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Fig. 1 - Basic Schemes of TFC and PFC QPCs
Fig. 2 - ByPass Switch (BPS) QPC reference circuits and if it implies a certain degree of over-rating, because
main components the thirteen QPCs will be all rated for 25.7 kA and
5 kV.
The JT-60SA superconducting magnet system is The TFC SCB has to interrupt uni-directional cur-
composed of 18 series connected coils (TFC) coils rent while the PFC one has to be bi-directional. For
divided into three units supplied by a single unidi- this latter scheme, Nidec ASI preferred the design
rectional thyristor converter, and 10 independent solution with two IGCTs in antiparallel with respect
Poloidal Field Coils (PFC) supplied by bidirectional to that with a single IGCT and a diode rectifier brid-
thyristor converters. To control the plasma bre- ge, mainly because the resulting total SCB voltage
akdown and flat-top phase, six PFC circuits are drop is lower, thus making faster the current com-
provided with Switching Network Units and four mutation from the BPS to the SCB; moreover, the
with Booster thyristor converters. Each TFC and resulting layout is much simpler.
PFC is provided with a dedicated QPC. Basic schemes of TFC and PFC QPC are represen-
The final scheme of the QPCs represents an advan- ted in figure 1 and hereinafter the main compo-
ced solution based on a Hybrid Circuit Breaker nents are described.
(HCB) composed of a mechanical ByPass Switch
(BPS) for conducting the continuous current, in The Bypass Switch
parallel to a Static Circuit Breaker (SCB) based on
Integrated Gate Commutated Thyristor (IGCT) for The BPS is a mechanical switch with main con-
current interruption. An explosive actuated breaker tacts in parallel to sacrificial contacts. When the
(PyroBreaker, PB) in series to the hybrid CB assu- main contacts open, the current is automatically
res the backup protection. This design allows be- transferred to the sacrificial ones, which are all in
nefiting from the fast current breaking and very low closed position at that time; then, the sacrificial
maintenance requirements of the static switches, contacts open themselves and an arc appears on
besides maintaining the advantage of the much each contact. The distance between the sacrifi-
lower power losses of the mechanical bypass in cial contacts is adjustable to guarantee the proper
normal operation. In addition, this scheme allows current sharing on all the sacrificial contacts; their
replacing can be done without changing the main
the minimization of power contacts.
losses in normal operation The BPS is designed so that the temperature of
and a better exploitation the contact staffs never exceeds a safety value that
of the current interruption guarantees the mechanical and electrical characte-
capability of the static de- ristics of the insulators, even in the worst ambient
vices, if compared with a condition; this can be assured with simple natural
fully static solution. air cooling. The BPS opening and closing are obtai-
Although the ratings of ned by means of a pneumatic actuator which assu-
TFC and PFC QPCs res a total opening time of 350 ms from the com-
present not negligible mand to the reaching of a contact distance which
differences, Nidec ASI allows withstanding a reapplied voltage of 5 kV.
proposed the same com- In normal operation, the current commutation cor-
ponents for both, with the responding to the arc duration depends on the
difference of unidirectio- arc voltage and the impedance of the connections
nal and bidirectional SCB between the BPS and the static CB, which were li-
for TFC and PFC QPC mited as much as possible to minimize the contact
respectively, therefore a erosion. In case of hybrid CB fault, the arc duration
single full-scale prototype is prolonged till the pyrobreaker intervention time;
was manufactured and however, this is much faster than the initial expec-
tested. This choice impro- tation, thus the additional stress is quite limited.
ves the modularity level The BPS is provided with a compressed air tank,
and gives advantages in so that it is possible to perform one opening ope-
terms of maintenance and ration even in case of full or partial loss of pressure
spare management, even from the compressed air supply system (figure 2).
58 Impiantistica Italiana - Settembre-Ottobre 2015
