By Paul Richardson
The use of fiber optics for test and monitoring of infrastructure is well known. Recent improvements in distributed sensing have brought new attention to its use in monitoring power generators. The sensing medium is fiber optic strands made of silica glass and is low cost and very pliable. Because they're made of glass, fibers are immune to electromagnetism present in the generators. This is a big advantage over the thermal couplers in use today, which are electrically powered. Overall, a fiber as a distributed sensor is not a new technology. Since 1995 fiber based distributed temperature sensing has been researched, field trialed, permanently installed and relied upon around the world. But new innovations over the last three years have resulted in systems that now are more accurate, more precise, faster to measure and lower cost than ever before.
The current method of monitoring heat of a generator with thermal couplers can completely miss a dangerous and eventual destructive localized build up of heat. If heat develops in a lead or coil, that localized heating event will likely go undetected. Many points of failure are not instantaneous, rather they are caused from a successive build up of heat over a short period of time. The current method of monitoring generator temperature really is measuring the hot gas temperature blowing out of the generator with a thermal coupler. Thermal couplers use electrically powered sensors and must be isolated from the 25,000 volts found in the generator, so a Teflon tube directs the hot gas past the isolated thermal coupler. That heat flow is from the widespread area. Consequently, small points of heat build-up are diffused with the rest of the hot gas coming off the generator. Given that a generator can be 25 feet long, the temperature of the hot gas is really a blend of all the heat sources together and is unable to detect localized hot spots. A better way would be to measure temperature throughout the generator directly. But localized measurements also need to be fairly rapid and accurate. Older fiber DSTS took as long as an hour to attain the accuracy needed and only measured every meter. New DSTS analyzers now can measure every 5cm to 10cm accurately and at a very fast increment.
There are several areas of interest for heat monitoring within a generator.
One area is the coils. Variable by generator, the number of coil slots varies but 27, 36, 42, 48 and even 60 slots exist in the upper and then again the lower coil. At the series connection point between the upper and lower coil is another troublesome hot spot that relies on a single point RTD that is very well insulated. Even though the coils are designed for a 20-year life span or more, coil tightness in the slot can slowly change over time, resulting in early failure. Strains of the coil along with temperature are of vital interest for the long-term health of the generator. Again, a need exists to better track temperature and tightness of the coils.
Brillouin Optical Time Domain Analyzer will measure a frequency shift in reflected light that is directly related to temperature and strain. A recent patent to OZ Optics LTD recognizes their B-OTDA unique approach to distinguish between temperature and strain instead of measuring the two together.
The parallel rings, where the main end leads from the 3 phase and ground meet, are currently unmonitored due to the difficulty of tying to a thermal coupler there. A good solution would provide measurement of these rings as well.
The newest generation of B-OTDA have the necessary speed and quality of measurement. With a temperature accuracy of 0.1°C, measurements are taken every 5cm over the entire length of fiber with a spatial resolution of temperature every 10cm, and at a speed of measurement needed to detect rapid heat build-up. The improvements are not limited to technical performance, but price has been reduced as well. Over the past three years, the cost of a B-OTDA has dropped by 50 percent.
For redundancy, multiple strands of fiber optic can be attached to the areas of concern such as the main lead, each length of each coil slot and coils in each half, the series connection, the parallel rings, main ends leads of each phase and their return ground. The B-OTDA then could be used to measure over the entire length of fiber every 5cm to give a complete 3D view of temperature and strain inside the generator. Strain is an important measurement as well, allowing for detection of slot tightness of the coils.
If fully installed and implemented, each generator will now have temperature measured every 5cm and spatially resolved every 10cm along every surface of concern, with an accuracy of 0.1°C, all without the need for electrical isolation. Comparing a 25-foot generator that may utilize roughly 100 thermal couplers, the DSTS approach would offer roughly 14,000 points of measurement along the fiber within the single power generator. Since the fiber is really a smaller than hair-thin piece of glass, the whole measurement system is passive to the generator's performance. With the 14,000 sample points, third-party software will be able to provide a 3D view of temperature, and along with that will be the strains inside each slot, and surface area. This should allow much more preventive maintenance and a big decrease of corrective maintenance. The financial pay-off is a reduced operational budget, longer up-time and full use of the engineered life span of the generator itself.
How It Works
The glass fibers are attached along the surface areas to be measured. As the surface heats, so does the fiber. Also if the surface changes shape, a strain will be introduced to the fiber as well. The shape of the core of the fiber will change slightly with change in temperature and strain. A naturally occurring back reflection known as a Brillouin reflection will change frequency as the core changes shape. It's easier to think of this as similar to the natural resonance of crystal glasses that you hear when you rub your finger on the lip. Just like that sound will change if you could change the shape of the glass, the Brillouin frequency changes as the core fiber changes shape. Not that you'll hear anything though; the frequency of Brillouin reflections are about 10 Gigahertz off from the light frequency, which is measured in Terahertz. The B-OTDA generates sweeping beat frequency that could exist looking for a match to the Brillouin frequency. Accurately measuring the change in frequency from one moment to the next, and knowing the nature of the fiber itself derives strain and temperature derived at the detector and source.
The new generation of B-OTDA models differ significantly from previous fiber optic reflectometers. The speed, accuracy, resolution, and cost have all improved to allow the use of distributed fiber-based sensing to move from very slow changing environments such as dams and buildings and into to applications that require near real-time information of the system being monitored. Today's power generators continue to rely on thermal couplers' point base systems and their necessary electrical isolation from the very areas that are most critical. With the improvements in optical technology, it is probable that the current rate of generator failure is proven to be needless and preventable with the use of a B-OTDA DSTS.
About the Author:
Paul Richardson works at OZ Optics Limited as Product Sales Manager, 12 years of product management and sales engineering in all areas of fiber optic testing, with direct management in fiber optics as a sensors including Raman, Brillouin and Fiber Bragg gratings.