RICERCA ITALIANA     

New methodological approach and innovative instrumentation for the characterization of energy meters opearting also in non sinusoidal conditions.

Seconda Università degli Studi di Napoli

Abstract

Though they play a crucial role in spheres of extreme significance from both the economical and technological point of view, electrical energy measurements and calibration of the related instrumentation still present unresolved technical and theoretical issues related to the request of guaranteeing metrological traceability through classical approaches. Difficulties originate from the wide spread of non-linear and time-varying appliances and general equipment that generate a host of electromagnetic disturbances, generally known as “power quality phenomena,” that flow through the electrical network making it unlikely to work under its nominal conditions. As a consequence, during normal operation the entire measurement chain devoted to the traceable computation of the absorbed electrical energy must cope with operational conditions that are pretty different from the nominal ones, upon which the measurement system has been designed.
Nonetheless, there is no reference to such scenarios in current Italian and international standards, which require that electrical energy measurement devices be calibrated only under sinusoidal conditions, but for some peculiar cases in which distortions are also taken into account. As for actual operational conditions, technical standards therefore lack an appropriate definition and management of such metrological items as: definition of measurement metric; choice and configuration of instrumentation; definitions of the methods for metrological confirmation and calibration intervals; management of measurement results and computation of the associated uncertainty.
All these topics have taken up an even more relevant role after two changes in technical standards’ framework: the complete reform and liberalization of the electrical energy market as of July 1st 2007, as a consequence of Directive 1996/92/CE about common rules for the internal market of electrical energy, and the Law Decree n° 22 of February 2nd, 2007. The latter has introduced in Italy the Directive 2004/22/CE about measurement instruments, and harmonized the procedures for admission on the market of measurement instruments, including electrical energy counters.
In the mentioned Directives, the largest allowed tolerances are defined with respect to different classes of instruments, the testing environments are set up, and specific constraints for the behavior of instruments under non sinusoidal conditions are fixed. Yet, no reference is given about the methodologies to assess compliance with specifications. The main point which is still unsolved is to adopt measurement metrics and compliance testing procedures that are agreed upon by all operators of energy market, with regards to the measurement of the portion of absorbed energy that is actually utilized by the end-user (active power), of the quality of energy absorption (power factor), and of the quality of energy supply (“power quality”).
Measurement metrics, levels of the absorption and supply quality should all be explicitly included and fixed in the contract (“custom power”) so to fix the cost of energy fairly with respect to both supplier and user, and to apply penalty for a low quality of absorption or to reduce the cost of energy if the supply is of poor quality.
In the proposed project, based on what has been mentioned so far, in a first phase the units will define measurement metrics for active power and power factor for both one- and three-phase systems that can be applied even in non sinusoidal or dissymmetric conditions. Then, based on different metrics and technologies, digital measurement systems will be developed which are capable to correctly evaluate absorbed energy and absorption quality in different operational conditions. Such task will strongly involve compensation of measurement transformers for voltage and current that are currently the most widely used transduction devices adopted for such kind of measurements.
Next, the research will deal with the topic of metrological traceability of the measurement chain, through the design of various methodologies for instrument compliance testing, the realization of reference instruments that can operate with signals commonly encountered in the electrical network, even if non sinusoidal, and the development of strategies aimed at allowing metrological confirmation at the site of installation and operation of the measurement instrument.
Such development will be tested through reference equipment at certified laboratories.
The obtained results, which will also be made available to the Central Metrological Commission and to Italian Electrotechnical Committee, will allow the evolution of technical and legal rules in the field of metrology that will lead toward the inclusion of electrical energy billing in sphere of certified measurements. <<<

