Modern measurement techniques for verifying electricity meters and measuring loads on site

This paper provides an insight into the measurement techniques available today.

Three phase portable working standard PWS 1.3 with scanning head and transformer clamps
Different equipment for differing needs

Measurements in situ are carried out for various reasons, and this article looks at available means according to user groups. We must establish whether the measurement will simply be carried out on the basis of a summary verification of the meter and its connections, or whether it will be complete verification at many load points, as in a calibration. It is obviously not necessary to use expensive precision equipment to test a standard household electricity meter of class 2, for example

Two basic types of procedure are used for on-site measurements. The first uses a reference standard meter to carry out a verification, using the existing customer load. This implies that measurements can only be carried out at values which are defined by the at-the-time loading which may or may not be typical values for the user. The second procedure involves a power source a phantom load to create the several desired load points at which the meter will be tested. In this case, the meter can be tested under exact, defined conditions.

Treatment of customer complaints

The proportion of a family's income which is spent on energy is growing. This is largely due to the increasing number of comfort units available today, such as microwave ovens and clothes' dryers. The energy they consume is often unknown, or not considered, and the resulting high billing is often blamed on a defective meter. In such cases it is usually enough for the electricity authority, using straightforward and uncomplicated measurements, to show the user that his installation is within the required limits of accuracy.

Various types of inexpensive measurement equipment exist to perform this function reference standard meters, power sources, or a combination of both within a single unit. They are characterised by their small dimensions and low weight, unlike the equipment of the past. Some earlier units were limited to a single load point for example, using a one kilowatt heater and measurements were carried out for a limited number of turns of the meter disc. The meter was judged accurate by such limited means!

Measurements using the available on-site loading

In domestic installations it is usual to find both single-phase and polyphase meters with currents limited to 60 A, or exceptionally to 80 A or even 100 A. The techniques require that measurements are carried out with the standard voltage, +- 15% and from 5% of the nominal current up to the maximum current value. It is also important to know whether the meter is running under no-load conditions.

Such meters are of accuracy class 2, although national meter regulations often allow the meters to be double the calibration error limit. [1] If it is necessary to carry out a measurement of these meters within the usual technical norms, a reference standard to the accuracy of 0.5% or 0.2% is required.

Many countries do not allow a meter to be calibrated outside the state authorised test laboratory. It is possible, however, to carry out a series of measurements with a reference standard of the required precision, in order to decide whether the meter reaches the precision defined in the local regulations.If it does not, or if the user contests the results, the meter can be removed for extensive examination. In this case, the results of laboratory tests will probably be identical to those found in the field.

Reference standards are offered in several models. Some can be connected directly to the meter via the voltage terminals, after removing the covers. The current circuits are opened and the reference standard is inserted in series into the current circuit. The mains power supply fuses must be removed beforehand to ensure the operator's security.

Measurements can start immediately after the connection to the supply. To carry out measurements at various load values, different domestic appliances can be switched on and off. Meter error is calculated by using a scanning head to detect meter revolutions or impulses, or by manual operation of a hand switch. The display of the reference standard will show the required values, which could include the voltages, the currents, the power and power factor and the error of the meter being tested.

An alternative procedure uses a reference standard which operates with transformer clamps in the current circuit. The great advantage is that it is unnecessary to open the current circuits. While error measurement using the current clamps is not as accurate as measurement using a direct connection, since the transformer clamps themselves have their own inherent error, the results in most cases reach the required level of accuracy.

A third option involves the measurement range adapters on transformer connected meters. Measurement transformer meters are often installed in such a way that a secondary circuit of the current transformer can be opened, allowing the higher measurement accuracy of the directly connected meter to be used.

There is a unit which combines the advantages of the units discussed above. It can be used for measurements with direct current connections in the range of 10 mA up to 10 A. In addition, further measurements may be made using error compensated transformer clamps in the range of 50 mA to 100 A. This unit weighs less than 2.5kg, including the current clamps and scanning head.

It can be used for a whole series of measurement operations: measurements of active power, reactive power, apparent power, voltage, current, power factor, energy measurements for register tests and error measurements for calculation of the relative error of the meter under test. The measurement precision lies within 0.2% or 0.5% accuracy.

Measurements with defined loads

In certain cases it is important to test the meter under defined load conditions, or to be able to define its complete load curve. Here it is necessary to combine the use of the reference standard with a power source - a load transformer. A single phase unit already exists, and the three phase version will soon be available.

