A Superconducting transformer system for high current cable testing Page: 2 of 10
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way, a nulling system is created that is independent of
the non-linear field dependence of the Hall-probe volt-
age and the measurement current is now proportional to
the secondary current. This method of secondary cur-
rent sensing has been applied successfully in a multitude
of superconducting transformer systems4-6, but requires
relatively complex cryogenic instrumentation, and is im-
practical to debug if any errors occur.
Advances in digital electronics have enabled the de-
velopment of digital integrators with sufficient long-term
stability to be suitable for inductive DC current mea-
surement systems. A more recent 50 kA superconducting
transformer system for the FRESCA facility at CERN,
Geneva, Switzerland, has demonstrated the feasibility of
a 12 bit digital integrator to integrate the signal of a nor-
mal conducting Rogowski coil7. A smaller scale, 100 A
system, constructed around a 16 bit PCI board that per-
forms a digital integration of the Rogowski signal, has
also been successfully tested8.
Here, we describe the development of a DC supercon-
ducting transformer system, which is designed to deliver
up to 50 kA for cable tests under transverse pressure
in the 12 T split pair magnet facility at the National
High Magnetic Field Laboratory, Tallahassee, FL, USA
(NHMFL). The system can easily be adapted for use in
other facilities. In section II the design of the supercon-
ducting transformer system is described, and test mea-
surements are presented in section III. Overall conclu-
sions are presented in section IV.
FIG. 1. Simplified schematic of the superconducting trans-
former and feedback system.
option for the inductive current measurement.
The primary current has to be accurately controlled
using a feedback system to allow for automatic compen-
sation of the parasitic losses that occur in the splice re-
sistances in the samples. The following sections describe
the implementation of the above general design consid-
A. Design considerations
Superconducting cables generally carry currents in the
range of 10 to 30 kA in the available background mag-
netic fields of 8 to 12 T. A suitable target current for
the transformer with sufficient margin is therefore on the
order of 50 kA. Such currents can be carried by a sin-
gle NbTi cable for the secondary. Output currents on
the order of 100 A are common for relatively compact
linear current supplies, and such currents can be carried
by simple copper current leads, and by superconducting
NbTi wires that are suitable for the primary windings.
A suitable transformer ratio is therefore on the order of
The superconducting transformer has to be compact
with limited height, to limit the required liquid helium
level above the magnet system that contains the cable
experiment. The system should allow for the use of two
Rogowski coils in anti-series to enable compensation for
possible parasitic pickup of the stray magnetic field of
the transformer, as was done for the FRESCA facility
transformer system7. Normal conducting Rogowski coils
with a room temperature digital integration are desirable
to simplify the cold parts of the current metering system.
The current meter system has to include a calibration
B. Transformer design specifications
A simplified schematic of a superconducting trans-
former and feedback system is shown in Figure 1. A1
represents a differential voltage amplifier in series with
a voltage controlled power supply, generating a control
voltage Up for the primary coil. A2 represents a dif-
ferential voltage amplifier, and INT represents a digital
integrator. LP and L, represent the inductances of the
primary and secondary coils, respectively, Lea represents
the inductance of the sample, and Lr the inductance of
the Rogowski coils. Mp, and Mr represent the mutual
inductances between the primary and the secondary coils,
and between the secondary cable and the Rogowski coils,
respectively. RP and R, are the total resistances in the
primary and secondary, and Ip and I, are the currents in
the primary and the secondary. U; is the input voltage of
the integrator, and Usec is a voltage proportional to the
secondary current I,. Usec is subtracted from Uset, an
externally generated voltage that is used to control the
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Godeke, A.; Dietderich, D. R.; Joseph, J. M.; Lizarazo, J.; Prestemon, S. O.; Miller, G. et al. A Superconducting transformer system for high current cable testing, article, February 15, 2010; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1012815/m1/2/: accessed July 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.