Concept level evaluation of the optical voltage and current sensors and an arrayed waveguide grating for aero-electrical system applications

In this paper we present, for the first time, the hybrid fiber-optic voltage and current sensors interrogated using an arrayed waveguide grating (AWG) device. Due to the excellent dynamic capabilities of an AWG-based interrogator and its improved robustness, the proposed system would be suitable for voltage and current monitoring within an aircraft electrical system. The voltage sensor comprises a multilayer piezoelectric stack, acting as a voltage-to-strain transducer, and a fiber Bragg grating (FBG) used to convert voltage induced strain changes within the stack into wavelength shifts. These wavelength shifts are then analysed by an AWG. To measure current, the same sensor type is used to monitor a specially designed ferrite-core current transformer. It is shown that the system is capable of measuring variable frequency of voltage and current waveforms, typical of those anticipated in the next generation aero-electric power systems. It is also demonstrated that the system can be used for voltage and current harmonic analysis and power quality measurement in such networks

FBG-based fibre-optic current sensors for power systems protection : laboratory evaluation

Conventional differential current unit protection schemes rely on a pair of electronic protection relays that measure current phasors separately at the boundaries of the protected zone. The scheme requires a separate, often optical, communications channel for the sharing of measurement information to enable the timely identification of and reaction to internal faults. The high voltage environment that the transducers must operate in poses a number of engineering problems stemming from the need for electrical isolation and requirement for transformation of high primary system current magnitudes. Additionally, when either the number of relays or distance between relays is increased, timing problems can arise due to the limited bandwidth, speed and changeable latencies of the communication channels and the increased computation requirements. Fibre-optical sensor systems are maturing as a technology and offer a number of advantages over conventional electronic sensor regimes, including the possession of inherent electrical isolation, chemical inertness, immunity to electromagnetic interference, and their small size and serial multiplexing capability. Fibre sensor systems are therefore experiencing increased uptake in industries that operate in harsh environments, such as oil and gas, or where specific requirements such as large step-out distances or resistance to radiation prohibit the use of electronic sensors. The Advanced Sensors Team within the Institute for Energy and Environment has developed fibre-optic point sensors for voltage and electrical current, based on fibre Bragg grating (FBG) technology, that have been applied successfully to power systems diagnostics. With the photonic systems capability to interrogate up to 100 km from source at kHz sample rates with up to 30 sensors in series, it is possible and highly desirable to adapt this technology for use in power systems protection, where immediate applications in unit and distance protection are clear. In this paper, the application of the FBG-based hybrid current sensor system to power systems protection is presented for the first time. Experimental tests of the response of an optical unit protection system to a range of internal and external fault scenarios are also reported. Secondary current inputs to the system are modelled using ATP and injected into the prototype test system via an APTS3 (Advanced Protection Testing System) unit. Fibre sensors, separated optically by 24 km of fibre, provide all measurement information via a single interrogation system situated at one end of the protected zone. Experimental results confirm high performance of the optical unit protection both in terms of sensitivity to internal faults and stability under external fault conditions. Therefore, the systems ability to overcome problems experienced in electronic relaying systems using conventional current sensing technologies is demonstrated. No separate communications channel is required in this configuration, with fault algorithms being deployed only at one location that need not be close to the protected zone. The fibre-optic current sensor systems capacity for long-distance interrogation and high sensor count qualify it for further applications in more complex protection schemes, or over larger distances, where a single fibre could form the basis of highly novel distributed protection schemes. This potential will also be discussed in detail in the paper

Preliminary characterization of an optical current sensor for HVDC networks

An optical current sensor (OCS) for distributed current measurements on high-voltage direct current (HVDC) networks is presented in this paper. The sensor prototype, comprising a piezoelectric transducer and a fiber Bragg grating (FBG) as a principal optical strain sensing element, is evaluated through laboratory testing and its performance is assessed within the context of the accuracy requirements specified by the relevant industry standards. It is demonstrated that the device has the potential to meet the requirements of the accuracy class 1 specified by the IEC 61869-14 standard

