Settlement metering - a new code of practice

Tony Wills - Applications engineer

One of the consequences of today’s relatively open energy market is that there is a vital need to monitor exactly how much energy is produced by each and every one of the numerous energy suppliers to ensure that the revenue from energy sales is distributed correctly and fairly. This onerous task falls to ELEXON, the organisation that oversees the operation of the so-called Balancing and Settlement Code (BSC).

The work of ELEXON is vital to the smooth operation of the wholesale electricity market. Essentially its role is to compare that amount of energy that generators and suppliers they are going to produce with the volumes of energy that they actually produce. Having made this determination, ELEXON calculates a price for the difference and transfers funds accordingly. This involves taking 1.25 million meter readings every day and handling sums that total over £1.25 billion every year.

To help ensure that its settlement calculations are based on accurate and reliable data, ELEXON issues the Code of Practice for the Calibration, Testing and Commissioning Requirements of Metering Equipment for Settlement Purposes. The current edition of this document is Issue 6, Version 9.0, which was published on 6th November 2014. It’s a long and detailed document that runs to almost 40 pages so discussing it in depth is beyond the scope of this short article. Section 5.2 of the document is, however, of particular interest as this deals with commissioning tests.

Among other requirements, Section 5.2 states that, as part of the commissioning tests carried out on site, the polarity and ratios of both voltage and current transformers must be measured and recorded. Many instruments are available for this purpose, of course, but among the most affordable, convenient and dependable are those that make up the latest generation of handheld turns ratio testers.

The best of these instruments, typified by the Megger TTR25, are powered by readily available AA batteries and feature simple one-handed single-button operation. They measure turns ratio and excitation current and, for single-phase transformers, they can also indicate polarity.

In addition to their use in the test and commissioning of potential and current transformers in measurement applications, these versatile instruments are also ideal for the routine testing of single- and three-phase power and distribution transformers. They perform all tests automatically, with three-phase transformers tested on a phase-by-phase basis. The results are shown on a large, clear liquid crystal display and are also available for printing in real time on a thermal printer, or for uploading to a PC for reporting and analytical purposes.

The Megger TTR25, which is an excellent example of this class of instrument, is capable of measuring turns ratios from 0.8:1 to 20,000:1 with five-digit resolution. It uses a low excitation voltage and, to ensure a consistent high level of accuracy, it performs an automatic self-calibration operation at power-up before each test.

In addition to a wide range of other applications, it can be used to record ratio errors for current transformers to an accuracy of ± 0.1%, making it ideally suited for use in demanding settlement metering applications. This compact hand-held transformer tester weighs just 890 grams including batteries, making it easy to carry around and use, yet its durable ABS construction protects it effectively against the knocks and bangs of everyday on site usage.

When the amount of money at stake is considered, it’s small wonder that ELEXON lays down strict requirements for the testing and commissioning of settlement metering installations. These requirements may at first seem daunting, but satisfying them can be made much easier by choosing appropriate modern test equipment and, as we have seen, the latest handheld transformer turns ratio testers are an indispensible tool for those engaged in such work.

For more information about the TTR25 click here

To find out more about ELEXON click here