Transformer Testing Procedures Flashcards
7 cards from real NETA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Transformer Testing Procedures flashcards as text
A transformer nameplate shows a BIL of 150 kV. During an applied voltage test, the correct test voltage per NETA MTS would be applied for:
Answer: 1 minute
NETA MTS specifies that applied (hi-pot) voltage tests on transformers are held for 1 minute at the rated test voltage.
When interpreting DGA results using the Rogers Ratio method, a ratio of CH4/H2 < 0.1 combined with C2H2/C2H4 < 0.1 is most consistent with:
Answer: Partial discharge (corona)
The Rogers Ratio pattern with low CH4/H2 and very low C2H2/C2H4 is characteristic of partial discharge or corona activity.
During a transformer load tap changer (LTC) resistance test, erratic resistance readings across tap positions most likely indicate:
Answer: Worn or pitted contacts within the LTC
Erratic or inconsistent resistance across LTC tap positions is a classic sign of worn, pitted, or dirty tap changer contacts.
The Karl Fischer titration method is used in transformer testing to determine:
Answer: Water content in transformer oil (ppm)
The Karl Fischer titration is the standard chemical method for precisely measuring water (moisture) content in transformer insulating oil in parts per million.
A three-winding transformer is tested for short-circuit impedance. Which winding combination provides the impedance between the primary and tertiary windings?
Answer: Short the tertiary, energize the primary, measure impedance
To measure primary-to-tertiary impedance, the tertiary is short-circuited, voltage is applied to the primary, and impedance is calculated from the measured values.
What does a low interfacial tension (IFT) value in transformer oil testing indicate?
Answer: Oil contains polar contaminants or oxidation by-products
Low IFT values in transformer oil indicate the presence of polar contaminants such as oxidation products, sludge precursors, or soluble decay products from insulating materials.
During a transformer core ground test, the insulation resistance between the core and ground reads zero. This finding indicates:
Answer: Multiple core grounds exist, creating a circulating current loop
A transformer core should have only one intentional ground point; a zero reading when testing the isolated core connection indicates an unintended additional ground, forming a loop that allows circulating currents.