What is the primary limitation of relying solely on key gas ratios in DGA for identifying transformer faults, and what supplemental information is crucial for accurate diagnostics?

The primary limitation of relying solely on key gas ratios in Dissolved Gas Analysis (DGA) for identifying transformer faults is that these ratios can often be ambiguous or inconclusive, particularly in complex fault scenarios or during the early stages of fault development. Key gas ratios, such as the Rogers Ratio or Duval Triangle, are used to interpret the relative concentrations of different gases dissolved in the transformer oil, like hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2H2). These ratios are intended to indicate the type of fault, such as overheating, partial discharge, or arcing. However, several factors can confound these ratios. For example, the same ratio value might indicate different fault types depending on the transformer's age, oil type, loading history, or previous repairs. Also, multiple simultaneous faults can skew the gas ratios, making it difficult to pinpoint the root cause. The ratios might also be within acceptable limits when a fault is just beginning, leading to a false sense of security. To overcome these limitations, supplemental information is crucial for accurate diagnostics. This includes the total combustible gas (TCG) level and its rate of change. A rapidly increasing TCG indicates a developing fault, even if the gas ratios are not yet conclusive. Historical DGA data is essential for tracking trends and identifying deviations from normal operation. A sudden change in gas concentrations is often more significant than the absolute values. Electrical testing, such as insulation resistance (Megger) tests, turns ratio tests, and excitation current tests, provides information about the condition of the transformer's insulation and windings. Oil quality analysis, including tests for moisture content, acidity, and dielectric strength, helps assess the overall health of the oil. Finally, operational data, such as transformer loading, temperature, and voltage profiles, helps correlate gas generation with specific operating conditions. Combining gas ratios with these supplemental data enables a more comprehensive and reliable assessment of the transformer's condition and the identification of potential faults.