Repeated Transformer Failures at a Solar Power Plant

Utility-scale renewable energy facilities rely on interactions between inverters, transformers, grounding systems, and utility interconnections. When equipment failures occur, identifying the root cause requires a forensic evaluation of the entire electrical system rather than a single component.

This case study provides an overview of a forensic electrical investigation at a grid-connected solar-electric generating plant that experienced repeated transformer-related incidents over a relatively short period of time, prompting a deeper technical evaluation. The forensic methods used to analyze those failures, and a discussion of how test data, field observations, and industry standards were synthesized to identify a probable underlying cause are included in the overview.

While the facts are specific to one site, the technical lessons and risk considerations are broadly applicable to insurers, adjusters, attorneys, and risk managers involved in renewable energy claims.

Key Findings

• Three transformer incidents occurred within one year.
• Failures involved equipment from multiple manufacturers.
• Dissolved Gas Analysis revealed abnormal electrical stress indicators.
• Voltage levels remained within acceptable industry limits.
• Evidence suggested a system-wide issue rather than isolated equipment defects.
• High-energy harmonics from inverter systems were identified as the most probable contributing factor.

Background of the Solar Facility

The subject facility was a utility-scale solar power plant consisting of multiple independently operating sections. Each section included power conversion equipment and a step-up transformer that increased the voltage produced by the solar array to a medium-voltage level suitable for utility interconnection.

The site had operated successfully for several years following commissioning. However, one section of the facility began experiencing recurring issues involving its grid-connected transformer. Over the course of approximately one year, three separate transformer incidents occurred in the same electrical section:

1. An abrupt transformer failure during normal operation
2. A second transformer failure during re-energization
3. A third transformer was removed from service after audible anomalies were observed shortly after re-energization

The repeated nature of these incidents raised questions about whether the transformers themselves were defective, whether installation or grounding practices were contributing factors, or whether another system-level condition was present.

Envista Forensics was retained to conduct a forensic electrical investigation, review diagnostic reports, assess available test data, evaluate prior corrective actions, and determine whether a common causal factor existed across the incidents.

Forensic Investigation Methodology

Review of Previous Failures

The first transformer failure was attributed, by others, to an internal low-voltage winding fault. That unit was rebuilt and later returned to service. A second transformer, sourced from a different manufacturer, failed during commissioning and exhibited insulation and tap-changer damage. A third transformer, rebuilt from the original unit, was later installed and energized. Within hours of being placed back into service, technicians reported audible “arcing-like” sounds originating from the rebuilt transformer. Although no protective devices had operated and no visible damage was observed, the unit was proactively taken offline to prevent a potential catastrophic failure.

Transformer Testing and Teardown

The third transformer was disassembled and inspected. No carbonization, insulation tracking, loose connections, or mechanical damage were identified. Electrical testing did not reveal conventional fault indicators that would explain the audible noise or the pattern of prior failures.

However, dissolved gas analysis (DGA) data told a more complex story. Oil samples from all transformers involved showed elevated levels of gases commonly associated with high-temperature events and electrical discharges, including gases indicative of arcing and thermal stress. Importantly, similar gas patterns were also identified in at least one transformer elsewhere at the site that had not failed, suggesting a broader electrical condition rather than isolated equipment defects.

This finding shifted the investigation away from individual transformer performance and toward potential system-wide electrical influences.

Grounding and Power Quality Assessment

Previous assessments had identified grounding irregularities and unintended current paths, which were partially addressed through wiring changes and the addition of equipment intended to stabilize neutral currents. While these steps were appropriate, they appeared to address symptoms rather than a fundamental cause.

Field measurements showed operating voltages within applicable industry standards. Overvoltage conditions alone were therefore unlikely to explain the repeated failures or the observed transformer behavior.

Further evaluation pointed toward the electrical characteristics of the inverter systems supplying the transformers. Modern DC-to-AC inverters inherently generate high-frequency harmonic content in addition to the fundamental power frequency, with the severity of the high-frequency content varying among manufacturers. When excessive harmonic energy is present and not adequately mitigated, transformers and other downstream electrical equipment can experience elevated thermal and electrical stress.

The investigation noted a convergence of indicators: excessive gassing, audible noise, high contact resistance identified in other electrical components, and repeated failures across different transformer manufacturers, all consistent with exposure to high-frequency, high-energy electrical content.

Root Cause Analysis

The investigation determined that the evidence did not support isolated transformer defects as the primary cause of the failures. Instead, the findings pointed to a broader system-level electrical condition. Specifically, the evidence indicated that high-energy harmonic content associated with the inverter systems was the most probable contributing factor to the repeated transformer failures and abnormal transformer operating conditions. This conclusion was reinforced by several factors:

• Multiple transformers from different manufacturers were affected
• Damage or distress on replacement equipment appeared quickly after energization
• Diagnostic oil analysis consistently showed abnormal gas generation
• Similar symptoms were observed in other electrical equipment on site
• Similar dissolved gas generation patterns were observed in transformers that had not yet failed

While the precise harmonic profile was beyond the scope of the investigation, industry guidance indicates that such conditions can lead to internal heating, partial discharges, insulation degradation, and audible noise if not properly controlled.

Key Lessons Learned for Renewable Energy Claims

This case highlights several important considerations for professionals involved in renewable energy claims:

• Repeated equipment failures warrant system-level analysis. When multiple components fail in the same location, the root cause may lie upstream.
• Traditional pass/fail criteria may not tell the full story. Equipment can meet voltage specifications yet still experience damaging electrical stress.
• Diagnostic data trends matter. Stable but elevated gas levels can indicate chronic abnormal conditions, not just isolated events.
• Modern power electronics introduce new risk profiles. Renewable energy systems require careful coordination between inverter design, transformer construction, grounding practices, site layout, and power-quality mitigation.

Why This Matters

Repeated transformer failures in solar power facilities can result in substantial equipment replacement costs, business interruption losses, reduced power generation, and disputes regarding loss causation. Forensic investigations help distinguish between equipment defects, installation issues, maintenance deficiencies, and broader power-quality concerns that may affect multiple assets across a site.

This investigation demonstrates how forensic engineering can move beyond component-level findings to identify broader electrical risks within renewable energy installations. By integrating field observations, laboratory testing, historical data, and industry standards, the analysis provided clarity in a situation involving repeated failures with no obvious electrical or mechanical cause.

For insurers, adjusters, attorneys, risk managers, and renewable energy asset owners, this case underscores the importance of evaluating electrical failures within the context of the entire system. As renewable energy technologies continue to evolve, understanding the interaction between power electronics and traditional electrical infrastructure remains critical to accurate loss causation analysis and effective risk mitigation.