In-depth Analysis of the DGA Test: The Perspective Eye for Transformer Fault Diagnosis

Introduction

In power systems, transformers serve as critical equipment for energy transmission and distribution, and their operational status directly determines the safety and stability of the power grid. Dissolved Gas Analysis (DGA) test, as a mature non-intrusive diagnostic technology, can early identify potential internal faults by detecting the composition and concentration of dissolved gases in transformer insulation oil, providing a scientific basis for equipment maintenance. This article comprehensively analyzes the DGA test technology from the perspectives of principle, core gases, analysis methods, application scenarios, standard systems, and practical cases.

Basic Principle of DGA Test: The Logical Chain from “Gas Generation” to “Diagnosis”

The insulation system of a transformer mainly consists of mineral oil (or environmentally friendly insulating fluids such as FR3) and solid insulating materials (e.g., insulating paper). During normal operation, insulating materials age slowly and produce trace amounts of gas; however, when faults such as arc discharge, partial discharge, and overheating occur inside the transformer, the high energy at the fault point accelerates the decomposition of insulation oil and solid insulation, generating characteristic gases. Most of these gases dissolve in the insulation oil, while a small amount exists in a free state in the oil or the equipment’s gas chamber.

The core logic of the DGA test involves a process of oil sample collection → gas separation → chromatographic analysis to quantitatively detect the types and concentrations of dissolved gases in the oil. Then, by combining the corresponding relationship between gases and fault types, it infers whether there are faults inside the transformer and the nature of the faults. Essentially, it restores the fault state through the “gas fingerprint”.

Core Analyzed Gases in DGA Test and Their Corresponding Faults

Different fault types produce significantly different gas types and proportions due to variations in energy intensity and operating temperature. According to international standards (e.g., IEC 60599) and industry practices, the DGA test focuses on the following 7 characteristic gases, and their corresponding relationships with fault types are shown in the table below:

For example, if the concentration of acetylene (C₂H₂) in the DGA result increases significantly, it usually indicates a high-energy arc (e.g., winding short circuit) inside the transformer; if the proportion of ethylene (C₂H₄) is prominent, it may be a high-temperature overheating fault caused by multi-point grounding of the iron core.

Key Analysis Methods of DGA Test: From “Single Value” to “Multi-Dimension”

Judging faults based solely on the concentration of a single gas has limitations (e.g., trace gases may come from normal aging). The industry usually adopts a combined strategy of “single gas concentration analysis + gas ratio analysis + graphical method + trend analysis” to improve diagnostic accuracy. The following are the analyses of 5 core analysis methods:

4.1 Single Gas Concentration Analysis Method: Basic Threshold Judgment

This method determines whether there is an abnormality by comparing the measured gas concentration with the standard warning value (specified in standards such as IEC 60599 and GB/T 7252-2017). For example:

In the insulation oil of a newly commissioned transformer, the concentration of acetylene (C₂H₂) should be close to 0; if C₂H₂ is detected, it is necessary to be alert to potential fault hazards remaining during factory production.

For an in-service transformer, if the concentration of carbon monoxide (CO) continuously exceeds 300 μL/L, the aging state of the insulating paper should be analyzed in combination with CO₂.

4.2 Gas Ratio Analysis Method: Fault Type Subdivision

Different faults generate different gas combinations. By calculating the ratios of characteristic gases (e.g., C₂H₂/C₂H₄, CH₄/H₂, C₂H₄/C₂H₆), fault types can be further subdivided. The corresponding relationships between common ratios and faults are as follows (refer to IEC 60599):

For example, if the ratios meet the conditions of “C₂H₂/C₂H₄ > 1 and C₂H₄/C₂H₆ > 3”, it can be confirmed as a high-energy arc fault; the detection of acetylene in the MVA transformer in Document 2, combined with the absence of visual arc flash signs, suggests that it may be a hidden arc (e.g., the development of partial discharge inside the winding).

4.3 Duval Triangle Method: Intuitive Graphical Diagnosis

Proposed by the Hydro-Quebec research institution in Canada, this method uses the volume percentages of methane (CH₄), ethylene (C₂H₄), and acetylene (C₂H₂) as the three vertices of a triangle. After calculating the proportion of each gas, it locates the position in the triangle diagram and judges the fault type according to the falling area. This method is highly intuitive and can effectively distinguish between “thermal faults” and “discharge faults”, and even subdivide the overheating temperature levels (low-temperature T1, medium-temperature T2, high-temperature T3).

The core area division of the Duval Triangle is as follows:

Area D1: Partial discharge; Area D2: High-energy arc;

Area T1: Low-temperature overheating (<300℃); Area T2: Medium-temperature overheating (300-700℃); Area T3: High-temperature overheating (>700℃);

Area DT: Combined fault of arc + thermal fault.

4.4 Rogers Ratio Method: Engineering Fault Classification

Proposed jointly by the UK’s CEGB and IEEE, this method establishes a fault classification matrix based on three sets of gas ratios (CH₄/H₂, C₂H₄/C₂H₆, C₂H₂/C₂H₄), and is suitable for rapid diagnosis of small and medium-sized power transformers. Compared with the IEC ratio method, the Rogers method can more accurately distinguish between “low-energy arcs” and “high-temperature overheating”, and Document 1 mentions that it is widely used in the North American power system.

