Dissolved Gas Analyzer (DGA): Early Fault Detection of Transformer & Reactor.

1. Introduction

A dissolved gas analyzer (DGA) is a component used to analyze the gas dissolved in the insulating oil of a transformer or shunt reactor. The presences of gases can be determined which faults or abnormal condition. It is used for conditional based maintenance (CBM) whereby it will send an alarm whenever there is abnormal detection in the insulating oil.

2. Overview

Applicable Standards: IEEE C57.104/ IEC 60599, IEEE C57.104, CIGRE 771 and ASTM D3612

Relevant Standards: IEC 60156 Dielectric Breakdown Voltage test method; ASTM D3487 – 16 Oil Color (among other methods); ISO 6295Interfacial Tension' IEC 62021-1, Transformer Oil Acidity (Total Acid Number); BS 61198:1994 Determination of Furan content; IEC60567 Dissolved Gas Analysis

Used in HV Components: Oil Filled Transformer and Shunt Reactors

To Detect Potential Early Faults of: Overheating, Corona/ partial discharge (PD)/ low energy arching, High Energy Electrical Arcing, Thermal Faults, Cellulose Insulation breakdown and Overload history of transformer

3. Early Fault Detection of Gases Types for Different Faults

The principal or key fault gases detected in DGA associated with each type of issue are shown below:

• Hydrogen (H2): generated by partial discharges and arcing

• Methane (CH4): generated by relatively low elevated temperatures (150° C).

• Acetylene (C2H2): generated by arcing.

• Ethane (C2H6): generated by high temperatures (300° C)

• Carbon Monoxide (CO): generated by oxidation of cellulose insulation

• Carbon Dioxide (CO2): generated by oxidation of cellulose insulation

• Moisture H20: insulation degradation

• Ethylene (C2H4): high-temperature thermal fault

4. How to choose a DGA?

The balance between performance (how many gases and accuracy) to price/ requirements ratio

• Portable DGA units offer on-site analysis for rapid fault detection. 

• Single gas units are basic for early detection of any faults (eg detecting only H2 and/or moisture), while multi-gas units provide more detailed diagnostic information with specific gases. 

5. DGA Detection Technologies

There are various gas detection technologies, namely:

• Gas Chromatography: Separates and quantifies different gases dissolved in the transformer oil. 

• Infrared (IR) Spectroscopy: Measures the absorption of infrared light by gas molecules, allowing for the determination of gas concentrations. 

• Sensors: Various sensors are used to detect specific gases, moisture, and other relevant parameters. 

DGA can be specialized to protect each sub components of transformer/ reactor including (which can be placed and analyze specific gas in these specific areas): OLTC, bushing, winding, core, radiator, fan, pump, tank etc.

6. Existing DGA Brands and Products

There are three main brands of DGA namely:

(1) General Electric (GE) Vernova: Transfix DGA500, Kelman DGA 900 (Normal, Plus, Taptrans, Multitrans), Kelman MINITRANS, Kelman TRANSPORT X^2, Hydran M2-X, Hydran 201Ti etc. See more: https://www.gevernova.com/grid-solutions/automation/monitoring-diagnostics/multiple-gas-transformer-dga

(2) MTE Meter Test Equipment AG: Portable DGA and Hydrocal DGA - GenX (Maintenance -Free), Intensive Care (With individual analysis of multiple gases) - 1005, 1008, 1009, 1005 Offshore, 1009 Offshore, 1011-3. See more: https://www.mte.ch/products/transformer-monitoring

(3) MR: MSense DGA 2, DGA, 5, DGA 9, See more: https://www.reinhausen.com/productdetail/sensors/msense-dga

Other DGAs brands includes Qualitrol, Megger, Optimus etc.

Some DGAs are online (real-time continuous) remote connection to SCADA with instant fault detection and alarm which connected to transformer and reactor at all time. Some DGAs are portable onsite. Some DGA systems utilize a single valve opening for both gas extraction and oil flow, while others employ separate inlet and outlet valves for improved accuracy and detection of a wider range of gases.

7. Continuous Online Monitoring: Key Gas Method

To analyze the gas detection and potential faults, we have various methods, namely: Dornenburg Ratio Method, Rogers Ratio Method, Nomograph Method, IEC Ratio Method, Duval Triangle Method, and CIGRE Method. In summary, the Duval Triangle and IEC Ratio methods are the most popular DGA methods due to their simplicity, effectiveness, and widespread acceptance in the industry. While other methods like Doernenburg are also used, the Duval and IEC methods are favored for their reliability and ease of interpretation. 

