Historically, DGA testing of tap-changers has not been considered particularly useful due to the large number of gases normally generated by the arcs. This position has been reconsidered in recent years, and the opinion today is that a considerable amount of data is gained by DGA testing of tap-changers.
As 26% of transformer failures are attributed to On-Load Tap Changers (OLTCs), a reassessment of the relationship between DGA and OLTCs stands to save stakeholders considerable time and resources.
DGA testing is necessary to understand the internal condition of any On-Load Tap Changer, and this article will highlight the benefits of monitoring OLTC compartments with DGA technology to further improve oversight of this crucial compartment.
As 26% of transformer failures are attributed to On-Load Tap Changers (OLTCs), a reassessment of the relationship between DGA and OLTCs stands to save stakeholders considerable time and resources.
OLTCs enable voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption.
Online condition monitoring for an OLTC needs to start with the tap changer controls and drive mechanism. Combining monitoring and control into a single device allows the control system to monitor itself, greatly improving the overall reliability of a tap changer and the control system.
When combined, control functions are no longer performed blindly, and monitoring is no longer a passive activity. Since the control system is aware when a command has been issued, if it has access to the monitoring information, it can verify that the command issued was successfully completed.
Due to the number of moving parts, tap changer operation can be one of the more important functions to monitor on a transformer. Historically, the majority of the focus on an OLTC has been placed on the high voltage components (e.g., the tap changer contacts and insulating oil). While the importance of monitoring the high-voltage components cannot be underestimated (including monitoring of tap changer controls and drive mechanism), proper online condition monitoring for an OLTC should include DGA testing.
Dissolved Gas Analysis (DGA) offers critical insights that are crucial for a complete diagnostic picture. DGA can detect issues in their incipient stages when they are difficult to detect with other testing methods.
Dissolved gas analysis focuses on the levels of the individual gases generated during an operation as well as the ratios at which these gases are present in the insulating fluid. The prevalence of certain gases or related gassing patterns can indicate the condition of the tap changer's internal components and dielectric medium.
Simply put, this diagnostic accuracy eclipses other diagnostic tools and requires fewer resources than other offline methods such as visual inspections, thermal imaging, and acoustic analysis. DGA offers critical insights that are crucial for a complete diagnostic picture. DGA can detect issues in their incipient stages when they are difficult to detect with other testing methods.
Part 1: Monitoring Motor Energy
Monitoring motor energy of tap changers is accomplished by recording the power a motor consumes as the tap changer passes through each tap. Tap changers usually have a 16-tap raise and a 16-tap lower.
As the motor changes the moving contacts between these, abnormal contact wear or alignment can occur, causing a variance in the amount of power consumed when compared to that of a tap that is in prime condition. Motor monitoring can also be effective for detecting problems with the tap changer’s linkage.
While non-DGA methods can provide a limited assessment of contact condition without performing a visual inspection, complementing them with a DGA can be more effective than monitoring of motor energy, monitoring the temperature differential between the OLTC and main tank, and even infrared camera inspections alone.
Part 2: Key Gas DGA (Hydrogen Only)
Hydrogen or key gas monitoring is using a hydrogen sensor to detect abnormal conditions occurring within the transformer, which is particularly effective as hydrogen is both the first gas generated when an abnormal condition occurs and also the only gas which is consistently generated as thermal conditions increase.
The levels of the other combustible gases being generated evolve as thermal energy is increased. For example, hydrogen and methane are generated at approximately the same thermal level but methane converts into ethane as the joules of energy expended begin to increase. As the thermal conditions continue to increase, that ethane is converted to ethylene and finally acetylene at approximately 700℃. This thermal progression and hydrogen’s generation throughout make hydrogen monitoring the most practical, cost-effective solution for detecting abnormal operating conditions within the transformer.
Performing Dissolved Gas Analysis (DGA) on On-Load Tap Changers is essential for several reasons, primarily to ensure operational reliability and optimize maintenance practices.
Regular DGA testing is a proactive approach that ensures the long-term health and efficient operation of OLTCs, contributing to the overall reliability and safety of the electrical grid.
Part 3: Multi-gas DGA
Multi-gas DGA is a monitor capable of measuring diagnostic gases saturated in oil in the ppm range, making for more precise measurements. These monitors are available in the following:
- Five-gas: hydrogen, methane, ethane, ethylene, and acetylene.
- Seven-gas: carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene, and acetylene.
- Nine-gas: oxygen, nitrogen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene, and acetylene.
Hydrogen, methane, ethane and acetylene are deemed to be the combustible gases most crucial for thermal diagnostics of a transformer. (Carbon monoxide is also included in the combustible gas but is not related to a thermal value due to fault condition.) These gases provide an indication of the severity of a thermal defect due to the approximate thermal energy temperature at which they are produced.
- H2:CH4~150℃
- C2H6~250℃
- C2H4~350℃
- C2H2~700℃
As previously stated, the ratios of these gases are a reliable diagnostic tool for determining the nature of the defect.
Hydrogen-Dominant
Partial discharge: this dominant hydrogen reading can also be related to stray gassing, core saturation due to excessive harmonics, higher than normal ambient, and load. To make an accurate diagnosis of which of these conditions are present, load and temperature readings need to be taken into consideration.
Also, examining which of the other combustible gases are being generated, and how they are evolving, helps determine if the condition is due to external factors or internal conditions. Hydrogen (which is accompanied by methane, that over time starts to convert to ethane or ethylene) is a solid indicator that an internal condition is becoming more severe and needs to be addressed.
