NETA Transformer Testing Procedures 5 — Questions and Answers
Question 1: 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:
- 1 minute (Correct answer)
- 5 minutes
- 10 minutes
- 15 minutes
Correct answer: 1 minute
NETA MTS specifies that applied (hi-pot) voltage tests on transformers are held for 1 minute at the rated test voltage.
Question 2: 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:
- Thermal fault above 700°C
- Partial discharge (corona) (Correct answer)
- Arcing of high energy
- Overheating of cellulose
Correct 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.
Question 3: During a transformer load tap changer (LTC) resistance test, erratic resistance readings across tap positions most likely indicate:
- Correct tap changer operation
- Worn or pitted contacts within the LTC (Correct answer)
- Incorrect test equipment calibration
- A shorted winding in the transformer
Correct 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.
Question 4: The Karl Fischer titration method is used in transformer testing to determine:
- Acid neutralization number of oil
- Water content in transformer oil (ppm) (Correct answer)
- Dielectric breakdown voltage of oil
- Interfacial tension of oil
Correct 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.
Question 5: A three-winding transformer is tested for short-circuit impedance. Which winding combination provides the impedance between the primary and tertiary windings?
- Short the secondary, energize the primary, measure impedance
- Short the tertiary, energize the primary, measure impedance (Correct answer)
- Short the primary, energize the tertiary, measure impedance
- Open all windings and apply voltage to primary
Correct 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.
Question 6: What does a low interfacial tension (IFT) value in transformer oil testing indicate?
- Oil is free of contaminants
- Oil contains polar contaminants or oxidation by-products (Correct answer)
- Oil has high dielectric strength
- Oil moisture content is acceptable
Correct 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.
Question 7: During a transformer core ground test, the insulation resistance between the core and ground reads zero. This finding indicates:
- Normal condition — the core must be grounded
- Multiple core grounds exist, creating a circulating current loop (Correct answer)
- The transformer has a failed winding
- The test leads are connected incorrectly
Correct 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.
NETA Transformer Testing Procedures 5
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: