Optical current sensors are advancing the modernization of the power grid because of the reduced costs, environmental savings, safety, reliability, and measurement fidelity improvements. The measurement range requirements are from 1 mA to more than 150 kA with frequency response extending from DC to 20 kHz and beyond. The accuracies required are often 0.1% over a temperature range from -40 ºC to +60 ºC. Several leading edge applications of the optical current sensor are presented, along with a discussion of some developing power system architectures that fully capture the advantages of this new technology. The main challenges to face are identified.
We have measured the influence of the electro-optic (EO) Kerr effect on the response of a spun highbirefringence (hi-bi) fiber current sensor in a simulated Gas Insulated System (GIS) environment. We show that the EO Kerr effect distorts the response of the sensor, and that the second-harmonic signal has a small dependence on the input polarizer angle. We also, have theoretically modeled a polarimetric current sensor using spun hi-bi fiber and compared the models to our experimental results. With the models we predict the response of a fiber current sensor in a 550 kV GIS application.
We discuss the influence of the electro-optic Kerr effect on optical fiber current sensors in a simulated Gas Insulated System environment. We also show that current, voltage, and phase can be measured using one transducer.
NIST has developed a Standard Reference Material that can be used to calibrate polarimetric instruments and improve measurement accuracy. The device, based on concatenated Fresnel rhombs, provides nominally 90 degrees retardance at 1319 nm, with the actual value known within +/- 0.1 degrees. The design of the retarder is reviewed, and the stability of retardance over time is discussed. A procedure for certification is outlined.
The National Institute of Standards and Technology is developing a quarterwave linear retarder for operation at 1.3 micrometers . It is expected to be stable to within +/- 0.1 degree(s) over practical ranges of wavelength, temperature, and incidence angle. A spectral range of at least 10 nm is desired to accommodate solid-state and diode laser sources and typical wavelength variation. Normal incidence operation with an angular tolerance of +/- 1 degree(s) allows alignment by retroreflection of a collimated input beam. Operation over a temperature range of 25 +/- 10 degree(s)C encompasses most laboratory conditions, and practical rates of temperature change must also be allowed.
A computer controlled optical thermometer has been built to demonstrate a self-calibrating optical sensor. The self-calibrating thermometer records the temperature with a fiber-optic polarimetric temperature sensor. The wavelength sensitivity of the polarimetric sensor is used to facilitate the recalibration. The system contains an optical source which can be tuned over approximately a 9 nm wavelength range, and a monochromator to measure any shifts in the wavelength of the laser. The monochromator is calibrated with the spectrum of a neon discharge lamp.
Recent research at NIST has greatly extended the capabilities of Faraday effect sensors for both magnetic field and electric current measurements. Current sensors using single-mode optical fiber show temperature stability near material limits, and are approaching commercial availability for applications in the power industry. The Faraday effect in iron garnets shows great promise for measureing current at low levels and/or high speeds. Sensors with noise equivalent currents of about 200 nA/(root)Hz have been demonstrated. Magnetic field sensors using iron garnets and flux concentration, have led to sensors with noise equivalent magnetic fields in the range of 1 pT/(root)Hz.
Recent research at NIST has greatly extended the capabilities of Faraday effect sensors for both magnetic field and electric current measurements. Current sensors using single-mode optical fiber show temperature stability near material limits, and are approaching commercial availability for application in the power industry. The Faraday effect in iron garnets shows great promise for measuring current at low levels and/or high speeds. Sensors with noise equivalent currents of about 200 nA/(root)Hz have been demonstrated. Magnetic field sensors using iron garnets and flux concentration, have led to sensors with noise equivalent magnetic fields in the range of 1 pT/(root)Hz.
We demonstrate that twisting a fiber a few turns per meter before it is annealed largely eliminates the residual linear birefringence. This dramatically improves the yield of annealed coils used for current sensing and makes it possible to use fibers that previously could not be successfully annealed. It is believed that twisting is effective because the residual birefringence is associated with core ellipticity and this contribution is averaged to near zero by twisting. We also show the temperature stability of sensors made with this new technique.
NIST is developing a quarterwave linear retarder designed to have a retardance stable within 0.1 degree(s) over a variety of operational and environmental conditions. In this paper we review several design strategies and early results of this effort. These have led to a promising prototype design consisting of a double rhomb TIR retarder constructed from a low stress- optic glass. We also review several measurement methods that are used in our evaluations.
We report a new design for a Faraday effect current sensor based on yttrium iron garnet. The improved sensor has substantially greater bandwidth than previous designs and is considerably easier to fabricate. The measured sensitivity is 0.7°/A with a -3 dB bandwidth of 500 MHz, giving a factor of 45 increase in sensitivity-bandwidth product. A noise-equivalent current of 840 nA/Hzl/2 was measured at 1.8 kHz using difference-over-sum processing. The use of newly developed turning prisms with phasepreserving coatings greatly simplifies construction, improves electrical isolation, and increases sensitivity through proximity effects.
Optical sensor systems have source requirements that can be significantly different from those of optical communications and other technologies that have generally driven the development of semiconductor sources. In this paper, we examine basic interferometric, polarimetric, and other sensors. Relevant semiconductor source data is reviewed to illustrate the impact of source characteristics on sensor performance. The effect of low-frequency amplitude and frequency noise on sensor precision is described. Errors in sensor calibration due to amplitude and wavelength drifts are discussed. Examples of sensor performance using typical source data illustrate these issues.
In this paper we describe the development of an optical fiber ac voltage sensor for aircraft and spacecraft applications. Among the most difficult specifications to meet for this application is a temperature stability of ± 1 % from -65 °C to +125 °C. This stability requires a careful selection of materials, components, and optical configuration with further compensation using an optical fiber temperature sensor located near the sensing element. The sensor is a polarimetric design, based on the linear electro-optic effect in bulk bismuth germanate (Bi4Ge3O12). The temperature sensor is also polarimetric, based on the temperature dependence of the birefringence of bulk SiO2. The temperature sensor output is used to automatically adjust the calibration of the instrument.
The class of ferrimagnetic materials known as substituted iron garnets display characteristics
which make them suitable for applications of magnetometry requiring high sensitivity, high spatial
resolution, or high speed. Diamagnetic substitution, in which specific iron ions are replaced by
diamagnetic ions, reduces the saturation magnetization and increases the sensitivity. We find that the
sensitivity of a composition of gallium-substituted yttrium iron garnet is six times greater than of
pure yttrium iron garnet. The noise-equivalent magnetic field for a sample of this material has been
measured as approximately 100 pT/THz.
While current sensors based on the Faraday effect in bulk materials have shown good success in
field tests, the use of single mode fiber as the sensing element has both technical and economic
advantages. In this paper we describe some of the practical problems that have inhibited the development
of fiber current sensors. Recent research suggests that most of these problems, including especially the
problem of linear birefringence in the sensing coils, can be solved. Instruments providing a measurement
quality approaching that set by fundamental material parameters can now be achieved.