Dispelling the Old Myth’s of Winding Hot Spot Measurement Using Fiber Optics Today Fiber Optic Temperature Measurement is Critical to the Health of Transformers

Brett Sargent, Sr. Director of Energy Solutions, LumaSense Technologies, Inc. LumaSense (formerly Luxtron) has been measuring winding hot spot temperatures accurately and in real time for more than 20 years. Since LumaSense was one of the pioneers in using fiber optic technology to measure hot spots, we have been on the leading edge of development and have had the opportunity to listen to our customers concerns for two decades. As with any new technology, there were “growing pains”, but we have been in a unique position of working with your customers to successfully develop a sound, robust system that is now being used in more than 1,000 transformers globally. However, due to these early “growing pains” and the traditional nature of our industry, some of these “Myth’s” still exist today, which is unfortunate, given the fact that recent industry polls show winding hot spot temperature monitoring is the most critical parameter to monitor within transformers. These “Myth’s” have prevented customers from trying fiber optics in transformer temperature measurements, or prevented customers from continuing to use fiber optics after initial trials. The purpose of this article is to try and “dispel” some of these “Myth’s” that still exist today. Myth #1: Fiber Optics Are Too Fragile For Transformers During the initial usage of fiber optics in transformers, there was a high percentage of failures due to breakage. Fiber optic breakage mainly comes during three points in the life of a transformer:

1. Transformer manufacturing and initial installation 2. Transport of the transformer 3. Internal inspections of the transformer after it's put in

service

Fortunately for the industry, LumaSense has had the opportunity to redesign the fiber optic probes with over a 95% success rate in fiber optic installation and transport without breakage. In 2003, LumaSense introduced the Rugged Probe™, which is our fourth generation design to allow for minimal breakage. We have had tremendous success with this probe over the past four years. Additionally, transformer manufacturers have gained invaluable experience handling and installing fiber optics over the years. LumaSense works with over 50 manufacturing sites globally, and through on-site training and feedback from the transformer manufacturing partners, we have developed a design and installation methodology that works very well in the transformer industry. This is also the result of 20 years experience in fiber optic temperature measurement.

Figure 1: Patented Fluoroptic Rugged™ Probe by LumaSense Technologies

Double and triple installation of fiber optics at select monitoring points is no longer required with the LumaSense Rugged Probe™. This eliminates additional material costs as well as transformer manufacturer’s efforts, which leads to more cost savings. Myth #2: Fiber Optics Are Too Expensive Any new technology that is introduced into the market has higher initial costs. This can be seen from our experience with DVD players, cell phones and flat screen televisions. The same has held true with fiber optic temperature measurement systems. LumaSense has reduced the costs of our systems by 75% over the past 15 years, while at the same time adding enhanced features and functionality to the systems. This year we have introduced our ThermAsset2 Winding Hot Spot Monitoring and Temperature Control unit that is 75% less expensive than our original WTS-11 system launched 15 years ago. Our new system has the capability to monitor 8 channels, has memory storage of up to 1 year, cooling fan and oil pump controls, self diagnostics, alarms, protective trips and requires no calibration. Given the fact that many transformers can run in excess of $2 Million USD, a fiber optic temperature monitoring system is a fraction of a percent of the costs associated with large power transformers. Myth #3: Fiber Optic Temperature Measurements Are Not Reliable The winding hot spot is one of the most critical parameters to monitor within a transformer. The life of a transformer is dependent upon the life of the insulating material located within a transformer. The life of the insulating material is directly related to the temperature to which it is exposed. The winding hot spot is the hottest region within the transformer, and it is typically where the maximum flux leakage occurs - usually located within the upper region of the winding on the Low

Voltage (LV) side of core type transformers. For years, WTI instruments (gages) have been used on transformers for top oil

Figure 2: The danger of not knowing your transformer condition and the expense of replacing it

