Distributed fiber-optic sensor for real-time monitoring

Invited Paper

Distributed fiber-optic sensor for real-time monitoring of energized transformer cores Ping Lu*a,b, Kevin Byerlya,b, Michael Buricc, Paul Zandhuisa,b, Chenhu Sune, A. Learyd, R. Beddingfieldc,d,e, M. E. McHenryd, , Paul R. Ohodnicki, Jr.*b,d a AECOM, 626 Cochran’s Mill Road, Pittsburgh PA 15236; b National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, PA, 15236; c National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, WV, USA 26505; d Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA, USA 15213; e ORISE, National Energy Technology Laboratory ABSTRACT Real-time temperature mapping is important to offer an optimized thermal design of efficient power transformers by solving local overheating problems. In addition, internal temperature monitoring of power transformers in operation can be leveraged for asset monitoring applications targeted at fault detection to enable condition based maintenance programs. However transformers present a variety of challenging environments such as high levels of electromagnetic interference and limited space for conventional sensing systems to operate. Immersion of some power transformers within insulation oils for thermal management during operation and the presence of relatively large and time varying electrical and magnetic fields in some cases also make sensing and measurement technologies that require electrical wires or active power at the sensing location highly undesirable. In this work, we investigate the dynamic thermal response of standard single-mode optical fiber instrumented on a compact transformer core by using an optical frequency-domain reflectometry scheme, and the spatially resolved on-line monitoring of transformer core temperature rise has been successfully demonstrated. It is found that spectral shifts of the fiber-optic sensor induced by the temperature rises are strongly related to the locations inside the transformer as would be expected. Correlation between thermal behavior of the transformer core as derived from standard IR-based thermal imaging cameras and fiber-optic sensing results is also discussed. The proposed method can easily be extended to cover situations in which high accuracy and high spatial resolution thermal surveillance are required, and offers the potential for unprecedented optimization of magnetic core designs for power transformer applications as well as a novel approach to power transformer asset monitoring. Keywords: Fiber optics sensors, optical frequency domain reflectometry, Rayleigh scattering, transformer core, temperature measurement

1. INTRODUCTION Power transformers play an important role in the power transmission and distribution system, with smaller single phase transformers being particularly relevant for small power systems for certain power-grid networks with low population densities. These types of small distribution transformers have

Micro- and Nanotechnology Sensors, Systems, and Applications IX, edited by Thomas George, Achyut K. Dutta, M. Saif Islam, Proc. of SPIE Vol. 10194, 101941S · © 2017 SPIE CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2263387 Proc. of SPIE Vol. 10194 101941S-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/20/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

some major advantages such as low production cost and high versatility [1]. On the one hand, the dimensions of these transformers are typically limited by an acceptable temperature rise due to the power loss dissipated by the exposed surfaces of the transformer in the form of heat. On the other hand, the performance and lifetime of such types of transformers under overload is limited in high operating temperature conditions [2, 3]. Thus it is necessary to offer an optimum thermal design to construct more compact and efficient transformers by accurately measuring the excessive temperature profile in complex shaped core layouts in a distributed and real-time fashion. In addition, real-time monitoring approaches that enable spatially resolved temperature monitoring at various locations on an energized magnetic core can pave the way for on-line monitoring of such electrical assets for identifying early evidence for faults to enable condition based maintenance programs before catastrophic failure is observed. Asset of monitoring of large power transformers is an area of particular importance due to the large associated direct economic and social costs as well as the opportunity costs for bulky, and expensive grid assets which are custom components having a limited domestic supply chain [4-6]. Demands for higher power densities, more modular solutions, and advanced functionality of power electronics converters are also increasing as a result of a number of emerging applications spanning grid integration of distributed energy resources and electrification of military, aviation, aerospace, and automotive vehicles as well as development of modular and compact solid state transformers [7]. To achieve an optimal combination of power density, efficiency, and functionality a detailed understanding of the thermal profile is critical for successful thermal management which begins to play an important role and even dominate designs in some cases where the transformer components are being pushed to higher operational frequencies for maximized power densities. With the emergence of new wide-bandgap based semiconductor switching devices, transformers and other inductive components are being pushed at combinations of frequencies, power levels, and power densities not previously attainable [8]. Integrating optical fiber sensors in harsh environment has attracted great interest during the last decades because of their unique advantages such as immunity to electromagnetic fields, high sensitivity, fast response, and the possibility to monitor multiple parameters simultaneously [9]. Distributed fiber-optic sensor presents a powerful sensing tool which is not generally possible using conventional sensing technologies. Optical frequency domain reflectometry (OFDR) provides spatial resolution ranging from a few microns over several meters to a few centimeters over a few kilometers of fiber length [10-12]. It fills the gap in the measurement resolution and range between optical time-domain reflectometry and optical low-coherence reflectometry that it becomes very attractive for practical temperature and strain monitoring applications in structural health monitoring. Rayleigh backscatter in optical fiber is caused by random fluctuations in the refractive index profile along the fiber length, and it varies randomly along the fiber length due to inhomogeneity of the fiber. Changes in the refractive index profile caused by an external stimulus such as temperature will induce spectral shift being measured by the OFDR. Here a high-sensitivity, high-resolution optical frequency-domain reflectometry (OFDR) system is employed to measure distributed temperature rise of a compact transformer core. The existing OFDR sensor performances are only focusing on stationary measurement due to the nature of its operation principle. Furthermore, the small size and complex layout of transformer core make it difficult to fully attach the sensing fiber to the core surfaces and even exacerbate the temperature-strain cross-sensitivity effect. This work aims to explore the presence of fiber bending, transient temperature effects, temperature-strain cross-talk effects, as well as their possible impact on OFDR measurements when the transformer is in operation. This work provides new approaches to achieve distributed real-time temperature sensing that can be further improved by adopting new optical fiber structures or data processing schemes.

