What Causes Partial Discharge in Power Transformers and Gas Insulated Switchgear Substations
Everything You Need to Know About Transformer Oil and Winding Temperature Indicators
Is Hydrogen Gas in Transformer Oil the Best Early Indicator of a Fault?
Many single-gas online Dissolved Gas Analysis monitors focus on Hydrogen as the correct gas to monitor. Why is this? Why is Dissolved Gas Analysis the only oil test that happens online? Should you be monitoring more than Hydrogen?
Hydrogen Gas in Transformer Oil and the Complexity of Testing Oil Filled Power Transformers
Oil filled electrical equipment is a complex mixture of chemistry and physics. Chemistry in the for of oil, paper, pressboard and metal and physics thermal and electrical effects. For that reason, monitoring of the transformer invariably involves both chemistry and physics.
Multiple test methods describe the parameters that should be tested for and the methods that should be used
| Method | Description |
| IEC 60 156 | Dielectric Breakdown Voltage test method |
| ASTM D3487 – 16 | Oil Color (among other methods) |
| ISO 6295 | Interfacial Tension |
| IEC 62021-1, | Transformer Oil Acidity (Total Acid Number) |
| BS 61198:1994 | Determination of Furan content |
| IEC60 567 | Dissolved Gas Analysis |
The above is a small sample of the tests that an electrical asset manager might request from an oil laboratory.
Samples of oil from a transformer would be collected and sent to a laboratory for testing perhaps once per year. An example of the range of tests that are typical for a lab are detailed here. The information would come back to the electrical asset manager in the form of a transformer oil testing report.
So why is it, if all these tests must be routinely performed that Dissolved Gas Analysis is the only physicochemical test that is also done online? What is unique about Dissolved Gas Analysis that it alone has become an online test, and for single gas monitors, what is the reason that all the monitors focus of Hydrogen?
It all boils down to one thing – The Cost-Benefit of Online Monitoring for Oil Filled Power Transformers
For most physicochemical tests, the recommended frequency for getting a measure of the current state of the electrical asset is every 1 to 2 years. E.g. Furan analysis is recommended every 2 years; Dielectric Breakdown Voltage every 1 year. The exception to this is Dissolved Gas Analysis.
For Dissolved Gas Analysis, the recommendation is that as conditions progress the frequency of measurement should be increased from annual testing all the way up to daily [IEEE Standard C57-104-2019_pg 87]
While the accumulation of acidic compounds or the generation of furans happen very slowly and only need to be tracked over the long term, the production of large quantities of gas indicating a severe fault can happen in a matter of hours. It is for this reason that online Dissolved Gas Analysis has emerged as one of the most important parameters to monitor (closely followed by online partial discharge monitoring). More information on partial discharge and Dissolved Gas Analysis can be found here.
The value of online Dissolved Gas Analysis can be seen in the many case studies that exist showing instances where it detected a severe and rapidly progressing fault that would almost certainly not be detected with an annually Dissolved Gas Analysis. A Qualitrol Serveron TM8 Online Gas Chromatography Dissolved Gas Analysis monitors was employed in all of the case studies shown below.
Why is Multigas Online Dissolved Gas Analysis (DGA) not installed on all transformers?
Simple, the Cost Benefit does not warrant it.
While Online Dissolved Gas Analysis of Generator Step Up Units and large power transformers is easily justifiable given the cost of replacement or repair of these of transformers the same does not hold true for smaller power transformer and distribution transformer
On the basis that multigas online Dissolved Gas Analysis should be used in a targeted fashion the next best option for online Dissolved Gas Analysis is single gas Dissolved Gas Analysis for detection of hydrogen gas in transformer oil. These type of monitors tend to be much lower cost to purchase and install and some including the ServeronTM1 and Qualitrol DGA 150 are maintenance free for the lifetime of the monitor.
Why is Hydrogen the gas of choice for single gas monitors?
This is because of the chemistry of Oil breakdown, and is best illustrated by a graphic borrowed from Transformer Maintenance – Facilities Instructions, Standards, and Techniques; United States Department of the Interior Bureau of Reclamation: Washington, DC, USA,
Figure 1: USBR.GOV
This graphic is a simple representation of the conditions that result in the production of gas, including partial discharge and elevated Temperature
What is immediately obvious is that as temperatures increase in the presence of a fault, Hydrogen gas in oil is created at all fault temperatures AND it increases in quantity as temperature increases. Other gases come in and out as temperatures change but Hydrogen gas in transformer oil increase at an increasing rate. Observing the bar to the left we can see that this also applies to partial discharge, whereby in the presence of partial discharges Hydrogen gas in oil is by far the dominant gas produced.
Another view of this relationship between Hydrogen and most fault conditions comes from the below graphic. Here we can see that most electrical or thermal fault conditions are evidenced by the presence of Hydrogen gas in oil.
Final Thoughts
- Dissolved Gas in oil monitoring has emerged as the primary online monitoring for electrical asset management
- Online Dissolved Gas in oil monitoring of multiple gases will provide for rapid detection and diagnosis of many faults
- The universal fault gas is Hydrogen, produced under all fault temperatures and conditions.
For more information on Dissolved Gas Analysis products or solutions, contact us.