Life extension for wind turbine structures and foundations

This paper presents economic life extension scenarios for wind turbines as well as complimentary structural health monitoring of turbine foundations based on an advanced optical sensor network. Demand for this is driven by an ageing asset base and the overall reduction in governmental support towards wind energy in Europe, despite the agreed 2020 and extended 2030 renewable energy targets. Consequently, this paper displays early work on economic evaluation of levelised cost of energy (LCOE) under simple life extension scenarios and concludes that reductions within the order of 5% of LCOE can be achieved by extending a turbine’s lifetime by up to 15 years. At the same time, an ongoing project is presented that aims to apply structural health monitoring to a wind turbine foundation aimed at providing operational load data that can justify or dictate lifetime extension of a wind turbine foundation

Identifying fabrication defects of metal packaged fibre Bragg grating sensors for smart pre-stressing strands

Metal packaged fibre Bragg grating sensors have the potential to provide reliable measurements of temperature and strain in high stress environments for the purpose of structural health monitoring. However due to the induction brazing fabrication process a small percentage of sensors are found to have defective brazed joints. We demonstrate a defect identification procedure derived from the batch temperature calibration of fibre Bragg grating sensors for temperature and strain measurement, allowing defective sensors to be identified before installation. The procedure was demonstrated on a sample of twelve temperature sensors fabricated for a small-scale smart pre-stressing strand validation test

A decision support tool to assist with lifetime extension of wind turbines

This paper is aimed at analysing the levelised cost of energy (LCOE) of onshore wind turbine generators (WTGs) that are in operation beyond their design lifetime. In order to do so, the LCOE approach is introduced and input parameters are discussed for a UK deployment. In addition, a methodology is presented to support economic lifetime extension and investment decision making at the end of an asset's design lifetime. As part of a case study, a wind farm consisting of six 900 kW WTGs is subjected to different combinations of i) lifetime extension (5- 15 years), ii) input assumptions (pessimistic, central, optimistic), and iii) re-investment types (retrofits). Results indicate that in the central lifetime extension scenario, LCOE estimates of 22.40 £/MWh are achievable

The effect of upscaling and performance degradation on onshore wind turbine lifetime extension decision making

Ever greater rated wind turbine generators (WTGs) are reaching their end of design life in the near future. In addition, first approaches quantified the impact of long-term performance degradation of WTGs. As a consequence, this work is aimed at discussing and analysing the impact of upscaling and performance degradation on the economics of wind turbine lifetime extension. Findings reveal that the lifetime extension levelised cost of energy (LCOE2) of an 18 MW wind farm comprising of 0.5 MW rated WTGs are within the order of £23.52 per MWh. Alternatively, if the same wind farm consists of fewer 2 and 3 MW WTGs, the LCOE2 reduces to £16.56 and £15.95 per MWh, respectively. Further, findings reveal that an annual performance degradation of 1.6% (0.2%) increases LCOE2 by 34-41% (3.6-4.3%)

All-optical busbar differential protection scheme for electric power systems

This paper proposes a novel implementation of a differential protection scheme using magneto-optic current sensors. The proposed all-Optical Differential Protection (ODP) scheme utilizes inherent properties of magneto-optic sensors connected in series to perform differential protection functionality. In order to demonstrate the validity of the proposed scheme, all constituent components such as optical fibre, polarisers and Faraday rotators have been modelled using the Jones matrix representation. Through selected simulation-based case studies, including external and internal (high resistive and solid) faults, the paper demonstrates that the proposed novel ODP scheme for busbar protection meets the protection relaying performance criteria in terms of discrimination, sensitivity, stability, as well as ultra-high speed of operation

Nonlinearity compensation of the fiber Bragg grating interrogation system based on an arrayed waveguide grating

In this paper, we report on nonlinearity compensation for a solid-state fiber Bragg grating (FBG) sensor interrogation system based on an arrayed waveguide grating (AWG) device. A lookup table with calibration data is used to improve system linearity. A reduction in the absolute value of the measurement error from 120 mu strain or 15 degrees C to 4.8 mu strain or 0.6 degrees C, respectively, is experimentally demonstrated

Fatigue stress resistance of some composite materials for dental fillings

The paper investigates fatigue stress resistance of some originally made composite materials based on Bis-GMA resin with fluoridated glass and YbF3. The material was used to fill in the cavities in teeth extracted for orthodontic reasons. For laboratory tests a mastication simulator was used. The tooth samples were placed in special holders with resin and exposed to cyclic mechanical loads (100 000 cycles) in order to assess their resistance to fatigue stress. The influence of repeatable mechanical loads on the teeth and the structure of the material were investigated. Additionally, the impact of cyclic loads on fluoride release from the composite materials was estimate