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2. EXPERIMENTAL DETAILS The experimental arrangement for fiber-optics sensing based distributed temperature rise measurement of a single-phase two-winding transformer is shown in Fig. 1. Planar flow casting is used for the production of FINEMET-type ribbon approximately 50.8mm wide and 20μm thick with a nominal composition of Fe72.9Nb3.0Cu1.0Si16.2B6.9. The ribbon is wound into a toroid core and then formed into a racetrack shape (AMCC-200 equivalent) using a steel fixture. After forming a tape wound core from the Nano-crystalline ribbon, the assembly (material and steel fixture) is annealed at 540°C for 3 hours in air. Finally, the core is impregnated with an epoxy resin to provide strength and shape retention. To lessen the impact of fringing flux and other imperfections, the core is left uncut. Each core section has a 3mm OD nylon served Type 2 Litz wire four-turn winding, insulated with Nomex® and Kapton tape. Wire terminations are made with a ring terminal by crimping and dip soldering. The core is attached to a custom extruded aluminum frame to allow for minimal impacts from strain. The constructed transformer is subjected to an open secondary test using a custom developed H-bridge based bipolar excitation circuit of the type described by previous authors [6, 7] to evaluate its core losses and thermal field distribution when excited with waveforms on the primary winding that are relevant for deployment in medium frequency power electronics and power conversion applications. The custom excitation circuit employs SiC-based semiconductor JFET devices and is rated for excitation voltages approaching 800V with a DC power supply rated for up to 15kW. When considering the high anticipated efficiencies of transformer cores which are typically targeted at 98% or greater, the testing system can in principle allow for excitations that are relevant for cores utilized in power converter applications at 50-100kW or even greater. For the purpose of demonstration of the application of optical fiber distributed sensor technology to transformer monitoring in this work, we restrict our investigations to a small prototype transformer core at low to moderate excitation conditions with sufficient losses to result in measurable temperature profiles that can be detected and quantified. Moving forward, we will apply similar methodologies to larger magnetic cores under higher power excitation conditions and compare our results with thermal models to allow for new tools to optimize the design of advanced magnetic core technologies for a range of power electronics and transformer applications.


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Figure 1. Schematic of distributed temperature monitoring in the compact transformer using the LUNAOBR and a commercial single-mode optical fiber as the sensing fiber. Custom H-bridge based excitation circuits can also be observed as well as a range of current and voltage probes as well as electrical testing equipment for monitoring the primary current and open secondary voltage.

