To Troubleshoot poor IR (Insulation Resistance) values in a transformer involves a systematic approach to identify and resolve the underlying issues. While testing and commissioning of transformer we often encounter that Insulation resistance (IR) values are not very good. Here’s a step-by-step guide for troubleshooting poor Insulation Resistance values:
1. Preliminary Checks
- Safety Precautions: We need to ensure that all safety protocols are followed, including de-energizing the transformer and properly grounding it before performing any tests.
- Visual Inspection: Check for any visible signs of damage, oil leaks, moisture ingress, or contamination on the transformer.
2. Initial Testing
- Measure IR Values: Use a Megger or insulation resistance tester to measure the IR values between windings, and between windings and ground. Record these values.
- Tan-delta testing of all bushings and windings also needs to be done and seen that the values are within limits.
- Compare with Standards: Compare the measured IR values with the manufacturer’s specifications or industry standards. Typically, the IR values should be in the range of several hundred megaohms or higher for a healthy transformer.
3. Identify Potential Causes
- Moisture Ingress: One of the most common causes of poor IR values is moisture in the insulation (paper or oil).
- Contamination: Dirt, dust, or other contaminants can reduce insulation resistance.
- Internal faults-Internal faults may also cause the IR value to be poor.
- Aging and Deterioration: Insulation materials can degrade over time, leading to lower IR values.
- Oil Quality: For oil-immersed transformers, poor oil quality can impact insulation resistance.
- Temperature: IR values can be temperature-dependent. Ensure measurements are taken at consistent temperatures or corrected to a standard temperature.
4. Detailed Investigation
- Due to some internal faults like fault in the windings or if any of HV,IV,LV or tertiary bushing leads touch internally with the body of transformer then also IR will be nearly zero. Thorough internal inspection needs to be carried out to check for these faults.
- Failure of bushing insulators may also cause the IR values to be very poor.
- Moisture Analysis: Perform a moisture analysis of the insulating oil (for oil-filled transformers) using methods like Karl Fischer titration.
- Oil Quality Test: Test the dielectric strength, acidity, and other parameters of the transformer oil and check that they are within permissible limits.
- Winding Resistance Test: Check for winding resistance to identify any shorted turns or inter-turn insulation failures.
- Sweep Frequency Response Analysis (SFRA): Perform SFRA to assess the mechanical integrity of windings and core.
5. Corrective Actions
- Drying Out: If moisture is detected, dry out the transformer using methods such as hot oil circulation, vacuum drying, by using oil filtration machine .
- Cleaning: Clean any contaminants from the transformer surfaces and insulation.
- Oil Filtration: If oil quality is poor, perform oil filtration or replacement. Ensure the oil is degassed and dried before refilling.
- Rectification of internal faults: Insulation of bushing leads or windings if found damaged needs to be properly repaired and insulated.
- Repair or Replace Insulation: If insulation deterioration is severe, consider repairing or replacing the insulation.
- Temperature Control: Ensure the transformer operates within the specified temperature range to prevent future degradation.
Sometimes we get good LV-E IR values but poor HV-E values in a transformer , in this case chance of contamination of oil in OLTC chamber is very likely, therefore we need to filter the oil in OLTC chamber , it has been seen that after filtration of oil in OLTC, HV-E IR values improve significantly. Filtration of OLTC chamber separately is a very challenging task , the equalizing valve may be opened and the oil may be filtered in the main body.
6. Re-Testing
- After implementing corrective actions, re-measure the IR values to ensure they are within acceptable limits.
- Long-Term Monitoring: Implement a regular monitoring and maintenance schedule to check IR values and other critical parameters.
By following these steps, we can systematically identify and resolve issues leading to poor IR values in a transformer, ensuring its reliable and safe operation.