Guide to Selecting and Purchasing Online Monitoring Devices for Dissolved Gases in Transformer Oil (Including Technical Comparison)

• This device (DGA) is a precision condition-monitoring instrument that connects directly to the oil circulation system of large power transformers to capture fault gas concentrations in real time through automatic online sampling.

• The device overcomes the time lag associated with traditional offline testing and offers round-the-clock real-time monitoring capabilities and extremely high detection sensitivity.

• These systems are primarily deployed at core nodes where power supply reliability is of the utmost importance, and are used to prevent catastrophic power outages caused by hidden partial discharges or overheating.

What is a Dissolved Gas Analysis (DGA) system for transformer oil?

During long-term operation under complex conditions, the internal insulating oil and solid insulating paper in power transformers degrade due to thermal and electrical stresses, releasing a specific proportion of dissolved gases. To detect these minute chemical changes, power grid asset management technology has evolved from early methods—which involved periodic manual sampling and offline laboratory testing—to highly integrated online monitoring devices. Currently, mainstream technologies on the market include classic oil chromatography, cutting-edge photoacoustic spectroscopy, oil spectroscopy based on infrared absorption principles, and innovative low-oil-content group detection technology. While these technical approaches each have their own strengths in terms of system sensitivity, daily maintenance costs, and suppression of cross-interference, they share a common ultimate goal: to provide the most real-time data support for modern transformer fault diagnosis, helping power grid operations and maintenance personnel identify potential faults in advance.

What characteristic gases can a transformer online monitoring device detect?

Under different fault types and energy levels, insulating oil and insulating paper undergo catalytic cracking to produce distinctly different combinations of characteristic gases. A high-quality online monitoring device is typically capable of accurately quantifying the following seven key gases:

• Hydrogen (H₂): An early indicator gas of partial discharge, arcing, or moisture and heat in insulating oil.

• Methane (CH₄) and ethane (C₂H₆): These are typically produced in large quantities during low-temperature overheating inside transformers.

• Ethylene (C2H4): The primary hydrocarbon gas associated with overheating of bare metal at high temperatures.

• Acetylene (C2H2): The most critical indicator, signaling a high-energy arc discharge at temperatures of several thousand degrees or severe partial discharge.

• Carbon monoxide (CO) and carbon dioxide (CO₂): Abnormal spikes in the concentrations of these two gases directly indicate that the insulating cardboard has become damp or is aging prematurely.

By precisely comparing the relative proportions of these gases within the detection chamber, this type of equipment translates what would otherwise be complex chemical reactions into intuitive early-warning signals, thereby significantly addressing the blind spots in conventional manual insulation oil testing.

Principles, Advantages, and Disadvantages of Traditional Chromatographic Monitoring Systems for Transformer Oil

Traditional online oil chromatography monitoring systems, which have been widely used in power grids for many years, feature a rigorous workflow that includes online cyclic sampling, efficient oil-gas separation, and precise gas separation and quantitative analysis. The system uses a vacuum degassing process to extract and separate the mixed gases; high-purity carrier gas then propels the gases into a temperature-controlled chromatographic column, where different molecules are sequentially separated and quantified based on their varying adsorption capacities. Oil chromatography technology offers extremely high detection accuracy and is regarded as the gold standard in various fault diagnosis guidelines. However, when considering procurement, its operation inherently relies on a high-purity carrier gas source, and chromatographic columns are subject to aging. The consumption of consumables and the need for periodic calibration result in relatively high long-term maintenance costs.

Why Have Photoacoustic Spectroscopy and Oil Spectroscopy Technologies Become Popular Choices for Procurement?

To completely overcome the pain points of traditional monitoring systems—namely, the consumption of carrier gas and the need for column maintenance—photoacoustic spectroscopy has emerged as a leading solution amid the wave of intelligent upgrades. Essentially an advanced derivative of oil spectroscopy, this technology uses an infrared laser of a specific wavelength to directly illuminate the released gas mixture. When specific hydrocarbon gas molecules absorb the corresponding photons, they undergo thermal expansion, which in turn generates acoustic waves. By capturing these acoustic signals with a highly sensitive microphone, precise concentration levels can be determined. Since it requires no carrier gas whatsoever and eliminates the issue of chromatographic column aging, it achieves “truly consumable-free” operation, extending maintenance-free intervals to several years, making it a new favorite among high-end substations and unmanned sites.

Selecting Online Monitoring Systems for Transformers: Comparison Table of Oil Chromatography, Photoacoustic Spectroscopy, and Low-Oil Grouping Technologies

When selecting and purchasing equipment, it is necessary to comprehensively evaluate both operating and maintenance costs and detection accuracy. In recent years, in addition to full-component chromatography and spectroscopy, sub-fraction analysis (which involves extracting a very small oil sample and performing combined analysis based on key gas groups) has emerged as a new trend that balances efficiency and cost.

