21-09-2016, 11:58 AM
Fiber Bragg Grating Sensor for Fault Detection in Radial and
Network Transmission Lines
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Abstract: In this paper, a fiber optic based sensor capable of fault detection in both radial
and network overhead transmission power line systems is investigated. Bragg wavelength
shift is used to measure the fault current and detect fault in power systems. Magnetic fields
generated by currents in the overhead transmission lines cause a strain in magnetostrictive
material which is then detected by Fiber Bragg Grating (FBG). The Fiber Bragg
interrogator senses the reflected FBG signals, and the Bragg wavelength shift is calculated
and the signals are processed. A broadband light source in the control room scans the shift
in the reflected signal. Any surge in the magnetic field relates to an increased fault current
at a certain location. Also, fault location can be precisely defined with an artificial neural
network (ANN) algorithm. This algorithm can be easily coordinated with other protective
devices. It is shown that the faults in the overhead transmission line cause a detectable
wavelength shift on the reflected signal of FBG and can be used to detect and classify
different kind of faults. The proposed method has been extensively tested by simulation
and results confirm that the proposed scheme is able to detect different kinds of fault in
both radial and network system
Introduction
Current transformers (CTs) and potential transformers (PTs) are widely used in monitoring power
systems by sending fault current/voltage information to relay and control rooms at substations. If
current/voltage in a particular line is out of a pre-set range, relays will send a trip signal to breakers to
trip. If a primary relay fails to operate and clear a fault, based on pre-set rules, a back up breaker will
clear the fault. Conventional CTs are iron based which subject to saturation and hysteresis. Most relays
make decisions and send trip signals based on the root mean square (rms) value of fault current
detected by the CT. Saturation in the CTs cause the rms value of fault current sensed by the CT to be
much smaller than the actual value and it can prevent relays from tripping and eventually cause
instability in the system.
Optical current transformers (OCT) are becoming more popular in power systems. An OCT can
offer a better transient response, better accuracy, and wider bandwidth in comparison to traditional CTs
due to the OCTs’ lack of iron core [1]. OCTs are light, small, less expensive, and immune to
electromagnetic interference (EMI) [2]. Already, a number of Faraday effect current sensors have been
investigated and successfully implemented [3,4].
Although OCTs can measure high current without saturation, lack of proper fault detection and
classification algorithm prevents them to apply in power systems. Previous researchers have discussed
fault effect in the FBG wavelength shift [5-7]. Authors here, for the first time, use wavelet transforms
and ANN algorithm to detect and classify the faults based on FBG wavelength shift signals. This paper,
proposes an FBG based OCT which can replace conventional CTS. A unique ANN algorithm is used
to first detect the fault based on FBG wavelength shift and then classify it. The proposed OCT does not
need any CT or PT for biasing and the proposed ANN algorithm works in both radial and network
systems.
2. Magnetostrictive Materials
Magnetostrictive materials are the part of ferromagnetic materials that transform from one shape to
another in the presence of the magnetic field. Magnetic field causes internal strain in the
magnetostrictive material with consequences of expansion of the material in the magnetic field
direction. In magnetostrictive materials, magnetic field strength is proportional to the square of applied
strain until eventually the magnetic saturation achieved. Since the basis of expansion is molecular, the
magnetostrictive materials are very sensitive to strain and have a very fast response [8,9]. Also, due to
the change in the crystal structure of the material, measurement is repeatable with in milliseconds.
Among these materials, Terfenol-D,
Tb0.3Dy0.7Fe1.95
, an alloy of Terbium, Dysprosium and Iron, has the
highest strain in magnetic field. At room temperature, Terfenol-D can produce about 1,000 ppm which
is large enough to apply to FBG strain sensor. Previously, Sun and Zheng [10] have shown that the
highest sensitivity in the Terfenol-D in the magnetic field up to 20 kA/m can be achieved with 6.9 Mpa
prestress. Due to the nature of all giant magnetostrictive materials, applying prestress can cause a better
sensitivity. However, their response is roughly proportional to the strength of the magnetic field.
Terfenol-D can be polarized by using a DC biasing field [8,9]. Performance of the Terfenol-D depends
on the prestress and the DC bias magnetic field.
Sensors 2010, 10 9409
Since AC magnetic field is measured in this experiment, the DC biasing is necessary to shift the AC
wavelength and prevent changing the polarity of the output while the input is changing. DC biasing can
be achieved with serial or parallel permanent magnet, or the DC biasing coil. In this experiment, the
DC biasing method is used due to its simplicity to change the DC magnetic field. In general, biasing
point of the magnetostrictive material is defined based on applied prestress and DC biasing point.
Figure 1(a,b) show Terfenol-D with and without DC biasing field respectively. As shown in
Figure 1(a), the DC biasing field,
H0
, is chosen such that the slope of the curve is in its maximum
point. Behavior of magnetostrictive materials is a nonlinear relation which is already described in
detail [10,11]. The structural design of FBG sensor using Terfenol-D has been shown in the
Figure 2(b). In this experiment, DC solenoid with 2,600 amp-turn produces the DC biasing field and
causes wavelength shift in FBG strain sensor attached to Terfenol-D material. The Hysteresis and eddy
losses are present in the giant magnetostrictive materials [12-14] and they are considered in the sensor
model
FBG Sensors
FBG sensors can work as arrays for real time measurement of temperature, strain, and pressure in
the systems. Optical fiber sensors have numerous advantages such as electrically passive operation,
EMI immunity, high sensitivity, and multiplexing capabilities which make them a perfect candidate to
use in power systems. FBG sensors are commonly used for strain and temperature measurement and
they can measure strain up to ±5,000 ?ε and temperature ranges from −40 °C to +120 °C Strain causes
change in the grating pitch and the fiber index of the sensor. The sensed strain in FBG sensor is then
coded directly into the wavelength and can be detected as wavelength shift. FBGs reflect a narrowband
of light and transmit all other wavelengths. In other words, FBG is an optical fiber that works as a filter
for a particular wavelength. The principal of a FBG based sensor is to detect the reflected Bragg
wavelength shift due to changes in temperature, strain, or pressure