Fiber Optic Sensing

It took nearly 25 years of research before the use of fiber optic sensors to monitor winding temperature became mainstream with transformer manufacturers and utilities.


It took nearly 25 years of research before the use of fiber optic sensors to monitor winding temperature became mainstream with transformer manufacturers and utilities. The technology's accuracy, powered by an ability to directly measure winding temperature as opposed to inferring it though sensing oil temperature, eventually helped utilities increase short-term loading by an average of 15 percent without fear of damage to the transformer insulation.

In the 1980s, manufacturers of fiber optic temperature measurement systems worked extensively with transformer original equipment manufacturers (OEMs) to improve ease of integration and reduce susceptibility to probe breakage during installation. By the early 1990s, significant design changes had been made to fiber optic probes to address these issues. These changes included smaller diameter fibers used for probe construction-made possible by the availability of more powerful light-emitting diode (LED) light sources-and the use of protective Teflon spiral overwrap to shield against compressive forces. These enhancements helped transform fiber optic hot spot monitors from a specialty product into robust, industrial-grade technology suited for transformer monitoring, which in turn gave more utilities a chance to realize its benefits. In addition to benefits such as thermal model calibration and recalibration, fiber optic hot spot measurement enabled utilities to verify transformer design integrity and safely maximize normal loading without damaging insulation or reducing transformer life.

The next evolution in fiber optic hot spot monitoring was the introduction of the hot spot controllers that enabled utilities to instantly activate different cooling measures or alarms when the winding reached specific temperatures. Temperature outputs, threshold alarm signals and system diagnostics could be integrated into the utility's supervisory control and data acquisition (SCADA) system. Advantages for this include the ability of operations personnel to use the temperature data to aid in determining how heavily they can load power transformers without jeopardizing their life value as well as protecting against short-term overloads.

Fiber optic technology is the most accurate method for determining, in real time, when the transformer winding hot spot is approaching a critical value. By contrast, traditional temperature indicator technology, which senses top oil temperature, can significantly lag or even misrepresent actual winding temperatures. Traditional temperature indicators react slowly to the winding temperature changes and use calculations to derive that temperature. This is less efficient than fiber optic technology, which allows direct and instantaneous temperature readings.

Market forces are now poised to accelerate the use of fiber optic winding hotspot monitoring to standard practice. First, many of the transformers currently in use are approaching the end of their operating life. As these transformers are replaced, utilities are expected to replace them with transformers that are far more instrumented than their predecessors. Secondly, as the smart grid era moves forward, many utilities are under greater pressure to ensure reliability with the most accurate maintenance data available. These trends are expected to make utilities take a closer look at using fiber optics in their units, which could mean more business for companies such as Santa Clara-based LumaSense Inc.

One of the company's flagship products, the LumaSMART fiber-optic monitoring system, uses a patented technique called Fluoroptic technology that has been used for decades in thousands of transformers. Fluoroptic technology is based on the decay time of an inorganic photo luminescent sensor material after it is excited by a light pulse. The resultant decay of the fluorescence varies precisely with temperature. Because it is a time-domain measurement, this technique is inherently stable and drift-free, making it ideal for measuring hotspots since it is immune to electromagnetic interference (EMI) and radio frequency (RF) environments. In addition, no required calibration means no maintenance is needed for the life of the transformer.

In addition to the common benefits of fiber optic technology, the LumaSMART control system includes an easy-to-use touchscreen interface that supports up to 16 channels of measurements and relays. LumaSMART is designed specifically for EHV/UHV/HVDC applications and reactors, or any large power transformers; it can, however, also be applied in distribution and transmission transformers. Aside from the instrumentation, LumaSMART's key differentiator is the technology's ruggedness and longevity. With decades of real-world deployments, the technology is proven to last the full life of a transformer. The company is confident LumaSMART has the right mix of durability and ease-of-use to enable transformer makers and utilities to optimally design, build and operate the next generation of transformers that will take the nation's power infrastructure well into the future.

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