7.1 IEC Ratio Method

This method uses specific gas ratios (C2H2/C2H4, CH4/H2, and C2H4/C2H6) to diagnose faults. It is a standardized approach, widely accepted and often used in conjunction with other methods. The IEC method is known for its reliability and ability to detect various fault conditions. 

7.2 Duval Triangle

Duval's Triangle is an excellent technique that has proven itself to diagnose internal faults in power transformers. It used the measurements of gases ppm and draw a line to intersect on the potential fault area.

As shown in the above figure, the areas are defined as:

• PD: partial discharge

• T1: thermal fault below 300 degrees celcius

• T2: thermal fault between 300 and 700 degrees celcius

• T3: thermal fault above 700 degrees celcius

• D1: low energy discharge (sparking)

• D2: high energy discharge (arcing)

• DT: mixture of thermal and electrical faults

7.2.1 Calculation Example

According to the calculation and Duval Triangle Analysis example from MR Official Website, (All Calculations are credited to MR official websites, you can refer to its website), the measurements of DGA values (for example) are the below:

• H2: 8 ppm

• CH4: 1313 ppm

• C2H2: 2908 ppm

• C2H4: 616 ppm

• C2H6: 420 ppm

• CO: 450 ppm

• CO2: 1069 ppm

Only the values of C2H2, CH4 and C2H4 are necessary for Duval Triangle only.

The percentage of C2H2 is 2908/(2908 + 1313 + 616) = 0.601 = 60.1%.

The percentage of CH4 is 1313/(2908 + 1313 + 616) = 0.271 = 27.1%.

The percentage of C2H4 is 616/(2908 + 1313 + 616) = 0.127 = 12.7%.

Then draw a corresponding lines parallel to the labelling of the axes and it intersects in the area called D1, meaning low energy discharge, or sparking.

The areas are defined as:

• PD: partial discharge

• T1: thermal fault below 300 degrees celcius

• T2: thermal fault between 300 and 700 degrees celcius

• T3: thermal fault above 700 degrees celcius

• D1: low energy discharge (sparking)

• D2: high energy discharge (arcing)

• DT: mixture of thermal and electrical faults

For more comparison between different methods: See - https://onlinelibrary.wiley.com/doi/10.1155/2023/9960743 & https://www.sciencedirect.com/science/article/pii/S1876610211044997

Overall, DGA is used to maximize the operational life of transformer and shunt reactor. It is for the safety operation and preventive maintenance purpose. Any ongoing faults and unwanted gases will weaken the insulation, winding and other aspects of HV components. Early detection of faults will allow corrective measurement to take place.

DGA is only used for online remote monitoring and it can be not as accurate as laboratory testing. Hence, offshore substation requires higher accurate DGA with more gases detection as manual laboratory oil sampling could be challenging in offshore. In short, DGA is used for preventive early detection of potential transformer/ reactor fault/ failure.

8. Challenges

Some technical limitations and challenges of a DGA includes: Calibration and accuracy, communication issues, integration limitation with other unsupported communication protocols, inaccurate gas detection and still requires manual oil sampling with laboratory testing. Offshore DGA is also challenging as the location itself in offshore is limited to routine checking and hard to analyze the oil via manual sampling and onsite checking. Hence, a better accurate and multi gases DGA is recommended in offshore application. Plus, DGA requires comprehensive proactive maintenance, proper Data Visualization and Analysis and proper Integration with Other Systems. It also requires OT security compliance to ensure security.

9. Existing DGA Monitoring Solution & Connectivity

SAP has a predictive maintenance solution specialized for DGA monitoring with oil quality analysis (OQA) [5]. GE, MTE and MR have their in-house web apps, GUI and other relevant oil monitoring software to be deployed in SCADA and HMI systems. Plus, on the DGA itself, it will have a local HMI display to see gas content on-the-spot.

Normally DGA has Modbus® over RS485 / TCP/IP, Standard 1Gb Ethernet (RJ45), DNP3.0 over RS485/TCP/IP, IEC 61850, ST/SC Multi-mode fiber converters, Option: GSM/GPRS/UMTS/HSPA+ modem and Option: Wi-Fi (802.11b/g/n). Its enclosure should have IP rated and coated with paint/ made of stainless steel.

