Meter test procedures for today and the future
Today, even a simple meter must be calibrated in an ever shorter time span, with higher accuracy. The modern multi-functional meter, on the other hand, must be tested in a wider range of functions.
Electronic power supply systems
The principal components of a meter test installation are power supply units, which create the voltage and the current furnished to the meter, and the reference standard meter. They govern the quality and the effectiveness of the testing.
Why an electronic power supply unit?
Conventional, transformerised power supply units have several disadvantages. There is a direct link between the power values which are supplied to the measurement circuits and those of the public power supply, both in terms of amplitude and phase displacement. Variation in the mains supply must be compensated for by adjustments in the measurement circuits. In a three phase test system, this constant adjustment can become a real game of patience. A voltage stabilisation unit can reduce these varying effects, but can also adversely influence the voltage symmetry.
It should be stressed that the distortion factor in the measurement circuit is directly dependent on the public supply network. The test station's transformers aggravate this distortion - non-linearity and consequent additional harmonic signals can be introduced.
It is not possible to decouple the frequency of the measurement circuit from that of the supply network when using a transformerised meter test station. This means that meters can only be tested with the frequency of the mains power supply. In networks with strong frequency variation there is a further influence on the results of the meter testing, since some types of meter have a high degree of frequency dependency.
A real improvement of the test conditions can be made by the introduction of a motor-generator. In this case, the influence of the voltage and frequency of the public supply network is practically eliminated. It is even possible, for example, to test customers' 60 Hz meters, when the system has been designed to test meters of 50 Hz nominal frequency.
However, introducing a motor-generator means additional - and onerous - requirements. First, the operating and the servicing costs are not inexpensive. Secondly, it is necessary to provide a synchronous generator having a very low distortion factor, in order to meet the requirements of the meter standards. Such generators are no longer available.
The demands of the IEC - for example, in relation to setup of the voltage to a value of ± 0.5 to 1%, the voltage and current symmetry to ± 0.5 to 2%, the frequency from ± 0.5 to 1% and the distortion factor 2 to 3% - cannot be fulfilled with such equipment. The requirements can only be met with constant supervision of the stabilisation, constant servicing of the transformers and switching devices, and constant verification of the setup values by the operational personnel, to name just a few.
The only true solution is the use of a static power supply unit.
Analogue amplifier technology
The most simple way to introduce a static power supply unit is to use one with classical analogue amplifiers. Such amplifiers have long been used in the field of audio technology. For meter testing, however, there are limitations. On one hand, the frequency range is usually limited to between 45 and 65 Hz, while on the other, a very high output power is required: this can be up into the kilowatt range. The amplifiers used in meter testing do not have a constant output impedance to count on, since the resistance of both the voltage and the current circuit may be varied by the effects of heating, for example, and even by the diverse types of meter being tested.
A static power supply unit is comprised essentially of the generators that create the sine waves for the voltage and current circuits and, as a rule, six power amplifiers, three for the voltage and three for the current (see figure 1).
Analogue amplifiers have several advantageous features, which provide:
- High stability of the output values.
- High linearity of the signal transmission.
- Low distortion factor of the output values.
In addition, they can easily be adjusted with conventional regulators, because signal shifting is slight. Useful, large output power values of several kVA can be created.
However, analogue amplifiers also have several disadvantages:
- A poor power efficiency, resulting in high temperature losses.
- A need to eliminate excess heat, especially in tropical countries.
- Their large dimensions. Because space is required for heat elimination, it is often necessary to install the units in a separate room.
- The need for constant cooling, resulting in background noise which leads to loss of concentration on the part of the operational personnel and possible errors in carrying out the meter test procedures.
Analogue amplifiers have been widely successful and are largely used. The disadvantages, however, were such that solutions were needed to eliminate them.
Pulse-width modulated amplifier technology
Pulse-width modulated amplifiers are less well known than analogue amplifiers and are not yet used world-wide. The main reasons are that the switching concept has only recently been developed, and that the necessary components - rapid and highly efficient, low loss electronic switches - were expensive and difficult to purchase. In addition, because of their basic characteristics, several disadvantages made their utilisation difficult (see figure 2). There are, however, major advantages.
The most important of these lies in their very high efficiency range - not less than 85%. An analogue amplifier has an efficiency of 50% at best, dependent on the operational principle. Pulse-width modulated amplifiers are therefore especially recommended for use in tropical countries, because of the reduced need for cooling - an important cost factor. Because large cooling elements, powerful ventilation units and special air evacuation construction are no longer required, the unit is more compact, allowing meter test systems with higher performance output to be used in the test area. The space requirements are dominated by the size of the meter test bench, and not the size of the power supply unit.
The background noise typical for systems with analogue amplifiers is minimised, as cooling systems do not have to be run continually.
Because of the enormous increase in the availability of electronic components in the field of motor control and mains power supply technology, the building block elements for high performance are also available for use in the area of meter test technology.
High output power of up to 5 kVA and more can be generated without high additional design costs, since only the output stage of the amplifier needs modification.
These important advantages are, however, accompanied by several drawbacks, which make the use of this type of amplifier more difficult.