Figure 3: Data collected from a polling question taken during two recent web seminars hosted by LumaSense Technologies

indication and winding hot spot indication. This measurement for the winding hot spot is only a simulation based on heating an oil bath via a current transformer that is connected to one of the phases of the transformer. No simulation is as good as the real thing. LumaSense has several documented instances where the actual temperature measured using fiber optics located directly at the winding hot spot region provides results that are accurate, and up to 20°C different than what a WTI indication provides. Additionally, a WTI instrument can include up to four hours of delay in indications from what is actually occurring within the transformer. Direct fiber optic temperature measurement provides immediate indication of any temperature changes. Another consideration is the affect of cooling on the winding hot spot. Many older WTI instruments have no correlation with what is happening to the cooling fans and oil circulation pumps, since they are based solely on the load of the transformer. Therefore, if the fans were to become inoperable, a WTI instrument would show no change from normal since it is only a simulation. Direct fiber optic temperature measurement would immediately give indications of cooling changes by measuring the real affect this has on the winding hot spot. The fact is that WTI instruments are not reliable. Myth #4: Fiber Optic Temperature Monitoring Systems Require More Maintenance Compared to WTI systems, maintenance on some fiber optic monitoring systems is nonexistent. LumaSense uses a long life LED as a light source within our fiber optics systems. This LED has the ability to last more than 50 years without replacement, and our experience in more than 25 years we have never had a light source burn out and require replacement. This is important because when a light source has to be replaced, recalibration of the system has to be performed. Some fiber optic temperature monitoring systems that exist in the market do not use long life LED’s (they use halogen lamps or gas filled bulbs), and these will burn out in a few years of operation. LumaSense has designed systems that never require calibration or maintenance…ever. In fact, our systems are designed so they can be moved from transformer to transformer without any calibration or maintenance. WTI instruments have to be calibrated continuously (usually annually), which means another trip to the field and potential replacement if the calibrations fail.

Figure 4: New ThermAsset2 Transformer Winding Hot Spot and Control System from LumaSense Technologies

Myth #5: I Want To Measure More Than the Winding Hot Spot, and Fiber Optics Cannot Do That LumaSense has used fiber optics to measure winding hot spots, top oil temperature, core temperature, bottom oil temperature, tank wall temperature, etc. Fiber optics offer a level of diversity that has not been available in years past. We have seen fiber optics used as a direct replacement for WTI and Top Oil Temperatures successfully. Since fiber optics are EMI, RF and Microwave resistant, and designed for high voltage applications using high voltage rated fiber, they can be placed practically anywhere in the transformer.

Myth #6: Fiber Optics Cannot Last for the Lifetime of a Transformer The material used in fiber optics is long lasting in an oil submersed (or SF6 filled / dry) environment. From LumaSense’s perspective, we have transformers that have been successfully using our fiber optics for more than 25 years. With two and a half decades of experience, I believe we have crossed the “longevity” threshold with fiber optics. Myth #7: I Do Not Overload My Transformer, So I Don’t Need Fiber Optics While initial new transformers put into service are rarely used at 100% capacity, there is no guarantee that you would be able to keep your transformer at low load through its entire life. Transformers do fail, which means other transformers have to pick up the load. Service areas and loading will grow, which means transformers will see an increase in load throughout their life. There are several reasons to use fiber optics for direct temperature measurement:

1. Design Verification 2. Dynamic Loading 3. Transformer Life Calculations 4. Transformer Cooling Performance 5. Condition Based Maintenance

Transformers are one of the most critical assets to own and manage. Any additional piece of information that you can obtain is vital. Direct winding hot spot measurement with fiber optics can provide one of the most useful pieces of information to help manage your transformer through both good and bad times. While this technology may be new to some users, it is not new to the industry. LumaSense has experienced the learning curve with fiber optic temperature measurements, and through our experience have been able to address those lurking “Myth’s” within the industry.

Figure 5 Fiber optic routing in a power transformer