The primary winding is excited with a square wave shaped waveform and a fixed frequency of 50 kHz of the duty cycle of 50%. A DC power supply (N8937A, Keysight) generates an input voltage amplitude of 100V and an input current amplitude of 0.20A. The current on the primary winding is measured to be 1.91A by using a AC/DC current probe and the voltage on the open secondary is measured to be 396V (pk-pk) by using a differential voltage probe with attenuation ratio of 1:20. The transformer excitation conditions described herein are fixed in the distributed real-time monitoring of temperature rise of energized transformer cores by using a Luna OBR 4600 optical backscattering reflectometer. Figure 2 illustrates a schematic diagram of the configuration of the OFDR system, the physical structure of the transformer core, and the design of optical path. The OFDR system consists of a wavelength tunable laser source (TLS), a main interferometer, photodetectors (PD) and a sensing data acquisition and processing system. The sensing fiber is a 5-m-long commercial SMF28 single-mode fiber fusion spliced with a single-mode fiber patch cable terminated with a FC/APC connector, and the rear end of the sensing fiber was coiled into a fiber ring of a very small diameter to prevent end-face Fresnel reflection. The sensing fiber is successively mounted on the four faces (front, inside, back leftleg, back right-leg, and outside), inner loop, and outer loop of the racetrack-core transformer which is defined by its inner and outer diameters, inner and outer radius, straight leg length, thickness and width.


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Figure 2. Schematic diagram of the configuration of the OFDR system, the physical structure of the transformer core, and the design of optical path.

3. RESULTS AND DISCUSSION In OFDR, a fast Fourier transform is performed to obtain the power spectrum of reflected light at different Rayleigh scattering points by converting data from frequency-domain to time-domain. In order to realize distributed measurement of temperature, a reference trace is required for relative phase change measurement under two temperature conditions at each location along the fiber under test. Segments of the Rayleigh backscatter pattern at the same positions are extracted from both the reference and the measurement profiles and then transformed back to frequency-domain by using inverse fast Fourier transform. A Rayleigh backscattering spectrum as a function of spectral shift can be obtained accordingly. Thus, the cross-correlation between the Rayleigh backscatter spectra of the reference and measurement segments presents a spectral shift which is proportional to the changes in temperature. Distributed temperature sensing can be finally achieved by calibrating the spectral shifts. The spatial resolution for an 85.5 nm laser sweep range is calculated to be 0.01 mm, while the sensor gauge length is selected to be 5 mm for cross-correlation based spectral shift calculation. A reference optical time-domain like trace is shown in Fig. 3, where the FC/APC connector at the front panel of the optical backscattering reflectometer, the fusion splicing point, Rayleigh backscattering in the sensing fiber are observable. It is noted that about 1.5 dB loss is caused by the sharp bend of the sensing fiber at the corners of the inner loop and no extra loss is found at the outer loop corner position because of a much larger bending radius. The transformer generates energy loss that is converted to heat once the


transformer is in operation as a result of various sources of core loss including hysteresis, eddy current, and anomalous eddy current losses as well as losses within the primary and secondary windings. For well-designed transformers the temperature eventually approaches a steady state value which varies spatially throughout the core and depends upon the details of (1) thermal management, (2) excitation conditions, and (3) core geometry amongst others. The temperature rise of the transformer impacting the sensing fiber at different positions will be recorded by calibrating the thermally-induced spectral shift, while the temperature of the portion of the sensing fiber that is not attached to the transformer core is maintained at room temperature. -90

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Figure 3. Reference backscatter signals versus distance along the sensing fiber instrumented on the transformer core.

Another Rayleigh spectra is obtained when the transformer is in operation with the parameters of transformer operating conditions described in the previous section of experimental details. Figure 4 shows the spectral shift of cross-correlation calculation between the reference and measurement Rayleigh spectra along the sensing fiber after 2 minutes of heating from the transformer core. Spectral shift quality, Q, as a measure of the correlation strength between the reference and measurement spectra is also evaluated by setting two thresholds which are Q=0.85 and Q=0.15, respectively. In the first region above the first threshold (0.85

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Figure 6. (Bottom Left) Spectral shift of cross-correlation calculation between the reference and test Rayleigh spectra along the sensing fiber after 28 minutes of heating from the transformer core; (Top Left) Spectral shift quality analysis result.

Figure 7. The heating profile along the sensing fiber after 28 minutes of heating from the transformer core.

