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.

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, Tb_0.3Dy_0.7Fe_1.95, 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.

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 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 με 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. The Bragg wavelength is defined as follows [15]. (1) λ B = 2 n eff Λwhere, Λ is the grating pitch and n_eff is the effective index of the fiber core. Bandwidth of the Bragg reflected signal depends mainly on grating length and it is typically around 0.05 to 3 nm.

OCT used in this experiment consists of FBG strain and temperature sensors and a giant magnetostrictive material bounded together with epoxy. As described earlier, biasing point of the sensor can be adjusted by mechanical prestress on magnetostrictive material and the amp-turn of the DC biasing solenoid. One of the main advantages of the OCT sensor, which can substantially reduce the cost of implementing power system, is that the OCT sensor does not require any CTs and PTs for biasing to detect the fault in the system. Terfenol-D (Tb_xDy_1−xFe_y) was chosen as the magnetostrictive material due to its fast response (<0.1 ms) [16,17] and capability of producing a substantial amount of strain in the magnetic field. A broad band source was used to illuminate the FBG through a single mode fiber optic and optical coupler. The principle of FBG is well known [15] and will not be explained in detail here. A small change in strain and temperature both cause wavelength shift on the reflected FBG wavelength by following equations: (2) Δ λ λ 0 = k ɛ + α nTemp Δ T . (3) α nTemp = δ n n δ Twhere, Δλ is the wavelength shift, λ₀ is base wavelength in nominal temperature and strain, ΔT is the temperature change, k is the gage factor which is normally 0.78, α_nTemp is the change in index of refraction with respect to temperature, and ε is strain. In this paper, the following parameters were measured through experiment: λ₀=1528 nm, Δλ=1.17pm/(μstrain), and wavelength shift is Δλ=13.48pm/°C. Also, separate FBG temperature sensor with 27.1 mm probe, ±1.7pm/°C sensitivity and 0.3 s response time is used to measure temperature and compensate temperate effect. The system is calibrated for ambient temperature of 25° degree.

In each phase of the overhead transmission line one sensor is installed and all the sensors should be placed as close as possible to its own conductor as shown in the Figure 2(a). They should be placed as far as possible from the remaining conductors. Detailed design of sensors and their placement have been shown in the Figure 2(b). Since magnetic field is inversely proportional to the distance squared, the effect of other phases in this sensor is so small they can be easily ignored in the simulations. AC current measurement with FBG has been extensively discussed earlier [18–23]. If overhead transmission line is assumed straight infinite conductor, its magnetic field can be expressed as: (4) H = I 2 π dwhere, d is the distance from the center of Terfenol-D to the conductor. In our study, two FBG current sensors installed at both ends of the transmission line sense strain in both ends of transmission line. The transmission line is an assumed bundle with geometer mean diameter (GMD) of 11.07 m and geometric mean ratio (GMR) of 0.127 m with length of 150 km aluminum conductor, steel reinforce (ACSR), 60 Hz, and 0.55 per unit impedance. Base values for the system are: 100 MVA and 138 kV.

The block diagram of the system and experimental setup are shown in Figure 3(a,b). V_reference and V_filter are the output of the reference photodiode and filter photodiode, respectively. The reflected light (V_reference) is sent through a separate path to the reference photodiode. Previously, Melle and Liu [24] have shown that the ratio V_filter /V_reference is proportional to the fiber Bragg wavelength shift: (5) V filter V reference = K ( λ B − λ 0 + Δ λ π )   .where, constant K is the slope of the edge or tunable filter, λ₀ is the wavelength at which the edge filter has zero transmittance, λ_B is the peak wavelength of the grating, and Δλ is the spectral width of the Bragg reflected spectrum. This method has been chosen to measure the Bragg wavelength shift because of its simplicity to install, less complexity and fast response. In this experiment, a DC coil made of copper AWG 24 wire with 2,600 amp-turn is used with excitation current of 0.55 A and resistance of 18.8 Ω.

A novel protection method in the overhead power transmission line dependent in FBG optical current sensor that uses magnetostrictive material is investigated. These FBG current sensors can monitor the whole power system simultaneously and they offer better accuracy, and wider bandwidth in comparison to conventional CTs. Both radial and network power systems are designed with ETAP, and FBG wavelength shifts are processed with MATLAB software. Simulation and signal processing results both confirm the effectiveness of the proposed method. Fault detection rules have been defined based on ANN algorithm to detect the faulted bus and the rules almost always were successful. However, misclassification can occur in the cases that the incident angle, or fault resistance are different than what is used during the ANN’s learning process.

Both the DC biasing point and the prestress should be defined based on the normal current operation of the investigated power system. Higher performance can be achieved by precisely defining prestress in magnetostrictive material, accurately adjusting the DC biasing point of the sensor, and evaluate all possible disturbances in the system. Sensors can detect all kinds of fault in both radial and network overhead transmission line irrespective of the fault type, fault location, and fault resistance up to 100 Ω. The main advantages of the FBG current sensors include their ease of installation, immunity to EMI, reduced insulation requirement, smaller size, and they are less expensive in comparison to conventional CTs. In this research high harmonic current, sensor mal-function, complex feeder network and backup protection in the ANN network were not considered which limit the research. Future research should address the harmonics effect at the sensor response, sensor mal-function, and sensor noise based on different pole structure and configuration.