Its outputs can have multiple Alarm setting/scenarios, absolute gas level and/or PPM parts per million, Rate of Change (ROC), moisture level, Total Dissolved Combustible Gas (TDCG) and user defined gas ratios alarms: with analog outputs: absolute/ ppm Level, and Rate of Change (ROC) and/or Digital inputs: status transition.

10. Future Development

In the future, the digital twins of a transformer or shunt reactor can be created, and with the assistance of AI, the preventive maintenance can be more advanced and predictive. In a Nature Paper, a real-time intelligent monitoring and maintenance system for transformer operation was developed, enabling multi-modal twin data collection, digital twin visualization, efficient status detection and recognition, and intelligent generation of operational decision recommendations. The historical data of DGA gas detections can be used to train AI model to predict when/ what is the next fault for preventive maintenance.

Further Detailed Technical Reading and References | More info:

[1] Dissolved gas analysis for power transformers: https://www.gevernova.com/grid-solutions/sites/default/files/resources/products/applications/dissolved%20gas%20analysis.pdf

[2] Applying dissolved gas analysis to distribution transformers: https://library.e.abb.com/public/583b157139b358c485257c2300640110/Applying%20DGA%20to%20Distribution%20Transformers(4)%20.pdf

[3] A Review of Dissolved Gas Analysis in Power Transformers: https://www.sciencedirect.com/science/article/pii/S1876610211044997

[4] Dissolved Gas Analysis Equipment for Online Monitoring of Transformer Oil: A Review: https://www.mdpi.com/1424-8220/19/19/4057

[5] SAP Oil Quality Analysis (OQA): https://help.sap.com/docs/SAP_PDMS_ADDON_UTIL_OP/f5b113983bdc45e58cbaa08765152e33/c18eb09d4544449a990e90a04521e817.html

[6] Explanation of Duval Triangle: https://www.reinhausen.com/the-duval-triangle-explained-in-3-minutes

[7] A novel method for intelligent operation and maintenance of transformers using deep visual large model DETR + X and digital twin: https://www.nature.com/articles/s41598-024-83561-7

[8] Research Paper on Online DGA: https://www.researchgate.net/publication/358667995_Online_dissolved_gas_analysis_used_for_transformers_-_possibilities_experiences_and_limitations

[9] GE DGA 900 Brochure 1: https://www.gevernova.com/grid-solutions/sites/default/files/resources/products/brochures/md/kelman_dga_900_plus-brochure-en-230221-grid-gea-33152-a4-r006-hr.pdf

[10] GE DGA 900 Brochure 2: https://www.gevernova.com/grid-solutions/sites/default/files/resources/products/brochures/md/dga900_grid-ga-l3_a4_hr.pdf

[11] GE Example User Manual of Kelman 900 Plus: https://www.gevernova.com/grid-solutions/sites/default/files/resources/products/manuals/ma-041-dga-900-plus-installation-and-commissioning-manual-rev1.1.pdf

[12] MR DGA 5 Installation and Operating Instructions: https://www.reinhausen.com/fileadmin/downloadcenter/products/sensors/msense_dga/ba/5/7045877_en.pdf

[13] MR MSense Brochure: https://www.reinhausen.com/fileadmin/downloadcenter/products/sensors/msense_dga/f0388901_msense_dga_en.pdf

About Author

Ir. Ts. Dr. Kah Yung Yap, CEng (UK), is currently a deputy project manager at Feng Miao Offshore Wind Farm & senior project engineer/ HVDC expert specialist at Fichtner GmbH & Co. KG, supporting & leading both Taiwan & Korea Offshore Wind Farms HV Components & Substations. He is also an industry expert advisor at Heriot-Watt & UCSI University, and VELLD Consultant. Previously, he was a Senior High Voltage (HV)/Medium Voltage (MV) Engineer at Ørsted, working on wind farm projects in Taiwan and Korea. He was also a lecturer at Xiamen University. He worked at Monash University as a renewable energy researcher and as a consultancy electrical engineer at Perunding Eagles. His R&D interests include HVDC, renewable energy, electric vehicles, energy storage, electrical power systems, smart grids, and artificial intelligence. His technical publications are listed in Google Scholar and can be downloaded via ResearchGate.