The Luna-OBR can complete a single temperature measurement including laser wavelength scanning, data acquisition and processing within a few seconds. Thus monitoring of monotonous temperature rise of the transformer can be realized in real time during a 30-minute test period recorded every 2 minutes. Figure 8 shows the heating profiles along the sensing fiber at 10, 20, and 30 minutes of heating from the transformer core. It is shown that the inner loop/inside face is hotter than the outer loop/outside face of the core at the beginning of temperature rising period, and then outer loop ramps its effective temperature up much more faster than the the inner loop while the inside face still remains hotter than


the outside face because of different levels of thermal strain. As a comparison, an infrared thermal camera (PTI-160, Process Sensors) measures the temperature rise of the transformer as shown in the inset of Fig. 8. It is obvious that unavailable temperature difference information obtained by the IR thermal imager at some regions such as the insulation barrier covered area, the back of the core, and the area blocked by the metal frame can all be achieved by the optical fiber sensor. The ability to measure local effective temperatures of excited transformer cores in regions which do not provide optical access for standard IR-based imaging methodologies is a significant advantage of the optical fiber based temperature monitoring approach demonstrated in this work. 45 -

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Figure 8. The heating profile along the FUT after 10, 20, and 30 minutes of heating from the transformer core. Inset is the infrared thermal image taken at 20 minutes.

Simple linear regression analysis results on the effective temperature changes as a function of heating time at different locations of the transformer core are summarized in Fig. 9 and Table 1. It is demonstrated that the winding regions have the fastest rise in temperature for a 30 minutes period before the steady state of the transformer is reached. This is because the insulation barrier regions surrounding the core makes a non-exposed heat dissipation area here and the windings can also generate non-negligible heat, which also leads to a great effective temperature gradient near those regions. As expected, the measured effective temperatures throughout the core are highly dependent upon the core geometry and thermal conduction paths for the core instrumented with windings and instrumented with the optical fiber temperature sensors. A clear difference can be observed between the outer/inner loops and the outside/inside faces of the core which will be the subject of significant future detailed investigations. One of the ultimate goals of the unique testing facilities established and described in this effort is the ability to locally measure and quantify core heating and steady state operational conditions to develop an improved and quantitative understanding of the relationship with core fabrication and processing methodologies as well as the detailed excitation conditions to which it is subjected and the associated core and winding losses which can also be quantified. In the short term, this unique capability will seek to provide transformer designers with new information which was previously inaccessible to assist in the design and realization of transformers with controlled temperature rise as required for a particular application. Moving into the future, similar methodologies


can also be adapted for the purpose of real-time asset monitoring of power transformers deployed in the field to provide early indications of potential failure to enable preventative maintenance. 45 -

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Figure 9. Effective temperature changes as a function of heating time at different locations of the transformer core.

Table 1. Linear regression fit of the effective temperature change curves at different locations of the transformer core.

Location

Slope

Standard Error

Outer loop – secondary winding

1.52242

0.0408

Outer loop – primary winding

1.42605

0.06361

Inner loop – primary winding

1.0384

0.06031

Inner loop – secondary winding

1.01067

0.05564

Outer loop

0.96251

0.03544

Inside face

0.91082

0.04811

Inner loop

0.91057

0.04162

Front face

0.89295

0.04072

Back face – right leg

0.87914

0.03746

Outside face

0.77627

0.03168

Back face – left leg

0.72062

0.03556


4. CONCLUSIONS We have proposed and experimentally demonstrated the OFDR based fiber-optics sensors for distributed real-time temperature rise monitoring of the transformer in operation. The dimensions of the compact transformer create a challenge for conventional sensing methods to access however the advantages of small size and high flexibility makes it possible to install the sensing fiber distributed over the whole core structures without affecting the precise effective temperature readings. In addition the dynamic temperature rising process can be effectively monitored by the optical fiber sensor while it becomes difficult to make a continuous thermal imaging sensing by using its conventional counterpart. Relationship between the fiber-optic sensing response and transformer operation conditions is still under review and optimization of the real-time distributed temperature monitoring is pending further investigation. In particular, the effects of combined thermal-stress will be further explored and techniques for eliminating cross-sensitivity will be explored moving forward. Nevertheless, the results here clearly demonstrate that the fiber-optic sensor provides an effective solution to monitoring the physical structures of the transformer core as well as accurately detecting the non-uniform temperature distribution inside the transformer, and thus provides informative feedback to the transformer design by minimizing the core losses. In the longer term, such approaches may open the door for novel approaches to real-time asset monitoring of power transformers while in operation.

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