By Y. K. Nama
The complexities of modern meters, which include the measurement of several parameters and recording at the same time of power quality related measurements, communication and additional functionalities related to tariffs and billing, etc., have increased the importance of meter testing during manufacture, acceptance and on-site. Poor quality meters, which are widely available in the market, have increased the role and responsibility of the meter testing division, to ensure that basic energy measurement at least is done properly. To understand the meter testing world, let us first understand who tests, when, why and how.
METER TESTING PRACTICES AROUND THE WORLD
Worldwide energy meters are being tested at several places in the chain from manufacturer to consumer (Table 1).
Testing in the laboratory
Energy meters are being tested in the laboratory by the manufacturer before shipping to utilities, and by utilities before installing the meters. As mentioned above, both the parties have more or less the same objectives and use a stationary test bench. While the technique appears simple there are still challenges in doing laboratory testing. The main challenges are:
- Efficient and proper testing of a large number of meters in minimum time. To achieve this objective, each aspect of the testing should be conducted with care, e.g. process optimisation, quick connection of the meter, multi-functional testing, data administration, etc.
- New design of meters, which includes testing or verification of complex features, functionality and communication with electronic meters using IR scanning head, PLC, RS485, Modbus, etc.
- Testing of closed link meters.
- Maintaining the in-house legal metrology system, without removing the reference standard from the bench. The new design of benches offers specialised hardware and software that saves the time and cost of sending the reference standard to an external laboratory.
On-site testing practices and methods
On-site meter testing is becoming increasingly important for electric utilities around the world. The reasons are to assure revenue, comply with regulations and improve customer satisfaction – all of which are based primarily on the correct recording of energy consumption. The correctness of energy recording is in turn dependent on the meters' instrument transformers.
On-site testing methods differ from country to country, and are influenced mainly by:
- Access to the meter site – for example in Asian countries access to the meter and removal of the meter connections are difficult and restricted to certain authorities to prevent energy theft
- Testing regulation and practices – some countries legally accept on-site testing when the meter is tested under similar conditions to laboratory testing, i.e. using a phantom load.
The main on-site testing methods are on-line testing at the consumer load and off-line testing using a phantom load. In the on-line testing method the tester connects the reference meter to the meter under test, i.e. voltage in parallel and current in series using a clamp on the CT. This is a quick and economic method, but the tester has to rely on a running consumer load. The objective of such testing is to check the connection and any existing tamper condition and to get a fair idea of the accuracy of the installation.
In the off-line testing method the operator disconnects the meter under test and connects it to an electronic phantom load. There are two kinds of phantom loads available to cater for different needs and practices:
- Current phantom loads, in which the test voltage is fed from the incoming mains supply. The magnitude and phase angle of the test current can be varied as per the test points and synchronised with the incoming mains supply
- Voltage and current phantom load, in which an electronic phantom load generates the required test voltage, current, power factor and frequency. These test parameters are independent of the mains supply voltage.
Automated uniform testing of all meters is possible using modern phantom loads along with a reference standard and software. The technique can be somewhat time consuming, however, and compared with on-line testing requires the phantom load, but it is closer to laboratory testing. Off-line testing is currently used in European countries which have developed their standards and regulations accordingly.
STANDARDS FOR METER TESTING
Meter testing in the laboratory
The tests, test conditions, and acceptance criteria for meter testing in the laboratory according to meter type and accuracy class are defined by the IEC 62052-11, IEC 62053-11, 21,22, 23 and EN50470 series. While many countries have adopted the IEC standards as is, some have modified them to suit the local conditions. Examples are the Indian standards IS 13779 and 14697.
Meter testing on-site
No international standard exists for such testing. Generally the same limit defined for laboratory testing or with some concession for environmental influence is used.
For example in Australia a class 0.5 meter should have a 0.7 percent limit for on-site testing. In Kenya a 3 percent error is allowed for class 2 electromechanical meters. In Hong Kong a +2.5 percent to -3.5 percent error is allowed for class 2 meters.
Various initiatives are under way to define the tests and test conditions. Among these are:
- Italian standard (in draft stage), which defines the testing of various metering installations including CT/VT
- German standard (in draft stage) specially designated for on-site testing and testing equipment.
NEW FINDINGS - ADDITIONAL ERRORS IN MEASUREMENT
The power quality has decreased dramatically for several reasons, including pollution in the power supply due to SMPS, rectifier loading, etc. Under these conditions the conventional meter will not be able to record the energy accurately. Studies conducted in recent years have revealed that if there is a small DC voltage (e.g. 200 mV) in the voltage circuit of the meter, then this mixed signal would cause significant additional error in energy measurement. This kind of situation can prevail in LT/CT operated and whole current installations.
Recent studies have also indicated that energy meters which use a mixed core CT to measure the current signal will give a large error in the power measurement when the signal is mixed. This situation may arise with dryers, vacuum cleaners or other loads using an asynchronous motor, phase shifting or diode for output control purposes.
NEW TRENDS IN METER TESTING
The latest design of test benches offers a high degree of process automation, including the facilities beyond conventional testing such as automatic handling and connection of meters, pulse i/o testing, adjustment, parameterisation, and other functional testing. These types of benches are useful for volume testing.
In recent years, the scope of on-site meter testing has been expanded beyond simple accuracy testing. This trend has been particularly apparent in European countries. Now on-site testing also includes the following task/testing/ analyses:
- Connection checking using vectorial display and instantaneous parameterisation. This helps to detect tampering or correct wrong wiring
- Harmonic analysis. Detailed harmonic analysis is done to investigate which harmonic is causing a problem and whether it is generated by the consumer or is already in the network
- Burden measurement, to ensure that the CT/VT are not over- or low burdened
- Accuracy testing of the instrument transformer (ratio and phase angle errors)
- Complete on-site testing using a high voltage sensor, i.e. the complete site including CT/VT and meter can be tested without dismantling.
CONCLUSION - VALUE ADDITION BY METER TESTING
Meter testing can provide a significant value addition for both manufacturers and utilities. The major long term beneficiary will be the electrical utilities, provided that they integrate this activity properly and systematically in their distribution management system. Meter testing at utilities can provide the following value additions:
- Statistical evaluation of meter type, brand, performance of meters after given number of years of operation (which provides further data for establishing test intervals)
- Qualitative feedback for future procurements of meters and instrument transformers
- Detection of theft through regular and extensive testing on-site
- Screening of a particular area or zone so that the surveillance frequency of the zone can be increased or decreased
- In terms of customer confidence, minimising customer complaints and legal challenges, and image building.
The revenue for a utility will come only via the meter and its measuring component, the CT/VT, hence its proper and accurate operation over the long term will be challenging and interesting for the utility.