Dry-type transformers are integral to the electrical infrastructure of industrial, commercial, and institutional facilities. Because these transformers power critical systems, regular electrical testing and maintenance are essential to ensure safety, reliability, and performance.
This article summarizes recommended electrical tests and maintenance practices for dry-type transformers rated 500 kVA and above, based on ANSI/NETA MTS-2019 (Maintenance Testing Specifications) and industry best practices.
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NETA Testing Standards Overview
The InterNational Electrical Testing Association (NETA) provides two key transformer testing standards:
- ANSI/NETA ATS-2017 – for acceptance testing of new equipment
- ANSI/NETA MTS-2019 – for maintenance testing of in-service equipment
This article focuses on maintenance testing per ANSI/NETA MTS-2019, as it directly impacts long-term transformer reliability.
Differences Between Dry-Type and Liquid-Filled Transformers
While the general testing philosophy is similar, dry-type transformers differ significantly from oil-filled transformers in both construction and insulation materials:
| Feature | Liquid-Filled | Dry-Type |
|---|---|---|
| Cooling Medium | Mineral oil or ester fluid | Air-cooled |
| Insulation | Oil + paper | Epoxy resin, varnish, Nomex, or other solids |
| Insulation Class | Class A (105 °C) | Class B (130 °C), H (180 °C), or C (220 °C) |
| Testing Sensitivity | Uniform due to oil dielectric properties | Variable; depends on material and humidity |
Because dry-type insulation systems vary widely by manufacturer, test results must always be compared against previous baseline data and similar units, not generic values.
Types of Dry-Type Transformer Construction
- Open-Wound – basic air-cooled design
- VPI (Vacuum Pressure Impregnated) – coils impregnated with varnish under vacuum/pressure
- VPE (Vacuum Pressure Encapsulated) – adds protective coating for harsh environments
- Cast-Coil Resin – epoxy-encapsulated coils, highly resistant to moisture and contaminants
1. Insulation Testing
Two key insulation integrity tests are recommended by NETA MTS-2019:
a. Insulation Resistance Test
Measures resistance between windings and ground using a megohmmeter (commonly at 500 V to 5 kV DC).
- Recommended Minimum Value: 25,000 MΩ for windings > 5 kV
- Perform test shortly after de-energization to avoid moisture absorption.
- Trend results over time to identify deterioration.
b. Power-Factor / Dissipation-Factor Test
Assesses dielectric losses within the insulation system.
- Record capacitance along with the power-factor reading.
- Differences in winding shape or prior faults may alter capacitance values.
- Avoid attempting temperature correction; results vary by insulation material.
- Retest near 20 °C if results are questionable at higher temperatures.
c. Tip-Up Test (for windings > 2.5 kV)
Detects partial discharge (PD) within insulation voids.
- Measure power factor at low voltage (< 2 kV) and at operating voltage.
- Acceptable difference: < 1.0 % (per NETA MTS-2019).
- A rise > 1 % indicates PD activity or insulation defects.
2. Visual and Mechanical Inspection
Before and after testing, perform a comprehensive inspection:
- Shipping Bolts: Loosen (but do not remove) bolts that secure core and coils during transport. Tight bolts can cause vibration and noise when energized.
- Grounding: Ensure the transformer frame and core are solidly grounded.
- Cleanliness: Remove dust and debris from air passages.
- Cooling Fans: Verify operation and airflow direction.
Proper mechanical setup reduces vibration, extends insulation life, and improves efficiency.
3. Winding Resistance Test
This test injects DC current through each winding and measures the voltage drop to calculate resistance: R = frac{V}{I}
- Detects open circuits, turn-to-turn shorts, or poor connections.
- Results should match previous tests within 1 % or be consistent phase-to-phase.
- Expect values in milliohms (low-voltage windings) or ohms (medium-voltage windings).
4. Transformer Turns Ratio (TTR) Test
Confirms the ratio of primary to secondary turns and verifies winding integrity.
- Acceptable deviation: ≤ 0.5 % from nameplate or adjacent coils.
- Detects shorted turns, open windings, or incorrect tap connections.
5. Excitation Current Test
Performed by energizing one phase at a time with all other windings open-circuited.
- Expected pattern for a three-legged core: two similar currents and one lower.
- Identifies core faults, inter-turn shorts, or magnetic imbalance.
- If abnormal, demagnetize the core and repeat the test.
6. Core Ground Test
Ensures the transformer core is grounded at one single point only.
- Lift the core ground (if accessible) and test insulation resistance using 500 V DC.
- Minimum acceptable value: 1 MΩ.
- Multiple ground points can cause circulating currents and overheating.
7. Surge (Lightning) Arrester Testing
Transformers are often protected by MOV (Metal-Oxide Varistor) surge arresters.
- Verify MCOV (Maximum Continuous Operating Voltage) rating matches system grounding (solidly grounded vs. ungrounded).
- Perform watt-loss or power-factor testing to assess arrester insulation condition.
- Compare results phase-to-phase and with similar units.
8. On-Line Testing and Condition Monitoring
While dry-type transformers cannot be oil-sampled, certain on-line diagnostic tools are effective:
- Partial Discharge (PD) Scans – identify early insulation degradation
- Infrared (IR) Thermography – detect loose or overheated connections
- Temperature & Fan Monitoring – verify cooling system performance
- Voltage & Load Trend Analysis – ensure balanced and stable operation
Regular monitoring complements periodic offline tests and extends asset life.
Conclusion
Routine electrical testing of dry-type transformers is vital for maintaining system reliability and preventing unexpected failures.
Because each transformer’s construction and insulation system are unique, baseline testing, data trending, and comparison between similar units are critical for accurate condition assessment.
A comprehensive maintenance program—including insulation resistance, power-factor, winding resistance, turns ratio, excitation, core ground, and visual inspections—provides strong assurance of transformer health and operational safety.