This paper presents a protocol that enables instrumentation of random wound electric coils with fiber Bragg grating (FBG) thermal sensors for the purpose of distributed condition monitoring of internal thermal hot spots.
Random wound coils are a key operational element of most electric apparatus in modern industrial systems, including low voltage electric machines. One of the major current bottlenecks in improved exploitation of electrical devices is the high sensitivity of their wound components to in-service thermal stress. The application of conventional thermal sensing methods (e.g., thermocouples, resistance temperature detectors) for thermal condition monitoring of current carrying random wound coils can impose considerable operational limitations due to sensor size, EMI sensitivity and the existence of electrically conductive material in their construction. Another substantial limitation exists in distributed sensing applications and is caused by what is often a considerable length and volume of conventional sensor wiring leads.
This paper reports the design of a fiber optic FBG sensing system intended for enabling real-time distributed internal thermal condition monitoring within random wound coils. The procedure of random wound coil instrumentation with the FBG sensing system is reported in a case study on an IEEE standard wound coil representative of those utilized in electrical machines. The reported work also presents and discusses important practical and technical aspects of FBG sensing system implementation and application, including the FBG array geometry design, sensing head and fiber packaging, the sensor array installation and calibration procedure and the use of a commercial interrogation system for obtaining thermal measurements. Finally, the in situ multiplexed FBG sensing system thermal monitoring performance is demonstrated in representative static and dynamic thermal conditions.
Random wound coils are a key design element of most electric apparatus in modern industrial systems and are commonly used in low voltage electric machinery. A major barrier to improved usage of wound coils in these applications is their sensitivity to in-service electro-thermal stress. Thermal overloads are particularly pertinent in this regard as these can cause insulation coil insulation system breakdown and ultimately its total failure1; this can arise due to excessive coil current levels, or other causes such as a coil electrical fault or a cooling system malfunction, where localized hot spots are induced in the coil structure leading to insulation breakdown. Enabling operative in situ distributed thermal monitoring of an in-service coil’s internal structure allows for development of improved utilization and condition based maintenance routines; it would allow for advanced understanding and identification of the coils’ operating status and any degradation process, and thus condition based corrective action to maintain the operating status and prevent or slow down further damage2,3.
The presented method is aimed at enabling in situ monitoring of electric coil structure embedded thermal conditions through use of flexible and electromagnetic interference immune (EMI) fiber Bragg grated optical thermal sensors. The method offers a number of functional advantages over existing thermal monitoring techniques used in electric coils: these almost invariably rely on usage of thermocouple (TCs) or resistance temperature detectors (RTDs) that are not EMI immune; they are made of conductive materials; and they are generally reasonably bulky hence not ideally suited for sensing applications within the structure of wound electric coils. The usage of robust and flexible fiber optic FBG thermal sensors provides a number of considerable improvements in this respect, not solely due to sensor EMI immunity but also its small size, multiplexing ability and its flexibility, which enables them to be embedded into and conform to an arbitrary wound coil architecture to achieve thermal sensing with pinpoint accuracy in desired structural locations4. These features are especially attractive in electrical machine (EM) applications where device thermal limits are defined by electric coil thermal conditions and are particularly pertinent in the light of the expected considerable growth in EM usage with proliferation of electric transport.
This paper presents the methodology of instrumenting a typical low voltage random wound coil structure with thermal FBG sensors to enable on-line monitoring of internal hotspots. A detailed protocol of the FBG sensor choice, design, packaging, instrumentation, calibration and usage is reported. This is presented on an IEEE standard random wound coil motorette system. The paper also reports the obtained in situ thermal measurements under static and non-uniform thermal operating condition of the examined test coil.
FBGs are formed by the process of ‘grating’ the optical fiber core to create periodic longitudinal imprints (usually referred to as sensing heads in FBG sensing applications); when the fiber containing FBGs is exposed to ultraviolet light each existing FBG head will cause its refractive index to be periodically modulated5. The sensing head reflected wavelengths will be affected by the thermal and mechanical conditions that the fiber is exposed to, and thus enable the grated fiber to be applied as a thermal or mechanical sensor assuming adequate design and application.
