Page 367 - Fundamentals of Magnetic Thermonuclear Reactor Design

344 Fundamentals of Magnetic Thermonuclear Reactor Design

to air breakers, the VCBs are more durable and reliable, but, as a rule, they have
a lower current action integral. Four unified switch-resistor modules are used to
switch 50 kA current at ∼30 kV. Each module has four VCBs connected in two
parallel groups, each with two connected-in-series devices. The series connec-
tion enhances reliability and allows the system to remain operable, in case of a
VCB reignition.
CBs based on semiconductors are preferable in terms of reliability, service-
ability and durability, but their application is limited due to the high cost.
IGCT-based static DC CBs were used in the PS systems of the KSTAR
PF coils for plasma discharge initiation [12]. Their application in this case is
due to their being relatively undemanding in terms of electrical requirements
(20–25 kA, 3–6 kV) and the short time (∼100 ms) it takes for current to flow
through the thyristors. Each power system has seven IGCT modules with a
nominal current of 4 kA. The structural design provides the uniform current
distribution between the modules. The modules are equipped with snubbers to
suppress overvoltages at switching and with reactors to limit the current deriva-
tive when the modules are turned on.
Mechanical CBs connected in parallel with IGCT modules are used for
emergency discharge of 500 MJ electromagnetic energy stored in the KSTAR
superconducting coils. In the normal operation mode, the current flows through
solid contacts of the mechanical CBs [13]. In the emergency mode, the contacts
of the mechanical CB open, allowing the current to pass into the IGCT switch
under the action of arc voltage and then, with the IGCT switch disabled, to the
energy discharge resistor. The switches of the fast energy discharge system are
rated for currents up to 40 kA and a voltage of 3.5–4 kV.

11.4.2 ITER Switching Equipment

Switching devices are used in SNUs to initiate the plasma discharge and FDUs
to protect the superconducting coils. The CBs of these units should withstand
long-continued current up to 45 kA (SNU) and 68 kA (FDU) and should com-
mutate these currents at a voltage up to 10 kV. Besides, the SNUs use MSes with
similar parameters. All ITER switches must be fast operating.
The only way to fulfil these requirements was to develop specialised switch-
es, superior to commercial products by the key parameters. Since 1993, relevant
R&D has been carried out at the Efremov Institute focusing on two core ar-
eas: (1) fast-response multiple-acting switches for SNUs and (2) single-acting
switches with expendable parts intended for back-up operation within FDUs
[14,15]. As a result, two groups of switches differing in the operating principle
and design have been developed.
Multiple-Acting Switches. The main requirement imposed on the switch-
ing devices is the enhanced durability. At a total number of the ITER working
cycles amounting to 30,000 the SNUs should operate at least 5000 times over
3 or 4 years without major maintenance. A three-stage current commutation