Philip Orr

High-Speed, Solid State, Interferometric Interrogator and Multiplexer for Fiber Bragg Grating Sensors

We report on the design and prototyping of a robust high-speed interferometric multiplexer and interrogator for fiber Bragg grating sensors. The scheme is based on the combination of active WDM channel switching and passive, instantaneous interferometry, allowing the resolution of virgin interferometric interrogators to be retained at MHz multiplexing rates. In this article the system design and operation are described, and a prototype scheme is characterized for three sensors and a multiplexing rate of 4 kHz, demonstrating a noise floor of 10 nε/√Hz and no cross-sensitivity. It is proposed that the system will be applicable to demanding monitoring applications requiring high speed and high resolution measurements across the sensor array.

An Optically-Interrogated Rogowski Coil for Passive, Multiplexable Current Measurement

We report on the design and implementation of a novel hybrid electro-optical sensor for the measurement of electric current. A fibre Bragg grating is utilized to passively interrogate an air-cored coil via a low-voltage piezoelectric multilayer stack. Peak Bragg reflections are strain-tuned by the Rogowski coil/piezoelectric transducer combination, allowing primary current reconstruction to be performed remotely and without active electronics at the sensor. The preliminary embodiment demonstrates long-distance, passive measurement of current for metering and protection relaying applications, and retains the fiber transducers capability for serial multiplexing.

Sensor response characteristics for UHF location of PD sources

Ultra-high frequency (UHF) systems are well established for monitoring partial discharges (PD) in gas-insulated substations (GIS) and are increasingly being used on power transformers. Different sensor designs such as disk sensors, spiral sensors, monopoles and dipoles have been used depending on the application. A primary concern is to ensure that the sensitivity of these different sensors can meet the CIGRE requirement for detecting 5 pC partial discharges GIS. Recent work on locating PD in power transformers has focused attention on recording the arrival time of the received UHF signal, which is important for locating insulation defects in three dimensions. An accuracy of 1 ns or better is desirable for time-of-flight measurements to locate a PD source. In some situations, a clear arrival time for the signal can be identified but in other cases the arrival time can be uncertain within a tolerance of several nanoseconds. Therefore, a number of sensors were tested for their speed of response using a transient calibration system. Results suggest that more complex sensor structures have a slower build-up of energy at their output, while simple sensors exhibit less of a dasiablurringpsila effect on the leading edge of the signal.

Compound Phase-Shifted Fiber Bragg Structures as Intrinsic Magnetic Field Sensors

We report on the simulation and characterisation of compound phase-shifted fiber Bragg structures for use as novel in-fiber magneto-optical point sensors. Through simulation we show that the Faraday rotation spectra of phase-shifted gratings can be tailored by tuning the ratio of substructure lengths. A design process for tailoring the magneto-optical spectrum is illustrated, and it is shown that a general optimum structure exists for producing a region of total reflection incorporating magneto-optical Faraday rotation that is enhanced both in strength and spectral width. A practical optical fiber system that exploits the proposed distributed feedback (DFB) structures to enable novel all-fiber sensors for the dual measurement of magnetic field strength and temperature is described in detail, and the sensor response is simulated. The study is supported by laboratory fabrication of the proposed fiber DFB structures which demonstrates the principle of enhancement in terms of tailored group delay spectra and highlights practical issues for sensor packaging.

Nanoscale Resolution Interrogation Scheme for Simultaneous Static and Dynamic Fiber Bragg Grating Strain Sensing

A combined interrogation and signal processing technique which facilitates high-speed simultaneous static and dynamic strain demodulation of multiplexed fiber Bragg grating sensors is described. The scheme integrates passive, interferometric wavelength-demodulation and fast optical switching between wavelength division multiplexer channels with signal extraction via a software lock-in amplifier and fast Fourier transform. Static and dynamic strain measurements with noise floors of 1 nε and nε/√Hz, between 5 mHz and 2 kHz were obtained. An inverse analysis applied to a cantilever beam set up was used to characterize and verify strain measurements using finite element modeling. By providing distributed measurements of both ultra-high-resolution static and dynamic strain, the proposed scheme will facilitate advanced structural health monitoring.

Single-Ended Differential Protection in MTDC Networks Using Optical Sensors

This paper presents a method for rapid detection of faults on voltage source converter multiterminal HVdc transmission networks using multipoint optical current sensing. The proposed method uses differential protection as a guiding principle, and is implemented using current measurements obtained from the optical current sensors distributed along the transmission line. Performance is assessed through detailed transient simulation using MATLAB/Simulink models, integrating inductive dc-line terminations, detailed dc circuit-breaker models, and a network of fiber-optic current sensors. Moreover, the feasibility and required performance of optical-based measurements is validated through laboratory testing. Simulation results demonstrate that the proposed protection algorithm can effectively, and within very short period of time, discriminate between faults on the protected line (internal faults), and those occurring on adjacent lines or busbars (external faults). Hardware tests prove that the scheme can be achieved with the existing, available sensing technology.

Development of power system differential protection based on optical current measurement

A theoretical evaluation of a novel, all-optical differential current protection scheme employing low voltage piezoelectric transducers, fiber Bragg grating sensors and Rogowski coils is presented in this paper. The optical power reflected from the sensors is monitored by means of a single photodetector, whereas fault occurrence is detected by a simple optoelectronic threshold detector. We demonstrate through simulations that an immediate response to an increase in differential current can be achieved even at low voltage levels such as these generated by Rogowski coils providing a low-cost and fast-acting fault detection system.

Distributed optical distance protection using FBG-based voltage and current transducers

We describe a novel approach to distance protection using optical fiber sensors. An optical protection system utilizing two pairs of hybrid fiber-optic voltage and current sensors is presented, allowing for distributed, multiplexed measurement of voltage and current, and hence impedance, at multiple points on a transmission network. Three separate phase-to-earth fault scenarios are tested in order to demonstrate the schemes successful operation in laboratory conditions. The deployment of the proposed scheme in electrical power transmission and distribution systems may allow for novel and flexible protection architectures while reducing the complexity of installation and isolation requirements in the high-voltage environment as well as the elimination of the requirement for a dedicated communication media to implement multi-ended schemes. The possibility of hybrid optical/conventional techniques also exists.

Distributed photonic instrumentation for power system protection and control

We report on recent developments in hybrid optical protection schemes that make use of passive fiber Bragg grating-based transducers for the distributed measurement of voltage and current. In addition to the details of the hybrid optical sensor technology, and its full integration with a commercial busbar protection relay, we report on the first-time laboratory demonstration of a centralized three-ended unit protection scheme featuring hybrid optical sensors capable of passive and wide-area coverage. Furthermore, we discuss the impact of this technology on present and future protection applications that may be improved or enabled by the proposed scheme.

High-Speed Interferometric FBG Interrogator With Dynamic and Absolute Wavelength Measurement Capability

A passive, interferometric wavelength demodulation technique has been extended to measure the absolute wavelengths of a multiplexed array of fiber Bragg grating sensors. The scheme retains its original strain resolution of 10 nε/√{Hz}. A proof-of-concept interrogation system was able to determine the absolute wavelength of Bragg peaks to within 20 pm (17 με). Static and dynamic Bragg grating strains were accurately demodulated in both absolute and relative wavelength measurement modes. This demonstration indicates that interferometric techniques are able to provide absolute, static and dynamic measurements of strain within a single platform.