4.5 IEC 60599 Diagnostic Method: Comprehensive Standard Process

As an internationally accepted standard, IEC 60599 does not rely on a single method but adopts a three-step process of “concentration threshold → ratio analysis → trend verification“:

First, check whether the concentration of a single gas exceeds the standard (e.g., acetylene > 5 μL/L requires vigilance);

Then, judge the fault type through gas ratio analysis;

Finally, verify whether the fault is developing by combining trend data of 3-6 months (e.g., the monthly growth rate of gas concentration > 10%).

This method balances accuracy and practicality and is the mainstream diagnostic basis in the global power industry.

Typical Application Scenarios of DGA Test

The DGA test is not only used for post-fault diagnosis but also applied throughout the entire life cycle of the transformer, mainly including three scenarios:

5.1 Fault Type Diagnosis and Localization

This is the core application of DGA. When an abnormality occurs in the transformer (e.g., increased oil temperature, increased noise) or gas exceeds the standard during routine testing, DGA analysis can quickly identify the nature of the fault (e.g., “arc” or “overheating”) and provide directions for on-site maintenance.

5.2 Early Fault Warning (Trend Analysis)

By monitoring DGA data for a long time and analyzing the change trend of gas concentration, potential hazards can be detected in the “embryonic stage” of the fault:

Slow increase in gas concentration (e.g., monthly increase of 5% in CO): Usually due to insulation aging, requiring enhanced monitoring;

Rapid increase in gas concentration (e.g., 10 μL/L new acetylene detected in one day): Indicates a sudden fault, requiring emergency shutdown;

Sudden appearance of new gas (e.g., no C₂H₂ detected before, but detected in a certain test): May indicate the occurrence of a new fault (e.g., insulation breakdown of the winding).

5.3 Routine Testing and Factory Verification (Key Requirements in Document 3)

According to IEC 60076-1 and the requirements in Document 3, the DGA test should be conducted before the transformer leaves the factory, after new oil injection, or after overhaul:

Before the test: Verify whether the new oil is qualified (e.g., no acetylene, low moisture);

After the test: Compare the chromatographic data before and after the test to confirm that there are no internal hidden dangers during the test (e.g., partial discharge caused by withstand voltage test);

Example: Document 3 clearly requires “no abnormality in oil chromatographic analysis after insulation test” to ensure that the transformer oil and equipment status delivered to the user are qualified.

5.4 Maintenance Decision Support

Based on the DGA result, a differentiated maintenance strategy can be formulated:

Normal DGA data: Conduct routine maintenance as planned;

Slight abnormality (e.g., trace CH₄): Shorten the monitoring cycle (e.g., from once every 3 months to once a month);

Severe abnormality (e.g., excessive C₂H₂): Shut down for maintenance immediately to avoid equipment damage or power grid accidents.

Domestic and International Standard Systems for DGA Test

The standardization of the DGA test relies on the guidance of authoritative standards. Different countries/regions have formulated adaptive standards based on the characteristics of their power grids. The core standard systems are shown in the table below:

A common requirement of these standards is to not rely on a single method but to make a comprehensive judgment by combining multiple analysis methods and on-site working conditions (e.g., ambient humidity, equipment load).

Technical Advantages of DGA Test

Compared with other diagnostic technologies such as dielectric loss test and partial discharge test, the DGA test has three core advantages:

7.1 Non-Intrusive Detection, No Power Outage Required

DGA sampling only requires extracting 50-100 mL of oil sample from the transformer’s oil sampling valve, without disassembling the equipment or cutting off power (except for special cases). It can be completed during the normal operation of the equipment, greatly reducing power outage losses-this is particularly important for industrial users and power grid companies.

7.2 Early Fault Warning, Preventing Hazards in Advance

It usually takes several weeks to months for a fault to develop from “potential” to “outbreak”. DGA can detect characteristic gases when the fault energy is low (e.g., H₂ generated by partial discharge), which provides an early warning period several times longer than traditional “oil temperature monitoring” and “oil color observation”, allowing time for maintenance.

7.3 Covering Multiple Fault Types, Comprehensive Diagnosis

Whether it is an electrical fault (arc, partial discharge), a thermal fault (low-temperature, high-temperature overheating), or even solid insulation aging, DGA can achieve coverage through characteristic gas combinations; while other tests (e.g., insulation resistance test) can only reflect the overall insulation state and cannot locate specific fault types.

Conclusion and Outlook

As the “eyes” for internal fault diagnosis of transformers, the Dissolved Gas Analysis (DGA) test realizes the transformation from “post-maintenance” to “predictive maintenance” by interpreting the “gas code” in the insulation oil. Its core value lies not only in fault confirmation but also in early warning and life-cycle condition assessment.

In the future, with the development of the Internet of Things and artificial intelligence technology, the DGA test will move towards the direction of “online real-time monitoring + AI intelligent diagnosis“: Real-time gas data will be transmitted through online oil sample collection devices, and machine learning models will be used to automatically identify fault types and development trends, further improving diagnostic efficiency and accuracy. However, no matter how the technology evolves, the “fault correlation logic based on characteristic gases” remains the core of DGA, and mastering traditional analysis methods (e.g., ratio method, Duval Triangle method) is still an essential skill for power operation and maintenance personnel.

For the power industry, attaching importance to the DGA test, following international/national standards (e.g., IEC 60599, GB/T 7252-2017), and establishing a long-term trend database are key measures to ensure the safe operation of transformers and reduce the risk of power grid accidents.