Patent Application
20130027030
1. Field of the Invention
This invention relates generally to a fiber optic magnetic flux sensor for measuring the magnetic flux in a stator bar of a high voltage generator and, more particularly, to a fiber optic magnetic flux sensor employing a magnetostrictive Bragg grating (MBG) provided within a fiber for measuring the radial component of the magnetic flux impinging on a stator bar of a high voltage generator.
2. Discussion of the Related Art
High voltage generators for generating electricity as a power source are well known in the art. A power plant may include gas turbine engines that each rotate a shaft by combusting fuel and air in a combustion chamber that expands across blades which rotate, and in turn causes the shaft to rotate. The output shaft of such an engine is coupled to an input shaft of a high voltage generator that is mounted to a rotor having a special configuration of coils. An electrical current provided in the rotor coils generates a magnetic flux around the coils, and as the rotor rotates, the magnetic flux interacts with windings in a stator core enclosing the rotor. The stator core windings include interconnected stator bars that have a special configuration to reduce eddy currents in the windings, which would otherwise generate significant heat and possibly damage various generator components.
It is generally necessary to determine the distribution of magnetic flux across the stator bars in a high voltage generator to more accurately calculate electrical losses, and therefore, more accurately model the overall losses of the stator windings. The usefulness of these measurements depends largely on how close the particular flux sensor can be placed relative to the stator bars since measurements obtained at increasing distances from the measurement location must be corrected for attenuation of the flux field over the distance from the sensor to the bars.
Monitoring the magnetic flux within large generators is typically accomplished using copper wire search coils inserted into the slots between stator teeth in which the stator bars are provided or mounted onto the stator coils. Search coils provided in the stator slots can be used to detect the presence of the radial flux that could give rise to circulating currents in the rotor that lead to losses in the stator windings. However, conductive copper coils tend to have large cross-sections that limit the ability to measure small flux areas, and thus provide an average measurement of local magnetic flux. Copper coils also provide a risk in that copper conductive leads can initiate a ground arc that can damage the stator windings.
It has been proposed in the art to employ fiber Bragg gratings (FBG) as sensors to measure strain, vibration and temperature for various applications. FBG sensors measure strain on an optical fiber at the Bragg grating locations. This strain slightly alters the spacing of reflective grating lines in the FBG, thus affecting its reflective property. A broadband infrared (IR) signal is transmitted through the optical fiber to the FBG sensor. The degree of strain on the FBG is measured by the wavelength of the IR radiation that is reflected from the FBG. As the strain spans the fiber Bragg lines, the wavelength of the reflected light is increased proportionately. As many as a hundred of such measurements can be provided on a single optical fiber by appropriately setting the spacing between the Bragg grating lines to prevent overlap in the reflected IR light from each Bragg grating. Such FBG systems can also operate in a transmission mode.
For an FBG sensor strain measurement, the FBG sensor is mechanically strained by bending the coil structure at the FBG sensor attachment locations. For an FBG sensor vibration measurement, a mass attached to the optical fiber alters the tension in the optical fiber as it responds to vibrations at the attachment site on the coil. For an FBG sensor temperature measurement, the thermal expansion of the Bragg grating itself changes the Bragg grating line spacing.
In accordance with the teachings of the present invention, a magnetic flux sensor is disclosed that measures the radial component of the magnetic flux impinging on a stator bar of a high voltage generator. The magnetic flux sensor includes a fiber. Bragg grating formed in an optical fiber and enclosed within a magnetostrictive coating. The magnetostrictive coating responds to changes in magnetic flux by applying a strain on the fiber that changes the reflected wavelength of the Bragg grating that can be measured to provide a measurement of the flux. In one embodiment, one or more of the magnetic flux sensors are positioned directly within an insulating layer of the stator bar.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to an MBG sensor for measuring the radial component of the magnetic flux impinging on a stator bar of a high voltage generator is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
As will be discussed in detail below, the present invention proposes an MBG sensor including an FBG for measuring the magnetic flux in one or more of the slots 22 from the stator bars 26 and 28. The MBG sensors discussed herein are placed as close as possible to the wire strands in the stator bars 26 and 28 to provide an accurate flux measurement.
As is known by those skilled in the art, the FBG 52 can be selectively designed so that the index of refraction n2 of the fiber core 48, the index of refraction n3 of the sections 54, and the spacing Λ between the sections 54 define which wavelength λB is reflected by the FBG 52 based on equation (1) below.
λB=2n3Λ (1)
The system 40 also includes a circuit 58 that generates the optical input signal and detects the reflected signal from one or more of the FBGs 52. The circuit 58 includes a broadband light source 60 that generates a light beam 62 that is passed through an optical coupler 64 and is directed into and propagates down the optical fiber 46 towards the FBG sensor 52. The light that is reflected by the FBG sensor 42 propagates back through the optical fiber 46 and is directed by the optical coupler 64 to a dispersive element 68 that distributes the various wavelengths components of the reflected beam to different locations on a linear charge-coupled sensor (CCD) 66, or some other suitable optical detector array, such as a Bragg oscilloscope. A system of optical filters can also be used to reduce system cost, while limiting the number of FBGs on the fiber 46. By providing the broadband source 60 and the dispersive element 68, more than one reflected wavelength λB can be detected by the CCD sensor 66, which allows more than one of the FBG sensors 42 to be provided within the fiber 46.
The MBG sensors 72 each includes an outer layer of a magnetostrictive material that changes in shape in response to a magnetic flux that either increases or decreases the strain on the fiber 74 depending on the flux intensity, which can be measured as discussed above.
A change in temperature of an FBG will change the spacing of the sections 54 in the FBG that alters the wavelength of the reflected signal. Based on this phenomenon, it is known to use FBG sensors to measure temperature to provide a temperature calibration. Once the MBG sensor 72 is calibrated for a particular magnetic flux, a change in temperature of the MBG sensor 72 will affect the flux measurement. Most applications for measuring the magnetic flux in a stator bar of a high voltage generator measures AC flux that alternates with time. An AC measurement will typically not require a compensation for temperature because a change in temperature will be an offset that is applied to all of the flux measurements as the signal osculates. However, for DC magnetic flux measurements, it typically will be necessary to know the temperature change of the MBG for an accurate measurement of the flux. Therefore, the present invention contemplates providing a second MBG sensor either in the same fiber 74 proximate to the MBG sensor 72 or in a separate fiber (not shown) adjacent to the MBG sensor 72. Therefore, as the temperature changes, and the temperature measuring FBG provides an indication of that temperature change, that temperature change can be used in the calibration to determine the DC magnetic flux being measured.
According to the invention, one or more MBG sensors 108 of the type discussed above, are provided in the filler area 106 for measuring the magnetic flux of the stator bar 92 at a desired location. In this non-limiting example, five MBG sensors 108 are provided to measure the flux at specific locations across the slot 94. However, this is by way of a non-limiting example in that any suitable number of the MBG sensors 108 can be provided for a particular application for the desired flux measurement resolution. The sensors 108 can be part of any suitable detection system, as discussed above, where the sensors 108 can be provided in a single optical fiber, multiple optical fibers, etc., and where some of the sensors 108 can be provided for temperature measurement compensation. In this non-limiting embodiment, the sensors 108 are provided in only one of the slots 94 of the stator core 10 to provide the magnetic flux measurements. However, the MBG sensors 108 can be provided in any number of the slots 94 at any desirable location along the length of the stator core 10 as would be feasible.
Although the MBG sensors 108 are very close to the stator bar windings 104 that generate the magnetic flux, they can be positioned even closer to provide an even more accurate reading of the flux.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the scope of the invention as defined in the following claims.
