We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.

SQUID magnetometers for low-frequency applications

On-line monitoring and diagnosis of transformers have been investigated and discussed significantly in last decade. This study has concentrated on issues arising while on-line transformer winding deformation diagnosis is going to be applied on transformers with various kinds of techniques. From technical perspective, before replacing off-line methods by on-line methods and eventually by intelligent approaches, practical challenges must be addressed and overcome. Hence, available off-line transformer winding deformation diagnosis methods are discussed precisely. Mathematical calculation in on-line short circuit impedance measurement is investigated. On-line transformer transfer function measurement setup is presented. A profound insight to the problems pertaining on-line transformer winding deformation recognition methods, characterizes existing online methods, explains the concepts behind online measurements and striving to open the discussion doors towards challenges are discussed. In the end a 400 MVA step up transformer has been taken as a case in order to clarify the capability of Frequency Response Analysis (FRA) method in fault detection while short circuit impedance could only demonstrate some rough understanding about transformer condition.

Advanced transformer winding deformation diagnosis: moving from off-line to on-line

On-line monitoring and diagnosis of transformers have been investigated and discussed significantly in the last few decades. Vibration method is considered as one of the non-destructive and economical methods to explore transformer operating condition and evaluate transformer mechanical integrity and performance. However, transformer vibration and its evaluation criteria in transformer faulty condition are quite challenging and are not yet agreed upon. At the same time, with the advent of IoT facilities and services, it is expected that classical diagnosis techniques will be replaced with more powerful data-driven prognosis methods that can be used efficiently and effectively in smart monitoring. In this paper, we first discuss in detail an analytical approach to the transformer vibration modeling. Nevertheless, precise interpretation of transformer vibration signal through analytical models becomes unrealistic as higher harmonics are mixed with fundamental harmonics in vibration spectra. Therefore, as the next step, we aim to support the Industry 4.0 concept by utilizing the state-of-the-art machine learning and signal processing techniques to develop prognosis models of transformer operating condition based on vibration signals. Transformer turn-to-turn insulation deterioration and short circuit analysis as one the most important concerns in transformer operation is practically emulated and examined. Along with transformer short-circuit study, transformer over and under excitations are also studied and evaluated. Our constructed predictive models are able to detect transformer short-circuit fault in early stages using vibration signals before transformer catastrophic failure. Real-time information is transferred to the cloud system and results become accessible over any portable device.

Transformer Fault Condition Prognosis Using Vibration Signals Over Cloud Environment

Frequency response analysis (FRA) has recently been developed as a widely accepted tool for power transformer winding mechanical deformation diagnosis, and has proven to be effective and powerful in many cases. However, there still exist problems regarding the application of FRA. FRA is a comparative method in which the measured FRA signature should be compared with its fingerprint. Small differences of FRA signatures in certain frequency bands might be produced by external disturbance, which hinders fault diagnosis. Additionally, the existing correlation coefficient indicator recommended by power industry standards cannot reflect key information of signatures, namely the extreme points. This paper proposes an improved FRA based on binary morphology and extreme point variation. Binary morphology is first introduced to extract the certain frequency bands of signatures with significant difference. A composite indicator of extreme point variation is adopted to realize the diagnosis of fault level. A ternary diagram is constructed by the area proportions of the binary image to identify winding faults, which has a potential to realize cluster analysis of fault types.

Detection of Power Transformer Winding Deformation Using Improved FRA Based on Binary Morphology and Extreme Point Variation

This paper addresses the problem of predictive diagnostic in high-power transformers. Particularly, this paper is focused on their use in uninterruptible power supply systems for safety critical applications, such as railway interlocking signaling installations. With respect to the state of the art, where typically only thermal and electrical faults are monitored, this work proposes a distributed network of measuring nodes where also vibration-based mechanical stress diagnosis is implemented. Mechanical degradation is tracked through vibration measures, using multichannel accelerometers, with sensitivity down to 0.5 mg and local signal processing in the transformed frequency domain, up to 1 kHz. A compact hardware-software implementation of the nodes is also presented. The performances of the diagnostic system are assessed through experimental measurements on real three-phase high-power transformers used in railway applications.

Predictive Diagnosis of High-Power Transformer Faults by Networking Vibration Measuring Nodes With Integrated Signal Processing

A strategy for online diagnosis of the state of the windings in power transformers is discussed. Earlier work showed that it is possible to separately determine core and copper losses of transformers in real operating conditions. A similar measurement is suitable for the online determination, with good precision, of the stray reactance variations in transformers. Since the stray reactance is indicative of windings geometry, this technique enables a diagnostic system to be set up that provides indications of conductor insulation reliability, recommending shutdown when necessary. The method was tested on a 25-MVA-150-kV/16.2-kV three-phase transformer, showing the possibility of performing this online determination with an accuracy at least on the same order as that obtained in the usual methods with a transformer not in service. >

Diagnosis of the state of power transformer windings by on-line measurement of stray reactance

A method for measuring power losses in transformers under actual working conditions is presented. The technique is based on an unusual correlation between input and output electric quantities and yields uncertainties lower than those obtainable by evaluating these quantities as the difference between input and output powers. The analysis is carried out for both single-phase and polyphase transformers, whether with sinusoidal or any other periodic waveforms. >

Measurement of transformer power losses and efficiency in real working conditions

On the application of the guide to the expression of uncertainty in measurement to measuring instruments

The problem of compatibility evaluation for measurements performed on the same measurand by different processes (different methods, laboratories, devices) is addressed as one of the most critical in the perspective to establish a measurement traceability. A new approach is proposed based on the representation of the measurement results by means of the probability intervals generated by the random variables representing the "measures". The parameters characterising the measurement compatibility criterion are focused and an operative procedure depending on such parameters is pointed out and tested in some applications.

Measurement compatibility: a novel approach based on the definition of probability intervals

The most recent and improved Josephson standards are obtained with cryogenic voltage comparators, i.e., by transferring the critical elements of the comparison circuit between the Josephson voltage and the reference voltage to the low temperature of the junction. A cryogenic voltage comparator not of the usual resistive types is presented, which is based on the conversion of the ratio between the two voltages to be compared into an equal ratio between two dc currents, measured with a superconducting dc ratio standard and related SQUID detector. This voltage comparator uses the irradiated Josephson junction as an ideal voltage generator and requires no calibration of resistance ratios. It looks very promising for realizing a cryogenic voltage standard that is simple in both construction and operation and with an uncertainty at 10−9 level. The preliminary implementations of the apparatus and the first results are presented.

http://ieeexplore.ieee.org/iel5/19/6312221/06312277.pdf

E. Arri

Papers

Diagnosis of the state of power transformer windings by on-line measurement of stray reactance

Measurement of transformer power losses and efficiency in real working conditions

On the application of the guide to the expression of uncertainty in measurement to measuring instruments

Measurement compatibility: a novel approach based on the definition of probability intervals

A novel cryogenic voltage standard