NETA Transformer Testing Procedures 4 — Questions and Answers
Question 1: When performing a turns ratio test on a three-phase transformer, the acceptable deviation from the nameplate ratio is typically:
- ±0.5% (Correct answer)
- ±5%
- ±10%
- ±15%
Correct answer: ±0.5%
NETA acceptance criteria require the measured turns ratio to be within ±0.5% of the nameplate ratio.
Question 2: A transformer's insulation resistance reading is taken at 40°C. To correct this value to 20°C, you should:
- Multiply by a correction factor greater than 1 (Correct answer)
- Divide by a correction factor greater than 1
- The reading needs no correction
- Subtract 20% from the reading
Correct answer: Multiply by a correction factor greater than 1
Insulation resistance decreases with increasing temperature, so readings at elevated temperatures must be multiplied by a correction factor to normalize to 20°C.
Question 3: During a sweep frequency response analysis (SFRA) test, a significant deviation in the mid-frequency range (1 kHz–100 kHz) most likely indicates:
- Core deformation or shorted turns
- Bushing capacitance change
- Loss of clamping pressure or winding deformation (Correct answer)
- Oil moisture ingress
Correct answer: Loss of clamping pressure or winding deformation
Mid-frequency deviations in SFRA are associated with loss of clamping pressure or mechanical winding deformation.
Question 4: What is the primary purpose of performing a dissolved gas analysis (DGA) on transformer oil?
- To measure oil dielectric strength
- To detect incipient faults through gas generation patterns (Correct answer)
- To determine oil viscosity
- To check moisture content only
Correct answer: To detect incipient faults through gas generation patterns
DGA identifies fault types such as thermal overheating, partial discharge, and arcing by analyzing gases dissolved in the insulating oil.
Question 5: A power factor tip-up test on a transformer bushing compares power factor at two voltages. An increasing power factor with increasing voltage indicates:
- Good bushing condition
- Partial discharge activity within the bushing (Correct answer)
- Incorrect test connection
- Normal capacitive effect
Correct answer: Partial discharge activity within the bushing
An increase in power factor with voltage (tip-up) suggests ionization or partial discharge activity inside the bushing.
Question 6: When testing transformer winding resistance, a resistance reading significantly higher than the nameplate value in one phase of a delta winding most likely indicates:
- A shorted turn in the winding
- An open circuit or high-resistance connection (Correct answer)
- Normal measurement variation
- Incorrect tap position
Correct answer: An open circuit or high-resistance connection
A significantly elevated winding resistance reading compared to nameplate data typically indicates an open circuit or a high-resistance connection such as a loose tap or failed solder joint.
Question 7: The Doble-style excitation current test is primarily sensitive to:
- Winding insulation moisture
- Core condition including shorted laminations and air gaps (Correct answer)
- Oil dielectric strength
- Bushing power factor
Correct answer: Core condition including shorted laminations and air gaps
The excitation current test is highly sensitive to core conditions such as shorted laminations, core damage, or increased air gaps from mechanical shifts.
NETA Transformer Testing Procedures 4
When performing a turns ratio test on a three-phase transformer, the acceptable deviation from the nameplate ratio is typically: