Secondary Voltage Drops in Dry-Type Transformers Caused by Coupled Magnetic Flux Effects of Voltage Unbalance and Harmonics in Isolated Offshore Power Systems

This paper investigates abnormal secondary voltage drops in dry-type transformers operating in isolated offshore power systems. While conventional analyses primarily attribute voltage deviations to load conditions and transformer impedance, this study shows that noticeable voltage drops can also occur under no-load conditions due to the combined effects of voltage unbalance, harmonic distortion, and residual magnetic flux. A comprehensive approach integrating on-site measurements, PSCAD simulations, and laboratory experiments is employed to systematically analyze this phenomenon. The results indicate a coupled electromagnetic effect in which source-side voltage imperfections induce asymmetric core flux distribution, which is associated with reduced secondary voltage. In addition, a relationship between synchronous generator winding pitch and harmonic voltage distortion is observed, suggesting its influence on power quality in isolated grids. Simulation results show that the interaction of these factors can lead to a secondary voltage drop of approximately 4–6 V under no-load conditions, even in the absence of transformer defects. Finally, mitigation strategies based on voltage balancing and harmonic reduction are experimentally validated, restoring the secondary voltage to 1.002 pu. These findings provide practical insights for improving voltage stability and power quality in offshore and other isolated power systems.