The FBG technology is especially attractive for distributed sensing applications: it allows for a single optical fiber to be grated to contain multiple FBG sensing heads, where each head is coded with a distinct Bragg wavelength and acts as a distinct sensing point. This type of FBG based sensing device is known as an FBG array sensor6 and its operating concept is illustrated in Figure 1. Broadband light is used to excite the array resulting in distinct reflected wavelengths from each contained FBG head; here, each head reflects a defined wavelength (i.e., Bragg wavelength) that matches its grating design and is also dependent on the prevailing thermal and mechanical conditions at the head (i.e., sensing) location. An interrogator device is needed to enable array fiber excitation with light and the inspection of the reflected spectra for distinct Bragg wavelengths containing information on localized thermal and/or mechanical conditions.
A particularly important aspect of FBG thermal sensor implementation is the mitigation of thermo-mechanical cross sensitivity effects to obtain as close as possible to exclusively thermal readings7. The FBG inherent feature of thermo-mechanical cross-sensitivity requires careful design of FBG sensors aimed at thermal only or mechanical only sensing applications. Where thermal sensing is concerned an effective method of mitigation FBG mechanical excitation sensitivity is to isolate the sensing head with a packaging capillary made of material suitable for a given application; in the coil embedded thermal sensing application examined in this work this not only reduces cross-sensitivity problems but also serves to protect the fragile sensing fiber structure from underside and potentially destructive mechanical stress8.
Figure 2A shows the random wound electric coil test specimen used as a demonstration vehicle in this paper. The coil is designed according to IEEE standards9 for thermal evaluation procedures of random wound coils’ insulation system; the resulting test system shown in Figure 2B is known as a motorette system and is representative of a winding and its insulation system in a low voltage electrical machine. In the presented case study, the motorette will be instrumented with an FBG array thermal sensor consisting of four thermal sensing points, to emulate typical thermal sensing hot spots of interest in practical machine applications that tend to be localized in coil end winding and slot sections. For calibration and performance evaluation, the FBG embedded motorette will be thermally excited using a commercial thermal chamber and a DC power supply.
1. Fiber optic thermal sensor design
2. Interrogation system and sensor configuration
3. Packaging preparation
4. Free thermal calibration
5. Test coil build and FBG instrumentation
6. In situ calibration and evaluation
7. Testing
Figure 5 presents the temperatures measured by the array sensor in the static thermal test. The four internal temperature readings, taken by respective array FBG heads in corresponding coil locations, are observed to be closely similar as is generally expected for the examined test conditions; there is a slight variation between the reported individual measurement of less that ≈1.5 °C between the observed average hotspot temperatures of ≈75.5 °C.
Figure 6 reports the array sensor measurements obtained in the non-uniform thermal condition test. These are shown first for the period where there is no excitation in the external coil (first ≈75s) indicating closely uniform measured thermal levels, as would be expected. The external coil is then excited resulting in additional localized thermal excitation: this results in a clear change in the observed measurements, with the sensing point in closest proximity to the external coil (i.e., FBG4) measuring the highest thermal level (≈128.6 °C) and that furthest away the lowest (≈117.6 °C); the FBG temperature sensors located between these report intermediate and closely similar temperature levels (≈122.7 and ≈121.6 °C). The observed readings clearly relate to individual sensing head distribution in the examined test coil geometry. Furthermore, the results clearly demonstrate the functional capability of the coil embedded array sensor for monitoring and identification of internal distributed thermal hotspot distribution in random wound coils.
Figure 1. The FBG array sensor operating concept. This figure has been modified from a previous publication4. Please click here to view a larger version of this figure.
Figure 2. IEEE standard motorette coil assembly. (A) Random wound electric coil; see IEEE standards9. (B) Assembled and varnished IEEE standard motorette. Please click here to view a larger version of this figure.
Figure 3. FBG thermal sensor array design. (A) FBG array fiber length, (B) FBG head locations in the array structure, (C) FBG array packaging design. Please click here to view a larger version of this figure.
Figure 4. The packaged array sensor FBG heads calibration characteristics. The characteristics are derived from the data obtained in the array free thermal calibration tests. This figure has been modified from a previous publication4. Please click here to view a larger version of this figure.