How to Choose on a Limited Budget? Single-Component Hydrogen Sensors Offer Exceptional Value for the Money

In certain cost-sensitive scenarios, or when dealing with small-capacity transformers or the retrofitting of aging power grids, deploying a full-component system may be constrained by budget limitations. In such cases, single-component hydrogen sensors based on solid-state semiconductors or electrochemical technology offer a cost-effective compromise. This is because, whether the fault is thermal or electrical, hydrogen is often the first precursor gas to be generated and the fastest to diffuse. Although these sensors cannot provide detailed David’s Triangle diagnostics, they offer extremely sensitive threshold alarms for early, subtle overheating, serving as an excellent first line of defense against primary failures.

What pain points can DGA equipment with an intelligent fault diagnosis system address?

Modern high-end online monitoring devices are no longer merely sensors; they are intelligent terminals that deeply integrate big data mining and machine learning algorithms. The system automatically applies theories such as the Rogers ratio method from IEC standards to rigorously analyze and evaluate the results. When selecting equipment, devices equipped with expert systems perform multidimensional correlation analyses between component concentration trends, transformer load, and top-of-tank oil temperature. This effectively filters out errors from single-sample readings, accurately identifies very early signs of paper thermal aging or minute partial discharges, and significantly reduces the burden of manual data analysis.

Installation Requirements and Precautions for Transformer Online Monitoring Devices

The accuracy of the equipment is highly dependent on rigorous on-site installation procedures. During installation, it is essential to ensure that the oil sampling and return lines between the transformer body and the monitoring equipment are absolutely airtight. Even the slightest negative pressure leak can allow external air to seep in, leading to serious diagnostic errors. At the same time, the piping layout must adhere to fluid dynamics principles to strictly eliminate dead oil zones, ensuring that every sample drawn is an active oil sample. During acceptance inspections, the purchaser should treat the quality of the piping installation as a key evaluation criterion.

How to Ensure Long-Term, Stable Operation After Purchase? A Guide to Equipment Maintenance and Calibration

Equipment based on different technical approaches requires tailored operation and maintenance strategies. For oil chromatography systems, micron-level filters must be replaced strictly in accordance with procedures, and standard mixed gases must be used for calibration. For photoacoustic spectroscopy equipment featuring a consumable-free design, the attenuation of the infrared light source and the sensitivity of the microphonic amplifier still need to be calibrated periodically. Consistently comparing and verifying equipment data annually against offline test reports issued by accredited laboratories is the lifeline for ensuring that this system provides reliable data support for emergency repair decisions.

FAQ: Frequently Asked Questions About Procurement and Product Selection

1. Once an online monitoring device is installed on a transformer, can manual insulation oil testing in the laboratory be completely eliminated?

It cannot be completely eliminated. The core advantage of online monitoring devices is their ability to detect abnormal gas production trends in real time, around the clock. However, after such devices issue a severe out-of-limit alarm, safety regulations require that manual samples be collected and sent to a laboratory for high-precision offline testing to verify the results; the two methods complement each other.

2. When selecting equipment for a new substation, how should one choose between traditional oil chromatography and the new photoacoustic spectroscopy technology?

Each has its own optimal application scenarios. Traditional gas chromatography has a long history and is well-suited for sites with convenient transportation and easy access to replacement consumables. Photoacoustic spectroscopy, on the other hand, is truly consumable-free and highly resistant to interference, which significantly reduces the need for manual maintenance and operation, making it ideal for unmanned sites.

3. Why does the system immediately trigger the highest-level alarm when it detects extremely trace amounts (a few ppm) of acetylene gas?

The production of acetylene requires extremely high energy (typically above 800°C). Even if the sensor detects only trace amounts of acetylene, this is conclusive evidence that a highly destructive high-energy arc or severe partial discharge is occurring inside the transformer. Such faults are highly prone to causing explosions, so the threshold settings are extremely stringent.

4. When installing this type of online monitoring device, is it necessary to shut down the main transformer?

This depends on the on-site valve configuration. If standard, non-interfering sampling valves have been installed, some devices featuring backflow-preventing quick-connect fittings can be installed while the transformer is energized in compliance with regulations. However, to ensure absolute safety, or if there is a risk of leakage from the existing valves, most companies still choose to complete the installation during transformer de-energization for maintenance.

5. Can a comprehensive diagnosis of transformer faults be performed by installing only a single hydrogen sensor?

No. A single hydrogen sensor is an excellent early warning indicator, but it cannot determine the specific ratios of multiple gases. Only by obtaining comprehensive data can the ratio method be used to perform an in-depth root-cause analysis of the fault.

6. How does the degassing efficiency inside online monitoring equipment affect the final analytical accuracy?

The degassing stage is the critical bottleneck. If the degassing device (or the microextraction chamber in the low-oil group) becomes clogged, the dissolved gases cannot be extracted in proportion, and the downstream gas samples will lose their representativeness, leading to severe distortion of the analytical data.

7. Is it normal for the background concentrations of certain hydrocarbon gases to fluctuate slightly from season to season?

This is normal. The solubility of gases in insulating oil is highly affected by temperature. During the summer, when temperatures are high, solubility decreases and some gases escape; in the winter, when temperatures are low, solubility increases again. Advanced monitoring algorithms automatically factor in oil temperature to compensate for these thermodynamic effects.