Ethylene-Dominant
This is an indicator that a T2 or T3 level fault is present. Severity is dependent on ppm values of ethylene being generated as well as levels of ethane and acetylene.
Carbon Oxides: Carbon monoxide and carbon dioxide are the carbon gases in this case. The ratio of these gases indicates the effect the abnormality has on the solid insulation system.
Nitrogen/Oxygen
These are atmospheric gases, which are a good indicator of the integrity of the transformer tank. Nitrogen is the expected dominant gas as it is inert and often used to provide a blanket of positive pressure for transformers that are not conservator tank designed. The presence of excessive oxygen is also detrimental as this can lead to excessive acid levels, which expedites the breakdown of both the solid and liquid insulation systems.
Multi-gas monitoring requires that oil be circulated through the monitor to ensure that a fresh sample is being measured. This is required for the monitor to provide a DGA measurement that is representative of the transformer’s bulk oil.
Contact Condition
When it comes to identifying the contact condition of the OLTC, DGA sample analysis is the most accurate method of assessment.
This is achieved by observing the ratios at which combustible gases are present. For an accurate analysis, one must first recognize the gas ratios associated with a good arcing signature. In a sealed tap changer that is in good condition and functioning properly, the following ratios would be expected:
- Hydrogen/Acetylene~ 2:1
- Acetylene/Ethylene~/>10:1
- Methane/Ethane~3:1
DGA analysis can also be used to diagnose faulty OLTC conditions in their incipient stages, allowing for costly failures to be avoided, making for better scheduling of labor-intensive internal inspections.
Arcing Signature
Just as these ratios are a good indicator of a healthy tap changer, DGA analysis can also be used to diagnose faulty OLTC conditions in their incipient stages, allowing for costly failures to be avoided, making for better scheduling of labor-intensive internal inspections.
For example, a DGA sample from a healthy OLTC would present a much different signature than one that was experiencing excessive contact wear. One of the best means of determining this excessive wear condition is using the Stenestam’s ratio, as follows:
CH4+C2H4+C2H6/C2H2
<0.5 no overheating
=>0.5-5.0 resample - the higher the initial number or if found to be increasing on resamples, sampling rate should be increased
5.0 active overheating – active overheating is present, and an internal inspection should be scheduled
Another method for analyzing these samples is to track changes in the ratios when compared to baseline samples. If ethane is growing to become dominant over methane and starts to convert into ethylene, this is a solid indicator of incipient contact overheating.
An understanding of OLTC design is also necessary to make an accurate assessment of these samples. For instance, a free-breathing tap changer will allow hydrogen to rapidly escape to the atmosphere due to hydrogen’s low solubility rate and molecular structure. Therefore, samples taken from these free-breathing designs will have lower hydrogen values than those of sealed tap changers.
Another reliable indicator of a free-breathing tap changer is the nitrogen-to-oxygen ratio, which is expected to be ~70% to 30%, or closely representative of the atmosphere. However, if these conditions are present in a sealed tap changer, it is a sound indication that a leak has developed.
Vacuum OLTCs
Over the course of the last three decades, Vacuum OLTC technology has become the predominant switching technology in the areas of medium-voltage substations and high- capacity power contactors, replacing oil- and SF6-based technologies. Today, more than 60% of the demand for circuit breakers in the medium power voltage segment worldwide is covered by vacuum-type circuit breakers.
Since all the arcing occurs in a vacuum bottle isolated from insulating oil, more operations are possible overall.
These operations can be monitored with hydrogen, as there should not be any gassing in the oil since arcing occurs in the vacuum bottles. In these vacuum tap changers, the presence of any combustible gases would indicate that one of the bottles has failed. For arcing in oil tap changers, hydrogen-only monitoring would not provide much value, as the combustible gases are needed to access contact condition. Essentially, hydrogen and acetylene should be the dominant gases. If ethylene becomes dominant, it indicates contact overheating.
As all arcing for this type of OLTC occurs within a vacuum bottle, the presence of combustible gas in the DGA samples is an indication of vacuum bottle failure.
If a sudden increase in combustible gases occurs during any vacuum OLTC inspection, repairs should be scheduled immediately in order to avoid overall transformer failure.
Concluding Thoughts
Fully functioning online condition monitoring for an OLTC should strongly consider DGA testing, be it Key Gas DGA or Multi-Gas DGA.
Power transformers equipped with On-Load Tap-Changers have been the main components of electrical networks and industrial applications for nearly 90 years. DGA testing offers internal insights that are crucial for a full diagnostic picture, as DGA can detect issues that other methods might miss.
Chris Rutledge is currently a Product Manager at Dynamic Ratings, Inc.
He joined the Dynamic Ratings team in July 2019. Prior to his present position, he served as Substation Asset Manager and chairman of the Substation Safety Committee at Memphis, Light, Gas and Water. He has 27 years of experience in the utility industry, primarily focused on the installation, service and maintenance of both substation and distribution equipment. He has done extensive research and published multiple papers concerning the interpretation of dissolved gas analysis testing. Chris is dedicated to assisting customers in finding creative cost-effective solutions for improving safety and reliability across their entire electrical system.
Tyler Willis is a former magazine editor and teacher. He holds a Master’s degree in English from the University of Toronto. He is the Founder of Sterling Content, a content solutions company.
Tyler has worked in SEO Writing since before SEO Writing existed and has over 20 years of experience in Content Strategy, SEO Project Management, as well as B2B & B2C Copywriting.