The design, operation, and performance of a 1000:5 ampere highlyaccurate, hand-held, damp-on type current transformer is presented. Thisprecision current transformer uses a sensitive magnetic circuit todetect transformer ampere-turn unbalance between the primary andsecondary circuits. The unbalance represents an error in the ratio andphase angle of the transformer. The largest of the errors is due to thecore magnetization current and magnetic reluctance caused by the cuttingof the core material. After sensing the errors, electronic feedbackthrough a magnetic circuit is used to provide an error correctingcurrent. This reduces the overall errors dramatically. These types ofdevices are referred to as “active” current transformers dueto the use of electronic amplifier and feedback circuits. The devicedescribed has a novel feature of an openable, split-oval core. This“clamp-on” capability enables use of the transformer on abus or cable without the complications or need to open thecurrent-carrying circuit to be measured. Commonly used“clamp-on” type current transformers generally haveuncertainties from about 1% to 5% at full-scale rated current. Thispaper describes a commercialization of active current transformershaving a ratio uncertainty of less than ±0.03% over a currentrange from full-scale rated to 0.5% of full-scale. Additionally, thisproduct has a small phase angle which is an important consideration whenmeasuring electric power, energy, and power factor intended to be usedby electric utilities, standards laboratories, testing laboratories, andin applications where high measurement accuracy and the split-core,clamp-on feature are attractive considerations
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