Technical Article in IEEMA Journal, Sep, 2024 issue: Basic Understanding of Negative Tan Delta Phenomenon and its simulation through experiments

Transformer bushings are vital and essential components of power transformers. Dielectric or thermal breakdowns can cause catastrophic failures of transformer bushings and can lead to fire and catastrophic failure of transformers, resulting in huge collateral damage to neighbouring substation equipment and significant revenue loss. A study shows that bushings account for 17% of all power transformer failures and are the third most common cause of breakdowns. OIP bushings are the most prevalent cause of transformer fires, causing nearly 50% of all serious ones. During service, OIP bushings experience some typical issues such as moisture ingress. Some other issues may also be found when routine electrical tests are carried out while manufacturing bushings. The measurement of tan delta qualitatively indicates such abnormality in insulation of OIP bushings.

Regular offline monitoring of transformer bushings is essential and crucial for detecting an abnormality in the bushing and for taking preventive actions that avoid catastrophic failures. Capacitance and tan delta measurements at 50 Hz is the most common method of offline monitoring. Offline tan delta monitoring detects early signs of insulation degradation in transformer bushings. An increase in tan delta values can indicate insulation deterioration, which if left undetected, can lead to equipment failure or breakdown. The capacitance and tan delta tests at site are generally carried out at 10 kV and are compared with factory test results of bushings.

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