New Developments in Meter Testing:Total Meter Quality Management
Electrical energy meter manufacturers and their customers have seen fundamental changes in their markets over the last three decades. Highly sophisticated meters with a wide range of functions have been introduced. At the same time privatisation of the power industry has led to enormous pressure on manufacturers to increase functionality and, simultaneously, to reduce prices. The effect of all this is that component and production costs have been forced down, and R&D times have had to be cut in order to give a reduced ‘Time to Market' for new products.
As a consequence of energy market privatisation, power utilities also started to investigate their non-technical revenue losses and to search for means by which these could be reduced. The results of these investigations showed that losses could be up to 20% of the total generated power in some countries.
The reasons for these losses were found principally in the areas of mistakes in connecting the meter (especially CT operated meters); poor quality of the meter; tampering with the meter, changing connections (open voltage links etc.); accidental mistakes in meter reading or transmission of the data; intentional inaccurate meter reading, i.e. fraud by the meter reading staff; and billing errors (both accidental or intentional).
Because of cultural and economic differences the proportions of these vary from country to country, but the same problems are seen to a greater or lesser extent throughout the world.
Different Approaches to Meter Quality Management
Meter qualification and advanced tests
Before a new type of meter can be installed, most countries require a type approval test according to IEC standards.
Some utilities also ask for pre-qualification of the meter suppliers in an attempt to avoid meter quality problems, and some utilities carry out acceptance tests on meters supplied, either 100% or by statistical acceptance sample testing.
Meter quality assurance during lifetime on circuit
Data gathering can be achieved by three different methods:
• Statistical method A: Test random sample of meters removed from site and measured in the laboratory.
• Statistical method B: Test random sample of meters on site.
• Periodic exchange: Test all meters removed from site after a predetermined installed lifetime.
The periodic on-site test (method B) requires more effort and is consequently more expensive, but it has the advantage that incorrect meter connections and tampering, as well as meter faults, can be detected.
Whichever method is applied, every quality management system needs good, reliable data on its existing installed meter base.
Modern meter test equipment for laboratory application
The principal components of a modern meter test system are power supply units, which synthesise test voltages and currents to power the meters, a high precision reference standard meter, highly sophisticated software, and a meter test rack.
These components essentially define the quality and efficiency of the test station.
New developments in electronic power supply units
Current state-of-the-art technology for electronic power supply units uses pulse-width modulated and digitally regulated amplifiers.
The advantages of such digital control systems include highly precise measurement and adjustment of the test voltage and current in amplitude and phase relationship, even without the often-used system of feedback control via an external measurement device (the reference standard). Adaptive adjustment algorithms can effectively eliminate load-dependent variations in the generated supplies, and distortion factors better than 0.5% can easily be achieved.
This makes available an almost perfect power source unit for meter testing. Nowadays metrological institutes, such as PTB in Germany and NIM in Beijing use this technology for approval testing of three-phase reference standards.
High precision reference standard meters
Wide-range digital standards with a precision of 0.05% or 0.02% are generally used. These can measure all the various parameters required almost simultaneously, allowing all tests to be carried out using a single instrument.
The measurement range of the voltage circuits covers values from a few volts up to 480 V between the phase and neutral, and in the current circuits from 1 mA to 120 A. Such a reference meter does not require any external current or voltage transformers. It can also be installed in any existing meter test station without any additional capital outlay for other modernisation.
Meter function tests
In addition to the basic energy measurement accuracy test of a meter, peripheral function tests on electronic meters are becoming increasingly important for testing of energy registers in different measurement modes (e.g. active/reactive) and at different tariffs, testing of maximum demand registers, pulse output contact tests and checking of data communication over optical or 20 mA interfaces. For such purposes flexible software is necessary, allowing parallel communication with the meters under test, and freely programmable meter test sequences.
Solutions for On-Site Sample Testing
In some European countries (e.g. Sweden and the Netherlands) power utilities carry out sample meter testing on-site. Separate test criteria have been developed for industrial and domestic meters.
In the Netherlands, for example, domestic meters are periodically checked on-site with a reference standard and internal electronic phantom load at the following load points:
Nominal voltage of meter (220V or 230V)
- 100 % load, PF=1
- 50 % load, PF=0.5
- Starting current test
- Creep test
In Sweden, CT operated meters are tested on-site. The customer load has to be more than 20% of nominal value, otherwise an electronic power source must be used to supply the load. All measurement data and also test energy is recorded during the test procedure as follows:
- Meter accuracy test
- Check of the meter connection using clamp-on CTs or flexible CTs
- Check of the burden of CTs and PTs
- Check of the CT ratio
Analysis of the results of such routine on-site tests can show a quick return on the capital investment in test equipment by way of a significant reduction in non-technical losses.
Most of the errors found in practice are due to wrong meter connections or wrong ratio of the instrument transformers.
Non-technical losses in power utilities depend upon meter quality and the correct use of the meter in the network. Modern meter test equipment is able to support a professional meter quality management system.
Modern portable multi-function reference standards provide the capability to check metering systems, including instrument transformers.