Core Frame Tank (CFT) Insulation Resistance Test in EHV Transformers: Procedure for evaluating transformer core to tank insulation. Includes DC test measurement and earth resistance.
Q1. What is the role of Core Frame Tank (CFT) insulation in a power transformer, particularly in terms of its dielectric properties?
In large power transformers, the core (magnetic steel assembly), frame/clamps (mechanical supports), and the main tank must be properly insulated to protect the transformer windings. This insulation ensures that induced currents in the core structure flow through a single controlled earthing path via the CFT bushing.
Without this insulation, uncontrolled current paths may form between the core, frame, and tank, creating risks of overheating, gas generation, and reduced reliability.
Q2. What happens when CFT insulation is healthy?
When insulation between the core, frame, and tank is intact:
- Circulating current flows in a single defined path through the CFT earthing bushing.
- The main tank remains isolated, with only one earth reference point.
- Heat generation is minimal, and no parallel loops are created.
This condition is stable and ensures long-term safety of the transformer oil and insulation system, provided the frame-to-tank insulation is intact.
Q3. What are the risks if CFT insulation breaks down?
If the insulation between core, frame, and tank fails, it can lead to double the risk of failure and significant damage to the transformer windings.
- Multiple current paths appear (through both CFT earthing and tank earthing).
- Circulating currents flow in unintended loops, rather than a single controlled path.
- Localized overheating occurs at the breakdown point.
- Combustible gases are generated and dissolve in transformer oil.
- Long-term reliability of the transformer reduces, with potential for catastrophic failure.
Q4. Why is the CFT insulation resistance (IR) test so important?
The IR test verifies that insulation between:
- Core ↔ Frame
- Frame ↔ Tank
- Core ↔ Tank
…remains intact and can withstand dielectric stress.
This test is especially critical before and after lifting the transformer on site, since mechanical handling and vibration can damage insulation (usually made of pressboard).
👉 General practice in EHV substations:
- Short core and frame together, and earth them at a single point through the tank.
- Use a megohmmeter to measure insulation resistance at standard test voltages.
Q5. What misunderstandings exist about circulating currents?
Among engineers, there are debates on whether the observed currents are:
- Induced circulating currents (due to magnetic coupling between windings and core), or
- Capacitive leakage currents (due to capacitance between windings, core, and tank).
In healthy CFT conditions, the dominant effect may be capacitive leakage current. However, under insulation breakdown, induced circulating currents through multiple earthing paths become significant and dangerous, particularly affecting the transformer core at no-load conditions.
Q6. What are the best practices for field engineers?
- Always perform the CFT insulation resistance test before and after transformer lifting/installation.
- Maintain single-point earthing of the CFT via the main tank.
- Record and trend IR values across different stages of the transformer’s lifecycle using a megger for accurate measurements.
- If abnormal gases (like hydrogen or acetylene) appear in DGA, suspect possible CFT insulation issues.
- During commissioning, confirm no multiple earthing paths exist to ensure effective grounding.
FAQ / Clarification Section (from engineer discussions)
Q: Are the currents through CFT induced or capacitive?
- Some specialists argue the current in healthy CFT conditions is mostly capacitive leakage current.
- Others interpret them as induced circulating currents. Both exist, but the critical risk arises when insulation breakdown allows uncontrolled induced currents to impact the transformer’s performance.
Q: What happens in a CFT earth fault condition?
- A direct core-to-tank fault creates multiple earth loops, leading to overheating and tank stress. This highlights the importance of IR testing and DGA monitoring.
Conclusion
The Core Frame Tank (CFT) insulation resistance test is essential for transformer health in EHV substations. Maintaining single-point earthing and verifying insulation integrity prevents circulating current loops, overheating, and gas generation. Consistent testing before and after site handling ensures safe and reliable transformer operation.
Extra Practical Tips
- Tip 1: Disconnect the CFT bushing before IR testing to ensure accurate readings of the insulation’s dielectric strength.
- Tip 2: Compare IR results with factory test values for early detection of deterioration.
- Tip 3: Test at multiple voltages (e.g., 500 V, 1 kV) for a better picture of insulation health.
- Tip 4: If IR values drop suddenly, investigate mechanical movement or oil contamination.
- Tip 5: Combine DGA findings with CFT IR test results for a holistic transformer diagnostic.
