IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 9000604 Contactless Loop Method for Measurement of AC Losses in HTS Coils Nuno Amaro, Student Member, IEEE, Ján Šouc, Member, IEEE, João Murta-Pina, Member, IEEE, João Martins, Senior Member, IEEE, Jose Maria Ceballos, and Fedor Gömöry Abstract—Measurements of ac losses in high-temperature su- perconducting (HTS) coils are of utmost importance for power applications because these are one of the most limiting factors in superconducting devices. There are two possible ways to mea- sure such losses: calorimetric and electromagnetic methods. An electromagnetic method using a contactless loop is reported in this paper. To verify the applicability of such method in medium- to large-sized coils, two different contactless loops were designed and built, and results are compared with those obtained from measurements made with different configurations of voltage taps. All tests were performed at 77 K, using a superconducting coil with a variable number of turns (up to a maximum of 128) wound from BSCCO tape. Currents ranging from zero to the critical value were applied, at frequencies from 72 to 1152 Hz. Index Terms—AC losses, ac losses measurement, contactless loops, HTS Coils. I. I NTRODUCTION H IGH temperature superconducting (HTS) devices like Superconducting Magnetic Energy Storage (SMES) sys- tems and Superconducting Fault Current Limiters (SFCL) have several applications in power systems that make them poten- tially advantageous when comparing to conventional technolo- gies [1], [2]. However, there are yet technical challenges that need to be overcome in order to disseminate the utilization of these devices. One of those challenges is related to AC losses, since all generated heat must be removed from the system by cryogenics. Measurement of AC losses is then an important topic of research in superconductivity. AC losses can be measured using two kinds of methods: electromagnetic [3]– [6] and calorimetric [7], [8]. In this work an electromagnetic method using a lock-in amplifier is addressed. When consider- ing medium/large sized HTS coils, the process of measuring AC losses is complex, because acquired signals are easily higher Manuscript received August 12, 2014; accepted November 17, 2014. Date of publication November 24, 2014; date of current version February 6, 2015. This work was supported in part by National Funds through Fundação para a Ciˇ encia e Tecnologia (FCT) under Project PEst-OE/EEI/UI0066/2011 and by EU COST Action MP1004. N. Amaro, J. Murta-Pina, and J. Martins are with the Centre of Tech- nology and Systems, Faculdade de Ciˇ encias e Tecnologia, 2829-516 Quinta da Torre, Portugal (e-mail: nma19730@campus.fct.unl.pt; jmmp@fct.unl.pt; jf.martins@fct.unl.pt). J. Šouc and F. Gömöry are with the Institute of Electrical Engineering, Slovak Academy of Sciences, 841 04 Bratislava, Slovak Republic (e-mail: jan.souc@savba.sk; fedor.gomory@savba.sk). J. M. Ceballos is with the Electrical Engineering Department, University of Extremadura, 06071 Badajoz, Spain. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2014.2374155 than the input range of typical lock-in amplifiers, commonly used in electromagnetic measuring methods [9]. Considering this fact, it is necessary to decrease the measured signal, with- out changing its phase value, which is an essential condition to achieve accurate results. One possible way to compensate the high inductive component is through the utilization of an external coil [10]. However, such approach introduces more complexity to an already complex system. An alternative might be the use of voltage taps and/or pick-up coils to measure such losses [11]. To decrease measured values, instead of measuring losses through the whole coil, a representative turn can be used [10]. If the chosen turn is a good representative of the full coil then the loss value for the full coil can be extrapolated from the measured value (in only one turn). The measured value can thus be decreased, making this a more feasible approach for measuring AC losses in larger HTS coils. However, placing voltage taps in the coil also has disadvantages, because it is necessary to remove the insulation from the HTS tape. This is problematic in large systems, in which the voltage (mainly inductive) can easily reach kV level. Considering these aspects, a contactless method in which there is no electric contact with the tape might be more suitable to measure AC losses. Besides this, such method has yet other advantage: it avoids a large voltage value (when compared to voltage taps placed at the ends of the coil). These aspects, together with the fact that there is no electric contact, make this an adequate process to measure AC losses. Contactless methods have already been used to measure losses in tape samples for several years [12] and Nguyen et al. [13] already demonstrated the applicability of a contactless method in HTS coils. However, in the particular work of Nguyen et al., the contactless loop was placed in the last turn of the coil, which resulted in the need to add a calibration factor. According to [10], the outer turn is not the most representative turn of a coil, and it is therefore necessary to evaluate if a contactless loop placed in the most representative turn improves the accuracy of these contactless methods. Ac- cording to the work presented in [10], the most representative turn is considered as the one located at two thirds of the total number of turns starting from the inner one. As an example, in a 32 turns coil, the most representative turn is the 21st. In addition to this, it is also important to verify if a change in the length of the contactless loop has any influence in measured values. These aspects were tested and results are presented in this work. To verify the accuracy of such results, a contactless method (using two different lengths of contactless loops) is compared with results obtained using voltage taps of several arrangements in the same coils. 1051-8223 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.