.
Electricity meters are dedicated measuring instruments with stabilized basic measuring
functions.
3.
There have been always efforts to use only reliable electricity meters with a long-term service
life and without necessity of regular checks of accuracy.
4.
A great many electricity meters are operated in distribution grids.
5.
Electricity meters, which are installed as dedicated measuring instruments at the interface
between a distribution grid and a customer, must comply with many standards, including those
relating to construction.
An electricity meter is perceived, not only by the public, as an exceptional instrument, unchanging,
proven by long-term experience with unchallengeable data, and any, even technologically well-meant
development may lead to discussions and consecutively to suspicions, what surely is in contradiction
with the interest of distributors. Due to these reasons, the usage of the term “electricity meter” in the
measurement of operational parameters, quantity of electric power and voltage quality should be taken
in consideration.
Except of such socially and formally discussed usage of the term “electricity meter” with regard to the
measurement of power quality, when such quality does not represent an invoice item, there are also
technical and economical reasons for the association of measuring functions, in particular in the area of
DTS. However, there are also opposite reasons.
A Distribution Transforming Station is usually installed in a distribution grid [i.e. out of reach of the
public]. This also means that there is no need to equip any measuring device in DTS with mechanical
elements which would make such device inaccessible for the public. From the same reason, there is no
need for technical measures which would make such device highly resistant against any intentional
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tampering (such as malicious use of magnetic field or tampering with covers, input conductors or
electromagnetic compatibility, etc.).
The measurement or metering in DTS has different optimum composition and different information
relevance, when compared with the metering at the interface between a distribution grid and a
customer. Examples of this are power registers [which are reasonable at a single customer] but which
lose their importance in a device for power consumption metering of hundreds of customers together
[as in DTS]. Also, the operational measurement and power consumption metering differ in their concept
of measuring error. When we talk about electricity meters, their metering error is related to the
metered value itself, whereas in cases of operation monitoring devices, the error is always related to
their nominal measuring value. However, we can notice that there is certain convergence in this area
and even those devices which measure operational variables now define their accuracy in relation to the
measured value. Surely, it is not free of charge and the cost-effectiveness is a big question here. As
mentioned above, the information relevance of operational measuring devices often consists in the
measurement of different characteristics when compared with the requirements on power consumption
metering. It concerns different ranges of voltage or current measurement as well as different time scale
of measuring and recording. The convergence is coming even in this area but this is conditioned by the
unification and codification for the purpose of increase of number of produced devices and limitation of
increase in prices.
The communication interface represents another area which is developing separately. It is not only a
physical layer but it includes communication protocols in higher layers. Whereas operational measuring
devices are linked with the protocol IEC 60870-5-104 for SCADA systems, electricity meters usually
communicate through the protocol OBIS (ČSN EN 62056-61, ed.2) which is optimised for reading, tariff
control and load regulation.
The above mentioned suggestions concern rather the association of operational measuring functions
and power consumption metering. When we intend to add also the measurement of voltage quality to
the above mentioned measurement or metering, it is necessary to reconsider required class of such
measurement. In compliance with the applicable standard ČSN EN 61000-4-30, ed.2, there are three
classes of measurement, namely A, S and B. The most demanding and therefore the most expensive
measurement of voltage quality, with the shortest recalibration time period, is that of the Class A which
is intended for the contractual application or for the resolution of disputable situations requiring the
verification of compliance with standards. This is also the reason why the Class A is used for the
measurement in the interface between a distribution grid and a customer.
The Class S is intended for operational and statistical measurements and it is closer to the conditions
of DTS. However, the methods of measurement of voltage quality parameters in the Class S are divided
[there is certain duality] in the applicable standard. What concerns voltage phenomena such as voltage
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drops, surges or breakdowns, it is permitted to measure URMS1 a URMS1/2. When measuring harmonics and
inter-harmonics, it is possible either to calculate harmonics with gaps and to perform harmonic analysis
just once in a second or to do the same without interruption and without gaps, i.e. five times in a second
(the harmonic analysis for the power quality measurement is always calculated from 10 periods). It can
be seen that the methodology of quality measurement and evaluation is still variable and needs to be
further improved in the next versions of respective standard. The same relates to quick changes of
voltage, measurement and evaluation of transient voltage and also to the measurement of current for
the evaluation of voltage quality. Quite neglected but important functionality from the financial point of
view is the requirement on the measurement of short voltage phenomena with the duration up to 1
minute (or up to 3 minutes) in case of measuring device power supply interruption. This requirement
applies both to the Class A and Class S. When measuring at the LV level (where the measured voltage is
generally identical with the supply voltage), it means that a UPS source is needed. This also relates to the
requirement of minimisation of power consumption of respective measuring device what is in
contradiction with the intent of association [or combination] of all measuring functions into a single
device.
What concerns the methodology of voltage quality measurement in the Class B, the applicable
standard defines this class specifically for various existing devices, so that they would not become
obsolete. The Class B is not recommended for any new designs or projects. Methods of measurement in
the Class B are not described in the applicable standard and their description is left to respective
producers. Most measurement methods applied in existing electricity meters pertain to the Class B.
Conclusion
The intent of association or combination of all operational measurements, metering of power quantity
and measurements of power quality into a single combined device is quite in order, as the development
of measuring technology, electronics, communication technology and SW applications gradually fulfils
required prerequisites for this concept. On the other hand, there are still some questions which remain
to be responded and may complicate the achievement of the above mentioned goal. Many questions
can be economically evaluated by elaborating suitable studies and economic analyses. However, there
are some issues which can be considered only after collecting certain amount of operating experience.
Necessary verification has been mainly focused on the interface between a distribution grid and a
customer and it is obvious that suitable solutions must be verified in DTSs which are located within
distribution grids and possible complications thus may have no impact on customers. Distribution
Transforming Stations are easily accessible for required updating and/or repairing, either directly on spot
or through remote communication. In recent times, distribution companies have fulfilled certain
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prerequisites consisting in particular in the concentration of all sorts of measurement / metering in a
single company branch. However, it is still necessary to respect the fact that the scope and importance
of individual measurements in DTS is different when compared with the interface between a distribution
grid and a customer. Also the price relationships for measurements in DTS and in the above mentioned
interface are completely different, in particular due to the number of installations (tens of thousands vs.
millions of units).
In the end, a time factor should be considered. It can be expected that an optimum solution for the
association or combination of all measurements / metering will be different in the conditions of existing
distribution grids which are in operation for many years, when compared with new distribution grids or
grids with rebuilt / renewed power facilities. This factor is quite important in any consideration on such
association.
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