It’s a question as old as time, well at least for the last 20+ years, “Do I need a single-gas or multi-gas monitoring system for my transformer?” And the answer is…yes. Yes, you need a gas monitor on your transformer. Yes, you need to know when something abnormal has occurred inside the unit. Yes, monitoring your transformer will help you to extend its reliable life. But “yes” does not answer the question of which type of monitor you should install. This paper will attempt to help you develop the best strategy to pursue.
This paper, think of it as a roadmap, is designed to transition asset managers from a time-based maintenance model to a reliability mindset. My intention is to provide transformer managers and custodians with the questions and considerations they will need to help guide decision making on what type of dissolved gas monitor they should install.
We will review why online transformer monitoring is required and understand the costs associated with various DGA monitoring systems. Asset managers should consider both the initial purchase cost combined with ongoing maintenance requirements and ask and answer the question: “Do all my transformers need online DGA monitors?” Considerations like asset criticality and current health should be evaluated to determine if a single-gas or a multi-gas unit is the right fit for your transformer reliability program. Let’s get started!
Why install online DGA monitors?
First and foremost, we need to understand why dissolved gas analysis (DGA) is important and why we should install DGA monitors on our transformers. Transformers are a key component of the electrical system. It has been said that “transformers are the heart of the power system.”
So why is it important to have DGA monitors installed on these units? Ask any transformer guru and they will tell you that DGA is one of the most important tools any asset manager can have in their toolbox to understand and determine the health of their transformers.
DGA is a key chemical test of the dielectric insulating liquids that tells us if an abnormal condition has occurred. DGA diagnostics can also provide an indication what type of thermal or electrical fault may have occurred based on the profile and concentrations of the gases produced. Online monitoring can tell you whether an internal fault is ongoing or if it was a one-time event. Online monitors can also provide information on the specific conditions present when the fault occurs.
Manual DGA analysis is done by taking a manual sample of transformer insulating fluid and shipping that sample to a laboratory where it is then analyzed and evaluated. While this process seems quite simple, there are many factors that need to be considered when sampling and evaluating the results. For example, when looking at all the steps associated with the sampling process, one needs to ask the following questions:
- Did the technician sampling the transformer follow proper sampling procedures?
- Have they been trained in proper sampling techniques?
- How careful were they in the handling of the sample after extraction?
- How was the sample stored?
- What type of container did they use?
- Was all relevant information recorded and included with the sample?
- How old is the sample?”
These are just a few of the questions that need to be answered with regards to the sampling process because the answers will impact the accuracy of the test results.
Now, assume that the technician in the field did everything correctly and the sample made it to the laboratory without incident. We then need to ask the following questions of the laboratory:
- Was the sample received, processed, and categorized correctly?
- What method are they using to analyze the gases?
- Was the equipment calibrated?
- When was the last time the lab calibrated the equipment?
- Is the lab certified?
- How are the results reported?
- What standards and evaluation method(s) are used?
- Does the lab have historical data of the unit to adequately compare and evaluate the current results?
- How long will it take to process the sample?
- If elevated gases were found, is there any chance high gases from a prior sample could have carried over to our sample?
Once this process is complete, you now have a report that, hopefully, is reviewed, understood, and placed into an easily accessible, preferably electronic folder for future use. Capturing a single snapshot of one moment in time only gives us a glimpse into the health and status of this unit. According to accepted industry standards, if this is an annual sample, you are considered compliant.
However, one must consider what may have happened the other 364 days of the year while that unit is in operation. Is the transformer being monitored? Without an operating online DGA monitor, how can anyone be sure that a fault has not occurred? From a reliability point of view, the asset manager is only performing minimal tasks to ensure the transformer is operating adequately. The true value of a continuous on-line DGA monitor is that it provides transformer health and reliability information the other 364 days of the year.
Providing real-time data that can be evaluated and correlated with internal and external conditions, such as temperature and load, which help the reliability professional better manage their fleet of transformers and transition from a time-based maintenance strategy to condition-based.
What are the costs associated with online DGA monitoring?
