28-09-2012, 11:12 AM
The Measurement of Reactive Energy in Polluted Distribution Power Systems: An Analysis of the Performance of Commercial Static Meters
1The Measurement of Reactive.pdf (Size: 302.35 KB / Downloads: 27)
Abstract
This paper presents a comparative analysis of the performances
of both a traditional induction meter and various types
of commercial static meters for reactive energy in the presence of
harmonic distortion. This is a topical issue, since there are several
types of static meters on the market, based on different principles
of construction. In accordance with current standards, these meters
are designed to operate in sinusoidal conditions and their performance
is not tested in the presence of harmonic distortion. However,
with the increase of pollution levels in power systems, the meters
may be used even in the presence of distorted voltages and/or
currents; in such cases, their accuracy may be very different from
the nominal conditions, and the various meters may lead to different
measurements of energy for the same load conditions. In this
paper, the analysis is supported by several experimental tests, performed
by means of a power calibrator.
INTRODUCTION
ELECTRIC energy billing is generally based not only upon
active energy consumption, but also upon reactive energy
demand (or, in other terms, upon the power factor). For this
reason, the measurement of both active and reactive energy is
generally required.
In the past, traditional electromechanical (inductive) meters
were used. Nowadays, static meters are preferred because of
their better stability, accuracy and multifunction metering facilities
(for example, the integration of both active and reactive
energy and the simultaneous measurements of other significant
electrical quantities), as well as because of the possibility of
transmitting the data measured at a distance, by means of power
line carriers or other data transmission technologies.
OVERVIEW OF STANDARDS FOR REACTIVE ENERGYMETERS
Reactive energy meters are constructed for operating in sinusoidal
conditions, as required by the International Standards EN
62053-23 [6] and EN 62052-11 [7]. These Standards refer to “a
conventional definition of reactive energy for sinusoidal voltages
and currents containing only the fundamental frequency”.
As regards the metrological characterization of metering
equipment, the aforesaid Standards define the requirements
that instruments have to comply with in sinusoidal conditions;
moreover, they provide the test conditions and the accuracy
requirements that the meters have to satisfy, when some influence
quantities change with respect to sinusoidal reference
conditions.
OPERATION PRINCIPLES OF STATIC
METERS FOR REACTIVE ENERGY
The static meters available on the market can be constructed
with different solutions. For single-phase applications, the
measurement of reactive energy (or power) can be obtained
by means of an analog or digital multiplication of current and
voltage; the voltage (or the current) is preliminarily shifted by
90 by means of an integrator circuit, a time shifting of a quarter
of a period, a filtering stage or another numeric technique (these
types of operating principles are schematized in Figs. 1 and 2).
The different solutions for voltage or current shifting are
worked out for the sinusoidal working condition; thus, they
work correctly for the fundamental frequency but they can lead
to a different result when harmonic components are present [4].
For example, in the presence of harmonics, if a time shift of a
quarter of a period is operated on the current.
EXPERIMENTAL TESTS
The experimental tests were carried out on three commercial
static reactive energy meters of class 2; the modalities of the
measurement of reactive energy were not declared by the manufacturers.
The meters under test (MUTs) were compared with
each other and with a traditional class 3 inductive meter. The
comparison was made by evaluating the percentage error.
CONCLUSIONS
This paper presents a comparative analysis of the performances
of various types of commercial static meters for reactive
energy in the presence of harmonic distortion.
The theoretical analysis and the experimental tests show that
the adoption of these meters instead of traditional inductive meters
could lead to different measurements in the same working
conditions with the consequent risk of a heavier penalization of
customers in terms of the cost of energy, as compared to the situation
in which inductive meters are employed. This fact could
give rise to disputes between customers and utilities, because of
the different costs paid for the same consumption of energy.
The opinion of the authors is that manufacturers should
always specify the principle on which their meters are constructed,
in order to assess the metrological characteristics of
the electricity metering equipment installed.