Portable AC sources for on-site meter testing
Many distribution authorities carry out statistical sample testing of meter types to ensure compliance with national regulations. The results of this sampling can provide good advance warning of any possible problems with particular meter types before they become critical.
When conducting meter checks on site, there are two fundamentally different test methods available to the distribution authority. Each has its own advantages and disadvantages.
Method 1 – Checking using customer load
Under customer load conditions, checks can be conducted using only a simple portable reference meter. The type of reference meter used and the degree of accuracy required will depend on the accuracy class of the electricity meter to be tested, and also on which additional checks may be required over and above simple accuracy testing.
The test load is obviously limited to the existing load condition in the customer’s installation. Voltages and currents are sampled in such a way that the reference meter and the meter being checked are subject to the same conditions of load, and therefore both are measuring the same power and energy levels. The reference meter normally interfaces with the meter being checked via a photo-electric probe scanning the LED-impulses or rotor disc rotations, directly from impulses from the meter (SO) or even more simply for slowly rotating Ferraris meters by means of a manually operated switch operated by the test engineer in time with the disk rotations.
Checking under customer load conditions offers the following advantages:
- The electricity meter to be tested does not have to be disconnected from the customer’s load. There is therefore no interruption to the customer’s supply.
- Testing of the customer’s meter takes place under realistic operating conditions.
- The testing time is relatively short in comparison with full testing with external loading equipment, as testing can generally only be done at one load point.
- The customer sees that no interference has taken place with the actual installation of the meter, and is therefore likely to believe the results of the test without question.
- Capital investment costs in test equipment are reduced and less weight has to be transported to site.
Method 2 – Checking using externally applied loads
The use of modern solid-state test load generation equipment can significantly reduce some of these apparent advantages, particularly with regard to weight and capital expense. In addition, significant advantages can be gained by using external test loads.
- Tests can be conducted in unfavourable or non-typical load conditions – for example where the customer load is very small or there is no natural load at all.
- Tests can be conducted covering the full load curve of the meter in compliance with international standards.
- Tests can be conducted automatically by controlling modern, fully electronic test load equipment from the reference meter.
- Multi-functional test load equipment incorporating supplementary functions can also simulate adverse operating conditions (for example harmonics on currents and voltages). Automatic switching functions within the meter on test (for example by transmitting ripple control signals) can also be checked if this function is part of the meter specification, thus giving a more complete picture of the meter performance.
The technology of fully electronic test load equipment
The most suitable and cost-effective technology for portable sources would appear to be a combination of a digitally synthesised signal generator combined with switch mode amplifiers using PWM technology. The important advantages of this technology are:
- It is more efficient than analogue amplification technology – better than 85% at full load.
- The resultant load system has less weight and reduced size compared to other technologies.
Test load generators must feature menu-driven control panels integrated into the equipment to avoid having to carry a separate PC to site. However all sources should also feature suitable electronic interfaces, enabling optional external control via a PC or an electronic reference meter.
Current-only test load sources
Most normal metering installations require only that the meter current and phase be varied while the voltage applied is that from the normal supply. This means that the load generator can be simplified to a system supplying only the current loads to the meter in a form that allows these load currents to be synchronised with the incoming supply, while allowing them to be varied in amplitude and phase. This means that the cost and weight of the load system can be reduced to an affordable level which can easily be carried considerable distances by a single test engineer.
Modern current loading equipment of this type weighs less than 6.5kg for a 6 amp three phase source and 11.5 kg for a full 100 amp version, has compact measurements, and is the ideal tool for on-site tests for all meter types. Any test current up to 100A can be generated at phase angles relative to the system voltage over the full 360°. This type of load can be used wherever it is possible to access a convenient mains voltage for testing.
Current and voltage test sources
There are, however, some test installations where it is necessary for the test load to generate both the voltages and currents to the meter on test. This is the case in new installations, where there is as yet no supply installed on site, or in a laboratory where it may be necessary to investigate the effects of varying the characteristics of the applied voltage. The addition of the three voltage sources, also using switch mode amplifier technology, still allows the weight to be kept below 16 kg.
For more complex testing requirements, this type of load generator is also able to generate harmonics and ripple control signals to allow testing of peripheral meter functions
Integration of test meters and test load equipment
Test systems are available where the load and the measuring system are integrated within a single unit. Separation of these functions is often preferred, however, as the integration of test meter and source has significant disadvantages:
- The operator must carry the load equipment, even if he only needs to conduct customer load tests.
- The supply and measuring components are sometimes internally wired together, so that it is not possible to separate them and conduct a test under customer load conditions.
- There are now different forms of test load generation equipment available on the market which are optimised for specific tasks. The operator can combine these as required with various types and accuracies of measuring equipment to suit each task.
Summary and outlook
In an increasingly privatised and deregulated world energy supply market, on-site testing of electricity meters is becoming more and more important. The different methods and requirements with regard to on-site testing can be achieved using suitable electronic load generation equipment, supported by compatible reference standards, allowing equipment to be specified to suit individual needs and budgets.