When considering a DGA monitor, cost is, and will always be, a prominent factor. The initial cost of an online single-gas hydrogen only (H2) monitor ranges from $5,000 to $12,000. Multi-gas units start around $25,000 for 3-gases and can exceed $70,000 for 8 or 9 gas monitoring solutions. After initial installation and set-up, depending on the technology used, there are costs associated with maintenance and/or component replacement after 3-5 years of operation. These costs may include replacement of gas bottles, sensor, pump, or other component replacement cost, as well as costs for qualified technicians to perform the tasks.
Often, budgeting for the initial purchase comes from capital expense budgets. However, failure to plan for maintenance and repair or replacement of the unit leads to many inoperable and unplugged DGA monitors installed in the field. It is estimated that up to 90% of online DGA monitors installed on transformers are no longer working.
So how does one decide between single-gas and multi-gas? Keep reading to find out.
Evaluating asset criticality: Which transformer should have online DGA monitors?
When it comes to evaluating the criticality of transformer assets, there are several questions that should be answered. The first question is what does this asset power? Knowing what the transformer feeds helps to know what the consequences will be should it fail unexpectedly. This information helps asset managers establish a level of criticality for that asset. For example, is the transformer providing power to the main office, lights in the parking lot, or the 15 industrial ovens at the bread factory?
The next question asks if there is a backup plan if this unit fails? Do I have generators or a spare transformer that can be easily installed (this spare brings its own host of reliability and storage questions) to restore power to the lights in the office / parking lot / bread factory, so the lights and ovens stay on, and the third shift does not have to walk through a dark lot?
Or maybe, there is a sister unit that powers 50% of the ovens so you are only losing half of your production. Understanding what is powered and how the failure will affect the operation can sometimes be difficult. However, taking the time to clearly identify the criticality of an asset to the operation can aid asset managers when developing maintenance and management programs for their unique facility.
Finally, it is worth the time for asset managers to periodically reach out to transformer manufacturers or rewind and repair facilities to understand replacement or rewind costs and turnaround times. Since the pandemic began in 2020, supply chain issues have greatly increased not only cost of materials but delivery times, resulting in an overall increase in shipment lead times of 443% according to the Electricity Subsector Coordinating Council for delivery of all transformer voltage classes.
Once criticality of each transformer in the system has been established, then the decision to install monitoring devices can be better achieved. One could argue that all transformers should have a DGA monitor installed; however, when considering the cost of the monitoring devise versus the total replacement cost of the transformer, the argument can be made that online monitoring is not justified. Understanding the impact of an outage can be the tie breaker in certain cases.
Which system is right for my transformers?
While the criticality of some transformers may justify all the bells and whistles of a multi-gas monitor to ensure the transformer is covered, not every transformer needs a $200,000 monitoring system. The reality is transformers are highly reliable pieces of equipment with about a 1% failure rate. For the majority of new and in-service units, a simple, low-cost, solid-state hydrogen monitor is the best solution for improving transformer reliability and management programs.
The need for multi-gas monitoring can be summed up into two categories, critical large power transformers that are difficult to replace (such as those found in nuclear plants) and transformers with a history of gassing. Should your transformer fall outside of the afore mentioned circumstances, it is not necessary to equip units with multi-gas monitoring devices.
It’s important to remember that when data is brought in from a monitoring device, it must be delivered and stored somewhere, typically in a dashboard. With multi-gas monitors, asset managers migrate from one set of data points annually to multiple data points daily or even hourly. This often overwhelms users when it comes to the management of the data. In some cases, data overload can cause the data to be dumped into folders and ignored or the instruments to be turned off.
When considering which type of monitoring to install, think about where you will get the most bang for your buck. How will you spend your money wisely to find as many transformer faults on your system prior to failure? What is the most effective strategy to implement online transformer monitoring?
Conclusion
Just like people, every transformer is unique and should be treated as such. Criticality and asset health of each transformer are key considerations when determining a transformer management strategy. Each maintenance and management decision will be based on budgets, experience, knowledge and ambitiousness of the asset manager and organization.
Due to the inherent reliability of transformers, they can be often overlooked, and consequences of failure not evaluated. A comprehensive transformer management strategy should be developed. Recent advancements in online DGA monitoring provide solutions that allow asset managers to find potential hazards early and avoid costly downtime. Solutions are now available for transformers down to 1 MVA. With the rapidly increasing costs and lead times of transformers, it’s more critical than ever to evaluate your system and determine the most effective strategy to manage overall costs and reliability.