Principal Investigator

Carmine Landi Seconda Università degli Studi di NAPOLI

Research Objectives

The aim of the proposed project is the implementation of new methodologies and innovative instrumentation for the "harmonized metrological qualification" of energy meters that could allow both the simplification of the procedures for metrological confirmation required by MID and the billing unification for the consumer. The project provides various coordinated research activities among the different research units (OU). For the sake of clarity the following themes will be classified: (i) definition of metrics, (ii) development of instrumentation (iii) metrological characterization and testing.
Theme 1:
Starting from the absence of reference for active and reactive power in non sinusoidal conditions, this theme proposes the study of a possible harmonization of the different metrics for active and reactive energy and power factor measurements proposed in literature and adopted by the industrial community. The aim is to define a suitable metric that, taking into account the actual regime in which the energy meter is involved, will allow referable energy estimation with the lower measurement uncertainty. Many scientific references, technical standards, general specifications, legal metrology standards, recommendations of the authority for electricity and gas and directives of the European community deal with this theme. The different metrics they propose will be studied, tested and related to the power quality indexes in order to find correlation coefficients able to harmonize al of them. In addition the theme will provide the proposal of standard testing waveform to be used in the non sinusoidal characterization of energy meter.
Theme 2:
The goal of this activity is the implementation of both measurement instrumentation for electric energy estimate in non-sinusoidal conditions and reference instrumentation to be used for energy meters calibration and adjustment during their operations. The proposing instrumentation will have two main features: (i) a favorable ratio between cost and performance; (ii) a very good stability and accuracy even in the presence of typical disturbances present on the transmission grid. As far as the (i) point is concerned, particular attention will be paid to the transducer section since its deeply influence in the overall metrological performance of the meter. In this context, the research activities aims to the construction of high precision and low cost current and voltage transducers suited to operate with typical of non-sinusoidal signals. This objective will be pursued realizing transducers that, through the compensation of the magnetization current of normal TA and TV, allow currents and voltages measurement with low uncertainty in a wide range of frequencies. The compensation strategies must be based on physically independent circuits, in order to make possible the use of actual transducers and without structural or operative expensive changes. In parallel to the realization of prototypes for the compensation of TA and TV, suitably testing techniques for the non-sinusoidal regime will be the developed.
The design of the transduction section will be accompanied by the deployment of digital energy meters. To this aim measurement sections based on different estimation methods and innovative devices will be implemented to improve the measurement performance and reduce costs. Two energy meters types will be developed. The former (i) is based on the analog multiplication of voltage and current waveform in order to directly acquire the instantaneous power signal; the processing of the acquired signal allows the estimation of active and reactive powers and energies. The latter propose to acquire voltage and current waveforms separately and then operate computational processing in order to obtain both energy measurements and power quality indexes; in this case, the energy meters can be based on DSP or FPGA hardware solutions. A complete characterization of solutions proposed by the OU will be carried out to establish their performance and the suitability to the different needs of the electric network.
Concerning with the implementation of measurement instrumentation for metrological confirmation of energy meters, the research activity aims to develop a reference electronic load “add on” device able to be connected to any energy meter will allow accurate, fast and traceable metrological calibration without deny of service.
In this theme is also planned the realization of an energy meter multifunction calibrator. In particular, the calibrator will be able to (i) generate typical voltage and current waveforms of a three-phase system; (ii) measure, with reference to national standards, the main electrical quantities with low uncertainty; (iii) make the metrological characterization of the energy meter under test.
Theme 3:
The metrological characterization of both the realized devices and the adopted energy meters under sinusoidal and distorted steady state is the first aim of this area.
Until now the normative are deeply oriented to sinusoidal steady state. In order to extend the compliance measurements in a steady state with distortion some procedures will be identified in this activity.
The results obtained could be the start up for the harmonized normative in order to standardize the compliance measurements on the energy meters also under distortion.
Another aim will be the characterization of the measurement instrumentation in presence of electromagnetic disturbs in compliance with EMC normative. Test in the semi-anechoic laboratory And in the GTEM cell will be made in order to characterize the electromagnetic compatibility of the devices realized during the research activity.
The identification on how the electromagnetic disturbs match with measured quantity is the goal of this aim. According to these results the produced devices will be optimized modifying the main circuit also with the use of filters and shields.
The optimization of the calibration intervals to obtain a good compromise between the number of calibrations and the over calibrations state will be conducted in order to minimize the testing costs. <<<

