Valence Electrical Training Services

Potential Transformer Testing (PT/VT)

I co-wrote an article for the NETA World magazine in 2004/2005 that covers several different manual techniques for potential transformer testing that explains several low-tech solutions for potential transformer testing.  I’m not sure if it was ever published because I can’t find it in the NETA world archives and it was a long time ago.

Back to the Basics – Potential Transformer Testing (PT/VT) 

As test equipment becomes more sophisticated with better features and accuracy, we risk turning our field personnel into test set operators instead of skilled field service technicians. A test set operator connects the leads, pushes the buttons, and records the results; hoping the numbers he records are good. A test technician connects the leads, pushes the buttons, and records the results. However, the test technician understands what the test set was doing while all the lights were flashing, and why. The technician can also evaluate the results and determine if a re-test is necessary with different connections or substitute external equipment for tests when the test equipment malfunctions. The purpose of this article is to help a test set operator understand the tests they perform, and hopefully add or re-enforce a test technique to the test technician’s repertoire.

Potential transformers (PTs) are necessary to a power system for metering and protective relaying to convert higher system voltages to lower control voltages that are more practical from an equipment, operation, and safety perspective

You can download the complete potential transformer testing article below which covers the following topics:

  1. The three basic potential transformer styles
  2. Visual and Mechanical Inspections
    • Compare equipment nameplate data with drawings and specifications.
    • Inspect physical and mechanical condition.
    • Verify that all grounding connections provide contact.
    • Verify correct operation of transformer withdrawal mechanism and grounding operation.
    • Verify correct primary and secondary fuse sizes.
  3. Electrical Tests
    • Perform insulation resistance tests winding to winding and each winding to ground. ‘
    • Perform a polarity test on each transformer.
    • Perform a turn’s ratio test on all tap positions if possible.

Download the potential transformer testing article with the following link



About the Author Chris Werstiuk

Chris is an Electrical Engineering Technologist, a Journeyman Power System Electrician, and a Professional Engineer. He is also the Author of The Relay Testing Handbook series and founder of Valence Electrical Training Services. You can find out more about Chris here.

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