The principle of operation, and the breakdown of the sine wave into a series of short `impulses', means that it is necessary to provide filter circuitry for the output values to ensure the output signals themselves are of pure sine wave form.
Because of system conditional signal delays, the amplifiers are difficult to adjust using conventional regulators. This means that a low distortion factor at the level of the output values - of less than 0.8 % - is only possible by increasing costs. In addition, these amplifiers have a lower stability at the output value level when compared to analogue amplifiers.
The advantages of the pulse-width modulated amplifiers may only be meaningfully assessed when a solution to overcome the disadvantages has been found.
New trends in the field of electronic power supply units
There have been substantial advances on the components market in recent years. The components necessary for use in an improved pulse-width modulated amplifier are now available at reasonable prices. This is especially so in the case of rapid and powerful signal processors, and rapid and precise analogue-digital and digital-analogue converters. Use of these elements makes possible the realisation of a digital adjustment system using the Fourier transformation in real-time (see figure 3).
The advantages of such digital adjustment systems are:
- Precise measurement and adjustment of the effective values and the phase displacement, which are established by calculation and therefore reduce the small errors to be found in the analogue-digital converters. No analogue multiplication is required.
- Precise and exact adjustment of all harmonics can be made over a wide frequency spectrum, using the Fourier transformation. This results in a very low distortion factor. On the other hand, when special meter test operations are involved an exact reproduction of a composed wave form can be generated as an output signal, when it is necessary to overlay harmonics on either - or both - of the current and voltage base signals.
- It is possible to attain stable signals, even with amplifiers which have signal delays, by the realisation of complicated adjustment algorithms at various moments. This is a distinct advantage in the case of the pulse-width modulated amplifiers.
- Using adaptive adjustment algorithms makes it possible to eliminate load dependent instability.
These measures mean that all the disadvantages of the pulse-width modulated amplifier technology can be completely overcome.
Even without the frequently used system of feedback of the output values to the external measurement device - the reference standard - these output signals can be generated with high precision. There is therefore a separation of the reference standard and the power source unit, an increase in the redundancy, and therefore also in the measurement security. The pairing of the pulse-width modulated amplifier and digital adjustment technology makes available an almost perfect power source unit for meter testing. Future power supply units of this type will probably supersede those using analogue technology.
Precision reference standard meters
Next to the power source unit, the reference standard meter plays a deciding part in this field of meter testing. Today, only wide-range standards with a precision of, usually, 0.05% are used to measure in the frequency range of 45 to 65 Hz.
The main advantages lie with the digital reference standard, since this can measure all the different values almost simultaneously. After transformation of the analogue voltage and current input signals to digital form, the desired values can be calculated using a digital signal processor working in real time.
When the digital reference standard is incorporated into a meter test system no further measurement instruments are needed, since all the values such as the voltages, currents, power in each of the phases, frequency, power factor, and others are available for transmission over the interface of the reference standard. They can then be displayed on the screen, or even used for further data processing operations.
Universal reference standards are available with a standard accuracy of 0.05 and 0.02%. The measurement range of the voltage circuits typically encompasses values from a few volts to 480 V between the phase and neutral, and a few milliamperes to 120 A in the current circuits. Such a reference meter does not need any external current or voltage transformers. It can be installed in any meter test station, without additional outlay for other modernisation.
The reference standard possesses a serial line interface for its operation by software. An error measurement can be carried out on a meter by simple connection of a scanning head to the impulse input socket. An impulse output socket makes a power proportional frequency available. This can also be used for verification of the reference standard meter in a metrological institute.
The so-called wide range comparator is available now for especially high precision requirements. This has similar characteristics to the reference standards described above, but is deliverable as a single independent instrument rather than as a functional part of a meter test system. The frequency range of these comparators lies between D.C. and up to 2000 Hz. The current entry circuits of the comparator are equipped with shunts rather than the conventional transformers. This feature means that the unit can also measure direct currents. The accuracy of the comparator lies close to 0.01%, this being related to the apparent power (see figure 5).
To test modern meters, especially electronic meters, the latest test methods are obligatory. To ensure the quality of the meter testing according to ISO 9002, the meter test systems themselves must also be verifiable. A traceability to the national standard must be guaranteed.
Administration of the rapidly increasing test data situation is essential. In addition, the verification of the measurement results must be prepared in the recommended manner. In order to guarantee the quality of the tests, an automated test procedure is necessary
For all these reasons, a modern meter test installation has user friendly and universal control software which makes automated testing and calibration of all types of meters possible. The measurements are carried out in parallel for all the connected meters via a set of scanning heads. This is valid for both Ferraris and electronic type meters. In general, such a meter test system would be provided with 10, 20, 40, 60 or 80 meter positions. The control software is responsible for carrying out the individual load point measurements in a desired order, switching the meters under test to carry out the tests automatically. At the end of the test run, the results are printed out in the form of a test report.
It is apparent that a generation change from classical meter test systems to modern electronic ones has been taking place. The latest trends indicate that the characteristics of electronic systems are being steadily improved. The modular built-up systems can be commissioned in all regions of the world, and the quality, the costs and the service requirements can be optimised.