First Results

The result of the research project, certainly more ambitious, is the definition of a measuring metric and a resulting test protocol that can serve as a reference for a new harmonized standard both for type approval and metrological confirmation of electric energy meters, also operating on electricity networks with low levels of quality. This ambitious outcome would make transparent the operations of measurement made by modern electronic energy meters and restore users confidence about about the proper allocation of charge even in the presence of non-sinusoidal conditions.
The first expected result is certainly to reach a measurement metric valid in sinusoidal, deformed and unbalanced conditions. In this regard, the various metrics proposals at the international level, even if show partial agreement when measuring active energy, are totally different in the definition of reactive power and power factor. These two values are used as cost parameters in price setting for energy billing. The only timid standardization attempt, is represented by the IEEE 1459-2000, which defines many parameters of the power / energy, usable for other purposes such as penalty and power quality evaluation. The measuring metrics provided in the literature and those suggested by national and international standards, including that above cited, provide results of the measure reactive energy and power factor severely dependent on voltage and current waveform. It is therefore of great interest to define a standard waveform that can describe the more severe operating conditions that can occur on power network. This waveform could be "parameterized" depending on the type of factory production to make specific delivery contracts (contracts for quality) that take into account both the effects of power quality on energy charged and any long-term accelerated failure, rewarding virtuous users and penalizing those less noticed. The achievement of this goal is certainly an important result for the whole electric energy market.
Another expected result is the realization of current and voltage high precision and low cost transducers suitable to work with the typical network signals under non-sinusoidal conditions and the development of related performance testing for the estimation of the ratio errors and phase displacement. Currently all the rules concerning energy meters refer to voltages and currents at energy meter terminals. If the meter is connected directly on the power line it can have an “ true image” of the system and then to "see" the network quantities with the consequences earlier recalled. When, instead, between the energy meter and the network are present voltage and current transformers, meter don’t have a "true image" of the proper energy flowing on the network. In fact, as well known, the instrument transformers behave like low-pass filters introducing attenuation at higher frequencies and especially by introducing strong phase displacement between the primary and secondary quantity. To solve the problem described it is planned to compensate magnetization transducers current of common CT and VT, allowing the measurement of current-voltage with low uncertainty and wide frequency range. In literature there are examples of compensated instrument transformers, some are based on a specific compensation winding, others use a mathematical model of CT or VT whose solution leads to a better determination of the primary quantity. Both cited solutions provide for the creation of ad-hoc transformer. This is unacceptable for the number of installed instruments and the high replacement cost. Therefore, we can say that there is nothing in the literature comparable to ensure operation, even without compensation, directly using the transformers installed.
Another interesting result is the definition of a numerical energy meter, capable of furnishing what is already provided in commercial devices –multi-rate price, remote management and reading, etc.), but with the addition of to correctly measure the energy that flows and take into account the proper power factor. The scientific advancement in this case is not easily assessable because manufacturers do not declare the metric of measurement used, even if prescribed by the MID. An indirect assessment was obtained by testing a number of energy meters, some of those installed recently by main electricity distributor in Italy and other commercially available. This examination for reactive energy, shown in some voltage and current conditions errors up to 70%.
Another expected result is the realization of an equipment for calibration and metrological confirmation of energy meters. This equipment can be seen both as an electronic load to be applied to the meter and test its performance with the network signals available and as a generator of current and voltage imposing known parameter for calibration. As the uncertainty is concerned, the device, will be referred to an certified laboratory. At present it does not exist in the literature and even less on the market such a device, there is on the market an expensive laboratory calibrator suitable for the purpose (Fluke 6100), but it does not present features such as portability and flexibility in defining the metrics. Achieving this goal would reduce the time needed for metrological confirmation and a reduction in the testing time with a consequent reduction in metrological confirmation costs both direct or indirect.
Another important goal is the establishment of a procedure for assessing the optimal metrological confirmation range to be used for verification but that could be integrated directly into meters. This allows to reduce, during the maintenance time, the occurrence of possible states of “out of tolerance” and at the same time increase maintenance time. This implies a significant cost savings without incurring the risk of having measures with uncertainty that exceeds the limits of tolerance. <<<

