Carbon monoxide, commonly referred to as CO, is one of the most important gases in dissolved gas analysis for oil-filled power transformers. While gases such as acetylene are often associated with arcing and hydrogen is commonly linked with partial discharge or general fault activity, CO has a special diagnostic value: it is closely related to the thermal degradation and aging of cellulose insulation.
In an oil-immersed transformer, the insulation system is mainly composed of mineral insulating oil and solid cellulose insulation, including paper, pressboard, and spacers. The oil can often be filtered, regenerated, or replaced, but the solid insulation is much more difficult to restore once it has aged or degraded. For this reason, monitoring CO in transformer oil is not only about detecting a fault; it is also about understanding the long-term health of the transformer’s paper insulation.
CO is mainly produced when cellulose insulation decomposes under thermal, electrical, oxidative, or moisture-related stress. During normal operation, a small amount of CO may be present due to natural aging. However, when CO concentration increases rapidly or shows a continuous upward trend, it may indicate accelerated paper insulation degradation.
Common causes of abnormal CO generation include:
Because CO can come from different sources, it should not be interpreted by a single measurement alone. The most reliable approach is to evaluate CO together with its trend, growth rate, CO₂ concentration, hydrocarbon gases, load history, oil temperature, moisture, acidity, and other transformer condition indicators.
The lifetime of a power transformer is strongly influenced by the condition of its cellulose insulation. When paper insulation is exposed to high temperature, moisture, oxygen, or electrical stress, its molecular structure gradually breaks down. This process weakens the mechanical strength of the insulation and releases gases such as CO and CO₂.
A rising CO level can therefore be an early warning sign of cellulose degradation. If CO increases together with CO₂, moisture, acidity, or furan compounds, the evidence for paper insulation aging becomes stronger. If CO rises together with thermal fault gases such as methane, ethane, or ethylene, it may suggest that a thermal fault has affected the solid insulation, which usually represents a higher-risk condition.
In practical transformer diagnostics, CO is most valuable as a trend-based indicator. A single CO value may be influenced by transformer age, oil treatment history, sampling conditions, and operating load. However, a continuous increase in CO, especially under similar operating conditions, deserves close attention.
CO should usually be analyzed together with carbon dioxide, or CO₂. Both gases are associated with cellulose insulation degradation, but their relationship provides additional diagnostic insight.
A slow and stable increase in both CO and CO₂ may indicate normal aging. A faster CO increase may suggest local overheating of paper insulation. An abnormal change in the CO/CO₂ relationship may indicate that the degradation mechanism has changed or that a localized thermal problem is developing.
For this reason, many transformer maintenance teams do not only ask, “Is the CO value high?” They ask more useful questions:
These questions help prevent misdiagnosis and allow engineers to make more accurate maintenance decisions.
Traditional laboratory DGA is highly useful, but it is periodic. A transformer may be sampled monthly, quarterly, or annually depending on its importance and condition. However, faults and abnormal thermal activity do not always develop according to the sampling schedule.
Online DGA provides continuous or frequent measurement of key dissolved gases in transformer oil. For CO monitoring, this is especially valuable because the trend and growth rate are often more important than a single concentration value.
An online DGA monitor can help users:
For critical transformers, online CO monitoring can be an important part of asset health management. It helps operators move from reactive maintenance to predictive maintenance.
MEMS IR technology provides a strong technical foundation for compact and reliable CO measurement in online DGA systems. Instead of relying on consumable chemical reactions, MEMS IR CO sensing uses infrared absorption characteristics of CO molecules. This makes it well suited for continuous transformer monitoring applications.
Key advantages include:
MEMS IR CO sensing is based on optical absorption. The measurement process does not consume the target gas, and it does not require chemical reagents. This helps improve long-term stability and reduces routine maintenance requirements.
MEMS-based infrared components can be made compact and suitable for integration into online DGA equipment. This is important for substation applications where installation space, system reliability, and ease of deployment matter.
By using the appropriate infrared wavelength and optical filtering design, MEMS IR measurement can target the absorption characteristics of CO. This supports selective CO measurement within the gas path of a DGA system.
MEMS IR components are suitable for low-power designs. This is useful for online monitoring devices that need stable, continuous operation in field environments.
Online DGA does not only need accurate gas values; it also needs timely trend information. MEMS IR CO measurement can support frequent measurement cycles, helping users observe CO growth patterns and respond earlier to abnormal insulation behavior.
Compared with some traditional gas detection methods, optical IR sensing can offer good baseline stability when properly designed with temperature compensation, calibration strategy, and gas path control. This supports long-term online monitoring with fewer maintenance interventions.
MEMS IR CO measurement can be combined with data algorithms, alarms, remote communication, and transformer health models. This makes it suitable for modern digital substations, smart grids, and predictive maintenance platforms.
Our online DGA solution is designed to monitor key transformer fault gases continuously, with a special focus on CO as an indicator of paper insulation health. By combining online dissolved gas extraction with MEMS IR CO measurement, the system provides stable, compact, and maintenance-friendly CO monitoring for oil-filled transformers.
This helps asset owners detect early insulation aging, identify overheating risks, and make better maintenance decisions before transformer faults become critical.
