A "transformer insulation resistance measurement" is a diagnostic technique used to assess the condition of the insulation system in power transformers. This method can indirectly indicate the presence of moisture ingress into the transformer insulation. The insulation resistance measurement is typically performed using a Megger or similar instrument.
Here's how it works:
Insulation Resistance Measurement: Insulation resistance is a measure of how well an insulating material can resist the flow of current. In a power transformer, the insulation system is designed to prevent the flow of current between different winding layers and between windings and the transformer core. When moisture or contaminants infiltrate the insulation, they can create conductive paths, allowing current to flow where it shouldn't.
Moisture's Impact: Moisture is a conductor of electricity, and its presence within the insulation can reduce its resistance to electrical current. When moisture enters the insulation, it effectively provides a low-resistance path for current to travel through. This can lead to leakage currents, partial discharges, and even insulation breakdown. Increased moisture levels can accelerate the degradation of the insulation material, potentially causing serious issues.
Insulation Resistance Test: During an insulation resistance test, a high-voltage DC voltage is applied across the transformer windings and insulation. The current that flows due to the voltage is measured. The measured current is then used to calculate the insulation resistance using Ohm's law (Insulation Resistance = Voltage / Current).
Interpreting Results: A decrease in insulation resistance compared to previous measurements or a lower-than-expected insulation resistance value suggests a potential issue with the insulation system. While there can be various reasons for a decrease in insulation resistance, one significant factor could be the presence of moisture or contaminants that have compromised the insulation.
Tracking Changes: Regular monitoring of insulation resistance over time allows maintenance personnel to track any downward trend in insulation resistance values. A significant and consistent decrease in insulation resistance could indicate an ongoing issue, such as moisture ingress, that needs attention.
It's important to note that while a decrease in insulation resistance can be indicative of moisture ingress, it's not the only factor that can cause changes in insulation resistance values. Temperature, humidity, and the condition of the insulation material itself can also play a role. Therefore, insulation resistance measurements are just one tool among several used in transformer diagnostics, and they are often used in conjunction with other tests like dissolved gas analysis (DGA) to get a comprehensive picture of the transformer's health.