Open Secondary Testing of Window-Type Current Transformers (NUREG/CR-7228)

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Publication Information

Manuscript Completed: July 2016
Date Published: May 2017

Prepared by:
M. Subudhi, P. Joshi¹, S. Dimaiuta¹ and W. Gunther²

Department of Nuclear Science and Technology
Brookhaven National Laboratory
Upton, New York 11973

¹ Superconducting Magnet Division
² Retired BNL Staff

G. Taylor, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

Two AMRAN Current Transformers (CTs) were tested to characterize the propensity of an opencircuited CT secondary to cause a fire in a location remote to the open circuit. Also evaluated was the magnitude of the secondary winding peak (crest) voltage1 when the CT's secondary circuit transitioned to an open circuit condition, and its dependencies on the primary voltage, primary current and the CT's turn ratio2. Parameters identified as having an effect on the CT's open secondary crest voltage include:

• Material and construction of the CT's magnetic core,
• CT's primary voltage,
• CT's primary current, and,
• CT's turn ratio.

The primary objective of this testing, however, was to better understand the following scenario:

Will fire-induced open circuit in the secondary circuit of a wound-type CT, which is operating within its rated continuous primary current limits, result in an excessively high voltage in the secondary circuit sufficient to start a fire in the form of explosion or arcing in the circuit's insulation at the location of the CT itself or at some other location in the secondary circuit?

For the configurations tested, the results indicated that an open circuited CT was incapable of starting a secondary fire based on the criteria established in the test plan. This observation applies to the CT, CT taps, cabling, and burden devices. Additionally, when an arc was deliberately initiated in the secondary circuit, it was observed that the crest voltage diminished significantly.

The test results provide supplemental information to the JACQUE-FIRE Volume 3 working group for developing technical recommendations to possibly extend the limit on the 1200:5 CT turns ratio limit as stated in the NUREG/CR-7150, Volume 1.