Single phase portable test system PTS 1.1

 

The single phase unit's technical characteristics, such as measurement mode, measurement ranges, accuracy etc, correspond to the three phase standards. The reference standard is incorporated into the same housing as an electronic power source, and this creates the desired values of current and the current-to-voltage phase angle directly from the measurement voltage. The current range from 10 mA to 80 A allows it to be used with most meters having such power requirements.

Measurement techniques for industrial users

The user in the industrial sector is in much the same position as the private user. In his case, however, it is necessary to carry out measurements over a wider range of loading values, in addition to establishing that the meter is in good working order. There will also be meter types which differ in the degree of accuracy required and in the operational functionality.

Portable test system consisting of reference standard and power source, controlled by a personal computer

Simple and rapid measurements

In most cases the requirements necessary for measurements by the industrial ser relate to meters which are connected to current transformers. The supplier of energy must know that the meter is in order, and also obtain information about energy consumption. Such information includes the values for voltage, current, power factor, the active, reactive and apparent power values and the phase sequence. It allows the authority to determine whether the meter is correctly connected to the current transformer, or even whether the transformer has been replaced without correction of the information on the faceplate of the meter itself.

The named, simple meters can be measured with an accuracy of 0.2%, provided they have been directly connected. It is also possible to do the measurements with a somewhat wider range of accuracy, but with fewer connection difficulties, using current transformer clamps. This presents an easier solution in the gathering of the required information.

Higher demands

An alternative would be to use a more comfortable and precise reference meter for example a portable standard with an accuracy of 0.1%. Here the measurement ranges and the functionality are greater than those described above. The technical data reflects the large possibilities available for example, measurement transformer meters can be connected directly and verified with an accuracy of 0.1%. In addition to the standard functions, this category of reference standard offers further features, such as phase angle measurement, star point control, frequency measurement and the phase sequence indication. It can also perform maximum measurements.

If the data is to be observed over a long period of time, or the measurement procedure automated, a link to a portable personal computer will be useful. Higher accuracy class equipment has a serial line interface to facilitate this connection.

Equipment used on-site must be as simple as possible. Here the operation of the units is carried out using clearly indicated function keys, and clear displays with large indicators make results and information easily accessible.

Verification of measurements for large users

The difference between the industrial user as described and the large user lies in the way energy is consumed and the way the energy or the power is drawn or even restored. Measurements within higher degrees of accuracy are required, and tariff agreements may have to be considered. The accuracy of the meters in this case start at class 1 [2], [3] and can reach up to class 0.2S [4], and therefore the measurement technology must be of a correspondingly higher accuracy, with reference standards with an accuracy of 0.05 or even 0.02%.

The latest generation of reference standards offers the necessary precision over the whole of the measurement range, with further capability such as frequency range and harmonics measurements, and even a direct current component analysis using shunts on the current inputs.

In the case of tests involving meters with an accuracy of up to 0.2%, an on-site verification providing a complete load curve can also be useful. For this purpose a portable electronic power source is recommended, which will create and supply an artificial three-phase network of voltage and current.

This portable power source can also be used within the framework of a meter test installation. It draws the power for its internal operation from a standard outlet socket, and puts a full three-phase network at the disposal of the user. Together with a suitable reference standard, it can provide exact information concerning the meter being tested. The source is controlled via a personal computer over the serial line interface connection, which allows the unit to be used for a completely automatic verification of a special meter at a customer's premises. If a personal computer is not available, the unit can also be used via stored load point tables to perform a semi-automatic meter test.

The electronic power source can also be used in the small laboratory - for example, to check the phase sequence before meters are wired, and to investigate the operation of new types of meter with complicated tariff structures. It is ideal for the instruction and training of personnel in the use and servicing of such meters.

Finally the user should be aware that there are reference standard units with a higher precision than 0.02 %. These are known as comparators, now available in a three-phase version. Comparators are the norm for the measurement of AC power and energy having the highest accuracy of 0.01%. The name comparator was used because a comparison is made with a certain quantity of direct current power, and comparators meet the needs of a company, a region or a country. They are portable units, with the same functions as the reference standards described earlier in this paper.

  1. PTB testing instructions, volume 6, Electricity meters (2nd edition) 1982.
  2. IEC 521: Alternating current watt-hour meters for active energy (Classes 0.5,1 and 2).
  3. IEC 1036: Alternating current static watt-hour meters for active energy (Classes 1 and 2)
  4. IEC 687: Alternating current static watt-hour meters for active energy (Classes 0.2S and 0.5S).