Bulk Revenue Metering Using Relaying CTs


Because it’s increasingly common for different companies to handle generation and transmission on a single system, the need to measure energy precisely at multiple new locations on a power grid has become a necessity. Very simply, generators need an accurate measure of the amount of power they deliver to the grid, and transmission companies need to know how much power they receive and pay for. Payments based on scheduled shipments of power, or by relying on traditional CT meters (5% – 10% accuracy) are no longer acceptable. Indeed, the continued use of inaccurate meters could result in multi-million dollar disputes between generators and transcos, with lawsuits all but inevitable.

While some utilities routinely install new revenue-metering-class CTs as a matter of policy to measure power delivered or received, and to avoid complications, many others find it difficult – if not impossible – to do so, because space at many substations is limited and the costs can be extraordinary. Indeed, installation costs can run as high as $500,000 per substation.

The power industry needs a cost-effective solution, and two years ago, as part of an EPRI base-funded project, EPRIsolutions began testing a new theory: that relaying CTs – which are commonly found in substation equipment – can be reclassified and used for revenue metering.


In 2001, Dr. George Gela, EPRIsolutions project manager in Lenox, Massachusetts, began developing preliminary procedures, assembling the necessary instrumentation, and assessing technical and cost factors involved with the in situ assessment of installed CTs. Dr. Luke van der Zel, the EPRI project manager, oversaw the project.

The first problem Dr. Gela’s team faced was how to adapt existing relaying CTs for metering. While these CTs have a high dynamic range (that is, they can handle a wide range of short-circuit currents) their measuring accuracy suffers at normal load current, typically from a few hundred to a few thousand amps.

Gela’s team, working with an idea first proposed by Southern Company, found that after careful adjustments or minor modifications to the test CTs, their measuring accuracy can approach that of revenue metering CTs. Right now, the theory appears solid. It may be possible to reclassify some existing relaying CTs for revenue metering, and thus save the utility industry millions of dollars.