Paragraph 3.8 is amended to clarify the intent and harmonize with the performance of commercially available current transformers built to the most stringent nationally accepted standards. Currently, section 3.8 can be interpreted to require capabilities that are not available and could be extended to require current transformers to be replaced every time the state of the art is advanced incrementally.
Reason:
ANSI C57.13 defines 0.3 percent as the most accurate class of revenue metering transformers, requiring demonstrated accuracy at 10% of rated current, at rated current, and at the maximum current allowed by rating factors. The maximum r.f. recognized by ANSI C12.11 is 2.0, allowing a ratio of 20:1 between the maximum and minimum currents that can be accurately measured. For example, some CTs are manufactured with an unrecognized œ0.15 Accuracy Clas€? claim that extends down to 5% of rated current and couple that with rating factors as high as 4.0. Even at these extremes, there is still an 80:1 ratio between maximum and minimum loads that can be measured. An EPS site may require accurate measurement of a 1000 MW generator and a 1 MW standby load when the unit is o—a 1000:1 ratio. Good metering practice has always been to size for the maximum current at the high end of the rating factor and accept the possible consequences of an uncertain accuracy at the lower end of the scale. Data is available to demonstrate that typical CTs are acceptably accurate far below the minimum current demonstrated by the standards. It is easy to demonstrate that the MWH and financial impacts of any inaccuracy at low current levels is negligible to the market. The proposed amendment clarifies that for existing bi-direction “resoceâ€? metering points, minimum expected load levels do not have to fall within the ratings of the CT. This amendment also clarifies that once a CT is determined to be appropriate for a site it does not have to be replaced simply to improve low-end accu
