Why measure transformer hot spots?

Transformer hot spot measurements are generally carried out for the following three reasons: Design verification, Lifetime calculations, and Full Dynamic Loading management.

• Design Verification: To calibrate the thermal model, verify design integrity and manufacturing quality of the transformer. This is usually done during heat run testing at the manufacturers site.  Gathering this data during the heat run test allows for a "base line" of inforation that can be used for comparison throughout the life of the transformer.  If fiber optics are left installed at the same locations (typical application), customers can compare to the exact as manufactured locations.
• Lifetime Calculations: If the hot spot measurement exceeds 140C, the life of the transformer is significantly reduced. Utilities will use the measurement to calculate end of life timelines for their assets.
• Dynamic Loading: During high loading the hot spot measured temperature can be used via a control scheme to switch on cooling fans and increase the circulation rate of the oil to keep the transformer as cool as possible, thus maximizing transformer life. This practice will also extend the life of the transformer, as you are controlling your cooling equipment based on the hottest spot in the transformer, therefore limiting thermal affects in that area.  During high load conditions, you can maximize transformer performance by knowing exactly what the hot spot is reading, versus an inaccurate simulated technique.

Other benefits include maintenance deferral by understanding the insulation life of the transformer, diagnosis issues related to the transformer and earlier detection of failures, ability to detect cooling system issues - just to name a few.

Is hot spot measurement standard practice?

Hot spot measurement techniques using fiber optics were developed by LumaSense (formerly Luxtron) in cooperation with EPRI in 1982 and have come into standard use since.

How long do the probes last?

Once the probes are built into the transformer and tested, the transformer filled with oil and sealed, the probes tend to last for the lifetime of the transformers. Probes first fitted in 1982 are still functioning correctly. Our light source is LED based, and therefore will never need replacement unlike white light sources.

What is a typical failure rate during installation?

LumaSense (formerly Luxtron) fiber optic technology has been used in transformers since 1982. Over the years the there have been significant improvement in the long-term reliability of the probes and installation techniques. Typical yields are now in excess of 95% using our superior rugged probe design and where installation training has been given, higher yields can be expected.  Experience matters when it comes to fiber optic technology, and this is one of the differentiators that LumaSense offers.

What are the materials in the fiber?

The materials used in our WTS rugged transformer probes are shown in the diagram below. The outer Teflon PFA jacket is perforated to allow oil to completely impregnate any voids in the Kevlar.

Our quality probes have a similar design, with the exception that a layer of Teflon and the Kevlar layers have been removed.

How strong are the probes?

The rugged probes are fitted with a Teflon spiral outer layer to significantly improve the strength of the probes. This allows the probes to withstand events like dropping, running over by forklift truck, ladders/tools dropping on the fiber, etc. It is also possible to bend the probes to a bend radius of 2–3 cm. The last few centimeters of the probe are not protected and should be handled with care.

Are there any special installation techniques?

Reliable installation techniques have been developed over a 20 year period through experience and sharing information with our customer base. A full training program is available.

The probe is usually placed close to the hottest winding. In shell form transformers it can be taped to the winding and in core form transformers it is usually inserted between windings using a modified spacer (as shown below).

Our current installation techniques allow OEMs and repair shops to quickly install the probes for maximum efficiency and minimal costs.  This again comes with more than 25 years experience. 

The rugged probes represent over 20 years of design refinement. Once installed and working in the transformer LumaSense warranties the probes for the lifetime of the transformer. We also carry out free training for Original Equipment Manufacturers (OEMs) on proper installation techniques and fiber handling. Any OEM that participates in this training is certified and hence qualifies for a 100% warranty on all defects during the installation process, even if the defect is caused by installation error.  No other fiber optic provider offers this!

Can the probes be repaired?

If the probe has failed very close to the feed through it may be possible to lower the level of the oil and carry out an in-situ repair.

Can the probes be retro-fitted to an older transformer?

Generally this is not possible as the transformer is completely filled with oil and is sealed. There can be an opportunity to retro-fit probes during a major overhaul of the transformer if the oil is going to be completely drained.  LumaSense is developing techniques to allow for fiber optic replacement within a transformer in field repair with partial oil drain.

What is the maximum length of the probes?

Theoretically there is no limit to the length of the fiber or cable as light transmission is very efficient. For every kilometer of fiber there tends to be a loss of signal strength of approximately 3dB. Of more significance is the number of connections or junctions. For every junction interface there is typically a drop in signal strength of 3dB as well. The number of connections will have more influence than the length of the probe.

What is the temperature range?

The temperature measurement range is –30°C to +200°C based on our measurement equipment.  The probes themselves can measure temperature in excess of 300°C.

The WTS and ThermAsset® products come in a compact NEMA enclosures and can be used in operating environments ranging from –30C to +65C. Accessorizing the enclosures further can extend this operating temperature range.

Do I need to calibrate?

No, the system does not need to be calibrated. In fact, you can move our ThermAsset2 monitoring equipment from transformer to transformer, and calibration is still not required.

LumaSense’s Fluoroptic Thermometry is based on the principle that a phosphor material can be made to fluoresce when excited with light and that the decay time of the fluorescence is proportional to temperature.

Two of the major advantages of this technology are reliability and stability. Reliability comes from using only components that do not deteriorate with time (LED light source). Measurement stability comes from the physical fact that the decay time of the phosphor is an inherent material property and never changes.

What is the measurement rate?

The technology allows a single channel measurement rate of 250ms. In practice most utilities measure the temperature every 2 or 3 minutes. If the transformer is running at 90% loading or above and the winding hot spots are over temperature a more rapid read rate of one reading every 10 seconds tends to be used. It should be noted this form of temperature measurement is considerably quicker than relying on top oil temperature measurements that have time constants on the order of hours.

How many channels?

The WTS-22 instrument supports 4 measurement channels. The m600 module is available in 1,2 or 4 channels as standard products. An additional module can be added into the WTS-22 to provide an 8-channel version. Our ThermAsset2 device can measure from 1 channel up to 8 different channels.

Does the module have any memory?

The ThermAsset2 product has approximately 16MB of memory that can allow for 90 days of data storage at 1 minute intervals (interval can be modified during the manufacturing process).  Data can be downloaded in real-time and saved to digital files using ASCII files or Excel.

What is the accuracy?

The technology accuracy is ±2°C uncalibrated.

What system for different sizes of transformers?

Generally speaking any of the 3 products, m600, ThermAsset2 and WTS-22 can be used with any size of transformer. The WTS system tends to be fitted to the large transformers and the ThermAsset2 to smaller, < 25MVA, transformers. This is mainly related to the relative unit cost compared to the overall cost of the transformer. However the ThermAsset2 can be installed on large power transformers and GSU's with no issues.

The m600 module is the “building block” of the technology and can be incorporated into the manufacturers control scheme. The m600 is fitted into the ThermAsset and WTS units. LumaSense’s  probes can be utilized with any of the 3 product offerings.