Guidelines for Root Cause Analysis of Failures in Transformers Connected to Wind Turbines

Wind Turbine Transformers (WTT). These transformers operate under electrical, thermal, mechanical, and environmental conditions that differ substantially from those of conventional transformers with comparable voltage and power ratings. Highly fluctuating load profiles, frequent start–stop cycles, harmonic distortion associated with power electronic converters, limited ventilation, and restricted accessibility create a demanding operating environment, which has been linked to a higher incidence of failures and a reduction in service life.

Consequently, WTT frequently exhibit abnormal operating conditions or may even fail unexpectedly. In such cases, failure analysis of WTT becomes a critical factor in understanding the underlying causes of the failure. The systematic application of Root Cause Analysis (RCA) is essential to achieve a comprehensive understanding of the mechanisms leading to failures in

WTT and, therefore, to prevent their recurrence. This paper proposes a structured RCA methodology specifically adapted to transformers directly connected to wind turbines, covering both dry-type insulation systems (WTTD) and liquid-immersed units (WTTL). The methodology is based on the outcomes of multiple failure investigations and integrates design review, assessment of technical specifications, analysis of Factory Acceptance Tests (FAT), evaluation of operating conditions, and post-failure diagnostic and laboratory studies.

As general outcomes of the RCA methodology presented in this contribution, recurrent deficiencies leading to failures are mainly associated with insufficiently detailed contractual technical specifications, inconclusive FAT procedures, and inadequate designs and operational conditions.

The paper presents an RCA methodology that has been successfully validated and, in the cases where it has been applied, has enabled the implementation of appropriate corrective actions.