Figure 5. FBG array thermal measurements obtained in steady state thermal condition test. The individual head thermal measurements reported by the FBG array sensor are shown with an inset detail steady-state measurement view. This figure has been modified from a previous publication4. Please click here to view a larger version of this figure.
Figure 6. Thermal measurements in the non-uniform thermal condition test. This figure has been modified from a previous publication4. Please click here to view a larger version of this figure.
Intercept | B1 | B2 | Statistics | ||||
Value | Standard Error | Value | Standard Error | Value | Standard Error | Adj. R-Square | |
FBG1 | 1555.771 | 0.0137 | 0.00855 | 2.85E-04 | 1.50E-05 | 1.34E-06 | 0.99978 |
FBG2 | 1547.669 | 0.0112 | 0.00851 | 2.34E-04 | 1.41E-05 | 1.10E-06 | 0.99985 |
FBG3 | 1539.852 | 0.0101 | 0.00871 | 2.11E-04 | 1.30E-05 | 9.90E-07 | 0.99988 |
FBG4 | 1531.768 | 0.0131 | 0.00808 | 2.72E-04 | 1.67E-05 | 1.28E-06 | 0.9998 |
Table 1: Calculated polynomial qudratic fit curve parameters. The calculated parameters standard error and individual head correction coefficientsare included; good linearity and a coorection factor coefficient in excess of 0.999 was observed for the four tested FBG heads. This table has been modified from a previous publication4.
The paper has demonstrated the procedure required to design, calibrate and test in situ FBG thermal sensors in low voltage wound coils. These sensors offer a number of advantages for in situ sensing applications within current carrying wound coil structures: they are fully EMI immune, are flexible and can conform to an arbitrary desired geometry to deliver arbitrary desired sensing point locations with high accuracy, and can provide a large number of sensing points on a single sensor. While thermal sensing within wound coils can be achieved with conventional thermal monitoring techniques employing thermocouple or resistance temperature detectors, the application of FBGs is shown to provide a number of attractive functional advantages.
Appropriate packaging of the FBG array sensor is key to its effective utilization. It is important that individual sensing heads or the entire sensing area of the fiber be appropriately packaged to ensure isolation of FBG heads from mechanical excitation in a rigid yet flexible thermally conductive capillary. It is desirable for the capillary to be designed of non-electrically conductive material as this ensures optimal performance in the EMI rich environment characteristic of current carrying coils.
Care needs to be taken during the process of packaging capillary installation into the coil to accurately position the package segments in their corresponding sensing locations. It is also essential to optimize the capillary geometry in case highly dynamic thermal conditions are to be observed.
It is vital to ensure accurate characterization of the coil embedded sensor. This is best done by performing free packaged sensor calibration before its installation within the wound coil geometry. While a high degree of protection from mechanical excitation is provided by the in situ packaging, the installation process can result in wavelength shift due to strain sensitivity. If performed carefully this can be negligible; however, it is good practice for this to be ascertained in in situ calibration tests where possible.
This application of FBGs within wound coils is relatively new and opens a number of opportunities for improved design, utilization, monitoring and health diagnosis of electrical machines. Further work is needed to reduce cost of these and make them a credibly viable option for large scale application in electric machinery.
This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) HOME-Offshore: Holistic Operation and Maintenance for Energy from Offshore Wind Farms Consortium under grant EP/P009743/1.
Name | Company | Catalog Number | Comments |
Cletop-S | Fujikura | 14110601 | Commercial optic connector cleaner |
Copper wire AWG24 | RS | 357-744 | Commercial insulated copper wire |
DC power supply | TTi | CPX400SP | Commercial 420W DC power supply |
FBG sensors | ATGratings | NA | Commerically manufactured FBG array to design spec |
Heat Shrink Tubing | RS | 700-4532 | Heat Shrink Tubing 3mm Sleeve Dia. x 10m |
Kapton masking tape | RS | 436-2762 | Orange Masking Tape Tesa 51408 |
PEEK tubing | Polyflon | 4901000060 | Commercial PEEK tubing |
SmartScan04 | Smartfibres UK | S-Scan-04-F-60-U-UK | Commercial interrogator system |
Thermal Oven | Lenton | WHT6/30 | Commercial thermal oven |
Winder machine | RS | 244-2636 | Commercial winder machine |
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