Application of Fiber Bragg Gating (FBG) Sensing Technologies in Power Systems
Written by Hossam A.Gabbar and Yasser Elsayed
The operating conditions of power systems are changing continuously. These changes reduce the life cycle of the expensive power network equipment and impact the revenue and reliability of the overall electrical power supply system. Failure of windings in power transformers lead to disruption in power supply and accidents.
The most likely cause of failure is the electrical and thermal stresses which raise the operating temperature in the epoxy core. Degrading the insulation is dangerous to both equipment and personnel. On the other hand, the complex structure of the power grid requires an efficient monitoring system for the optimization and improvement of the operating conditions and life cycle of the equipment. Temperature and vibrating sensors are crucial in power utility systems to avoid the thermal and vibrating effects on the equipment. Conventional sensors are not isolated from the electromagnetic interference (EMI) and high voltage (HV) environment. In addition, conventional sensors are physically big and cannot be embedded internally in the sensed part of the equipment. The Fiber Bragg Grating (FBG) based sensors have been utilized in multiple engineering fields. The FBGs can measure a variety of parameters such as strain, temperature, current, voltage, vibrating and pressure, which makes them ideal for different sensing applications such as structural health monitoring, vibration, and temperature monitoring. Furthermore, numerous researches are being carried out for the application of FBGs sensors due to its high sensitivity, easy handling, accuracy, secure remote data transmission, and immunity to High voltage (HV) and electromagnetic interference (EMI). Important parameters such as hot spots on high voltage bus bars can be easily and safely monitored using FBG based sensors. Moreover, it also allows multiple parameters such as voltage, current, frequency, temperature, vibration, tilt and torque to be accurately and simultaneously monitored with lesser equipment when compared to traditional sensors. Sensors can be installed at any potential points in generators, breakers, transformers, Transmission Lines (TL) buses, and loads as shown in Figure 1.
Figure 1. Potential points for FBG sensors in power systems
Working principles of FBG sensors and applications: An optical FBG acts as an electric field sensor and is fabricated on a semiconductor diode that emits light of wavelength 1577.5 nm around its maximum reflectivity point . The intensity of the strong electric field can be easily detected with this sensor. The wavelength of FBG and strain of electrostrictive material change correlatively with the change of the applied electric field. Therefore, an electric field can be measured accurately as a function of the strain changes of the wavelength of FBGs . The direct application of FBGs and a Perovskite Lead Lanthanum Zirconate Titanate (PLZT) ceramics stack for the measurement of high voltage was discussed in . The results proved that a linear relationship exist between the high voltages applied to the sensor and the displacement of the reflective spectrum. In , a high voltage transmission line monitoring application using the fiber optic sensor was carried out to analyze and report line loading current, temperature, and voltage sag relationship. Significant work on the design of hybrid FBG piezoelectric voltage sensor for monitoring of electrical submersible pump (ESP) motors was also validated in . Another application of FBGs to measure the critical parameters of electrical machines such as speed, torque and temperature using multiple FBGs were presented in . The temperature dynamics of the three-phase induction motor under different operation conditions were also investigated and presented in . The temperature field distribution is monitored by the uniformly distributed FBGs in the stator. A similar application of temperature and harmonic measurement and estimation for different operating conditions for the equipment such as induction motor and power converters were reported in [8-10].
Application of FBGs in power systems: There is still a knowledge gap in developing the application of FBGs in power systems. The relation between the critical electrical parameters with the optical spectrum of optical fiber sensor as well as transduction circuitry and data acquisition system will convert these optical signals for monitoring and control purposes. In addition, multiple electrical parameters conversion over a single module of FBG sensor is also one of the great advantages of FBG. Furthermore, power utilities will gain benefits from implementing FBG technologies for performance monitoring of parameters in electrical power grid. The feasibility of utilizing FBG technologies as a measurement technology can be evaluated by modeling a high-fidelity demo system of a power network of the Canadian electrical grid. The demonstration of FBG will be complemented using OPAL RT-based SCADA, automation, and control system for monitoring, operation, and control applications. The demo power system network will resemble the same operational functionality as compared to the existing power network of the grid and can be used for research activities on various operational performance parameters of the power grid network. The system setup can be used to capture the operational data through the sensor to predict and improve the performance of the grid network and equipment. The main components of the demo system are a scaled-down version of transmission line, transformer, breaker, dynamic loads, motors, generators, capacitor banks, and power converters to form a small dynamic power system network in the controlled environment of the lab. The scaled-down version transmission line is made in such a way that it will affect line capacitance and inductances depending on the length of the line, similarly, the function generator will resemble a synchronous generator to generate conditions such as voltage unbalance, harmonic distortion, voltage/frequency fluctuation. The static and programmable loads will also be designed and prototyped for the grid network. The measurement units and control circuits such as CT, PET, transducers, sensors, and relays will also be commissioned for remote monitoring and control of the power system network. A solar PV system is also integrated with the conventional power network. For remote monitoring, operation, and control of the electrical network, a powerful real-time power system simulator, OPAL-RT, will also be interfaced through DAQ boards as shown in Figure 2. It shows the data collected by the conventional sensors and FBGs can be applied to the analog inputs (AIs) of OPAL-RT which is running the power system Simulink model and then the outputs of the simulation model can be applied to the analog outputs (AOs) of the OPAL-RT to be displayed by connected scopes.
Figure 2. The block diagram of the interface between OPAL-RT, FBG, and conventional sensors
The benefits of utilizing FBGs sensors in power systems: Utilities will gain several benefits by integrating FBG in their facilities for monitoring electrical parameters of power grid network equipment as below.
Data Integrity: The interface of multiple electrical parameters in power systems with a single module of FBG sensor can support different deployment in remote locations. Utility companies should perform selection & identification of desired measurement points and the best ways to deploy FBG.
Accuracy and response: The FBGs are isolated from the EMI and HV impacts when compared with the conventional sensors. In addition, they have a higher resolution and faster response time (1 ms to 200 ms) when compared with the traditional sensors. In addition, they have a smaller size and can be mounted inside the machinery i.e. inside the power transformer winding so they can sense accurate temperature and vibrating that is not affected by the surrounding environment.
Safety: FBGs sensors have a higher dielectric strength, ~1pC that is tested up to 1500k, hence they are safe to be used in HV environment while conventional sensor like RTD are generally not utilized in HV environment to ensure the highest standards of safety at workplace for people and equipment.
Linearity: Fiber optic sensors transmit light signals through glass, which is the purest form of silica. The sensors are linear and does not need any compensation and special algorithms. Figure 3. depicts the linear relation between the temperature and the sensor’s wavelength (mm).
Figure 3. The FBG sensor (TSMN-s) wavelength (mm) vs temperature
Power systems have many physical parameters that should be monitored to ensure the optimal and safe operation. The conventional sensors like temperature, vibrating, current, voltage, tilt, pressure have some issues like accuracy, repeatability, safety, EMI and HV impacts. FBGs sensors are an alternative solution for more accuracy, safety, immune to HV and EMI, linearity, and fast response. Utilizing FBGs in power systems requires optimum selection of sensors and transducers and data acquisition systems to capture the critical parameters of power systems and equipment. The evaluation of such system could be performed using prototyping of a demo lab-scale system. The analysis and verification could be achieved using the real-time simulator OPAL-RT (e.g. OP4510). The parametric analysis of captured operating parameters from FBG sensors will provide a basis for performance evaluation and technology assessment. This is followed by the selection of prototype sites and a deployment plan for wider implementations.
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This article edited by Vigna Kumaran