Timescale

24 months

National and international background

The electrical energy measurement and the definition of the issues of instrumentation calibration is currently one of the most complex aspects of the electrical metrology, both for the inability to carry out direct measurements, and the difficulty of guaranteeing in the traditional form a proper metrological traceability. This often translates in procedural deficiencies in the management of equipments and measurements that affect important metrological aspects such as the choice and metrological confirmation of instrumentation, the measurement process and the estimation of uncertainty. Alongside this, the electrical energy measurement is more important for many commercial transactions as the power consumption billing, the penalties infliction and the estimation of additional energy needs and it represents a fundamental milestone to the development of a free electric energy market. The availability of correct energy measurements may grant the government intervention of the authority of the public utility service regulation. In addition the 2004/22/CE MID directive expects the metrological confirmation of energy meters in operative conditions very close to the real ones. This is an important novelty with respect to the past since the actual CEI standards only consider sinusoidal regime and, only in one of the cases they impose the use of an additional current and voltage fifth harmonic.
At the same time, the deregulation of the electrical industry and the expansion of the world market for trade in electrical energy has triggered the need for greater precision and accuracy in its measurement. In addition, phenomena related to Power Quality (harmonic distortion, noise, transients, holes, overvoltages, etc.) have increased the degree of difficulty in the execution of accurate measurements. These trends are changing today's measurement technology, which is further hampered by the lack of effective and precise standard under which calibrate the measurement instruments.
In this context, characterized by innovations and technological improvements, new standards and rules have been introduced. The complete energy market deregulation occurred by the July the first 2007 (date of the putting into effect of the last step foreseen by the DL #79 of March the 19 1996) receiving the 1996/92/CE directive issued by the European Parliament and Council of December the 19th 1996 regarding with public rules for the local electric energy market, and the February the two 2007 DL N°22 (published on the Gazzetta Ufficiale N°64 of March the 17th 2007) have been introduced. The latter receive the European Parliament and Council 2004/22/CE (MID) directive concerning with the measurement instruments. With the aim of creating a single market, the MID implemented harmonized rules regarding procedures for the marketing of special measurement instrumentation, including active energy meters. MID defines the requirements to be complied the devices and measuring instruments that fall within the scope of specific directives adopted on the basis of Directive 71/316/EEC in connection with their marketing and/or put into service. This directive defines accurately maximum acceptable tolerances for categories of instruments, test environments and also imposes constraints specific about the requirements for electromagnetic (with regard to emission requirements continue to apply the Directive 89/336/EEC).
The MID observation, the electrical energy market deregulation, and problems concerning with calibration and measurements under non sinusoidal conditions will promise to impose huge technical and commercial spin-offs that will implicate the whole development and measurement of active energy meter market
In this context a first problem concerns with the definition of typical operating conditions. To this aim both MID 2004/22/CE and CEI EN50160 give out some definitions about the maximum values of the power quality parameters but don’t give detailed indications. A further problem concern with the metric to be adopted for accounting non sinusoidal conditions, disturbances, and unbalances in the active energy measurements.
In this case non sinusoidal conditions will rapidly aging the powered user’s appliances and, in the same time, could case problems to the power consumption billings.
A further problem is related to the lack of precise norms regarding the reactive energy or the crest factor in non sinusoidal conditions measurements. In the scientific community the theoretical approach to the electrical system studies under non sinusoidal conditions is a disputed and wide research field. It aims to define electrical quantities able to rightly determine phenomena related to the energy balancing in accordance to which happens in sinusoidal conditions. Several authors proposed definitions of electrical quantities in non sinusoidal conditions with the aim to extend to those quantities the same properties applicable in the sinusoidal regime. Notwithstanding, at the moment, the unique valid reference in this theme is the IEEE 1459-2000 standard.
It furnish guidelines for realizing innovative measurement instrumentation, also suggesting quantities to be used as reference for energy billing, power quality measurements, harmonic disturbance sources, and active filters or dynamic compensator development.
This standard proposes some definitions of “non-active” powers to be used on disturbed or unbalanced systems for the estimation of both the harmonic distortion and the unbalancing in the measurement section. A generalized concept of electrical power should be achieved when power measurements in non sinusoidal conditions have to be performed for billings, power quality estimations, disturbance sources identifications, harmonic compensations, etc.
The lack of this theory allows several degrees of freedom to the manufacturer of electrical measurement equipments in the choice of the measurement algorithm. For this reason, it is possible to find on the market measurement instruments that, implementing different algorithms, give out to different billing results. Another consequence is the unavailability to traceable calibration procedures since the unit under test and the reference meter may adopt different metrics.
This implies that a reference energy meter shall implements all the required metrics in order to calibrate any kind of equipment present on the market. <<<