Veera Palanivelu Rajendran

High spatial-resolution temperature monitoring of an industrial motor using a distributed fiber-optic sensing technique

We report the use of a fiber-optic distributed sensing system to monitor temperature at a multitude of discrete points on an industrial motor undergoing qualification after a rewinding. This technique involves using optical frequency domain reflectometry to demodulate the reflected signal from multiplexed Bragg gratings that have been photoetched in the core of an optical fiber. In this work, high-resolution optical sensing fiber was applied along the stator windings and end-windings of the motor to assess their suitability for long-term temperature monitoring. Performance tests were conducted at different heat loads representing different electrical conditions. Results indicate excellent agreement with collocated Resistance Temperature Devices (RTDs) and demonstrate significant potential for mitigating costly motor failure due to insulation breakdown resulting from highly localized hotspots.

Use of fiber optic-based distributed temperature measurement system for electrical machines

A fiber optic based distributed temperature measurement system was implemented in stator windings (straight copper bars) as well as in the end-windings (curved copper bars) of a motor. Usually, in electrical machines such as motors or generators, only a few conventional temperature sensors are used, whereas the distributed temperature system has the potential of providing very detailed temperature distribution by having hundreds of sensors in a single fiber. The sensors were made of Bragg gratings etched onto the fiber itself. For the present study, the spatial resolution of the sensors is 6 mm (nominally at 1/4” apart). The technique uses Optical Frequency Domain Reflectometry (OFDR) to process the back-reflected light signal indicative of the thermal filed. A prototype fiber optic system was implemented in a motor made by GE industrial systems. The sensing length (length of the stator) for the motor was 0.75 m containing approximately 150 sensors thus providing very detailed temperature data. Performance tests were conducted at different heat loads representing different electrical conditions. Continuous tests for the duration of 19 hours were conducted. The temperature of stator windings varied from ambient (~ 23°C) to approximately 85°C. As reference, Resistance Temperature Devices (RTDs) were installed in adjacent slots to the slot where fiber optic sensors were installed. A total of 8 sensors were installed but data were collected on only 3 fibers. Fiber sensor measurements were found to track the temperature trends very well. The fiber data agreed with RTD data within ± 3°C in the entire duration. The RMS value of difference between the fiber and RTD on one side was 0.3°C, and with the RTD on the other side was 0.5°C. The fiber measurements also showed how hotspots could be missed by using few RTDs, as is done in the industry. The fiber measurements also showed the temperature distribution in the endwindings, an area not normally monitored. The maximum temperature was an acceptable 110°C. The feasibility of this technique for measuring stator-winding temperatures is proved. Still some of the problems faced during the installation and experiments are (a) robustness of fiber and sheathing fiber and (b) fiber survivability during manufacturing process and repair.

Application of Novel Speed-of-Sound Based Technique to Measure Steam Wetness With Potential Application Into LP Exhaust

Wet steam is a common occurrence at the exhaust of the LP turbines in fossil-fired steam plants. In nuclear turbines, wet steam will be found right from the high-pressure sections. The presence of moisture in steam reduces the aerodynamic efficiency of the turbine sections, thus reducing the overall efficiency of the turbine. Additionally, water droplets also cause erosion and corrosion of buckets and other components. LP turbines account for a significant portion of the total cost of the turbines (due to the enormous sizes required by the expanding steam) and produce significant portion of the power output. Measuring and controlling wetness will help improve both the performance and reliability of turbines. A novel way of measuring the composition of wet steam using a speed of sound based technique is being developed. The technique, based on technology developed for measuring two-phase flow compositions in down-hole (oil-field) applications, relies on measuring acoustic pressures propagating in a one-dimensional wave-guide (pipe or tube) using an array of axially located pressure transducers. The technique is non-intrusive to the flow field and relies on passive listening of the noise generated by the flow itself (and, hence differs from the conventional ultrasound based techniques). The current study is an ongoing effort and the paper will focus on the feasibility of this technique for wet steam application. The eventual aim is to be able to measure steam wetness in the range of 0–10% with an accuracy of ± 0.2%. Initially, the ability of the technique to accurately measure the wetness in air-water mixture was established using an air and water mist facility. Next, high subsonic flow conditions were evaluated in single phase (air only) flow using a wind tunnel facility. Excellent agreement between speed of sound calculated for air, based on conventional pressure and temperature measurements in a wind tunnel, and that measured directly by the probe was obtained. The wind tunnel tests showed that the SOS measured by the probe and conventional instrumentation agreed within ± 1.5%. This establishes that the technique is capable of accurately measuring the speed of sound, which is the primary variable to calculate the flow composition. The technique can also be used to measure volume. Although the wind tunnel tests were not specifically designed to assess the accuracy of the flow rate measurement, comparisons were made between the flow velocities given by the probe and reference measurements. The additional motivation was to assess the ability of the probe to monitor volume flow/mass flow at high Mach numbers where only shorter straight sections are available. The flow velocities measured by the probe agreed with those calculated using the wind tunnel instrumentation (wall-static taps) within the estimated uncertainty levels introduced by the flow blockage and profile distortions. Additional tests are planned to assess flow rate accuracy. Effort is continuing to study steam flows representative of exhaust of low pressure steam turbines in steam plants.Copyright