Page 366 - Fundamentals of Magnetic Thermonuclear Reactor Design

Power Supply Systems Chapter | 11 343

11.4 SWITCHING EQUIPMENT
11.4.1 Switching Equipment for Experimental Facilities

Switching equipment allows for generation of the initial voltage at breakdown
and plasma current initiation and for energy discharge from the superconduct-
ing magnets at the quench. In both cases, it is necessary to quickly open a
high-inductance circuit, which transmits direct current of a few tens of kiloam-
peres. This task usually involves upgrading of existing commercial switches or
development of unique purpose-made switching devices [10].
When the main circuit opens, the current from the coil is switched to the
resistor connected in parallel to a CB. The CB is either a high-current mechani-
cal switch, a vacuum switch or a static device based on semiconductor devices,
such as a thyristor with artificial suppression system or an integrated gate-
commutated thyristor (IGCT; Table 11.8). The CB type is chosen on the basis of
specific operational requirements.
In the PS system of the JET inductor the current is switched into the energy
discharge resistor by two mechanical switches with forced air flow at 80 atm
[7]. At an initial current of 80 kA a voltage of 40 kV appears on the coil. The
main disadvantage of this version is the limited lifetime of the switches and
the necessity to maintain them frequently, as the contacts suffer erosion under
the action of electrical arc.
In the Ricerca Formazione Innovazione (RFX) machine, high-current keys
based on VCBs are employed to switch current into the resistors [11]. Compared

TABLE 11.8 Main Characteristics of Switches for Energy Discharge Circuit

Facility JET RFX KSTAR
Circuit breaker type
Mechanical AIR- Vacuum circuit IGCT circuit
Parameters blast circuit breaker breakers breaker
Nominal 80 70 25 (PF1–PF4)
current (kA) 20 (PF5, PF6)
Nominal voltage 40 17.5 3 (PF1–PF4) 6
(kV) (PF5, PF6)
Breaking time (ms) 2 30 100 (operation
time)
10
10
Max current action 10 (3 × 10 in 2.3 × 10 9 —
2
integral (A s) abnormal mode)
Energy absorbed by 200/120 150 —
resistors (MJ)
RFX, Ricerca formazione innovazione.