Page 369 - Fundamentals of Magnetic Thermonuclear Reactor Design

346 Fundamentals of Magnetic Thermonuclear Reactor Design

Initially, the movable contact element is held in the lower position, pressed
down to the wedge-shaped surfaces of the current leads by compressed air in the
upper chamber. Stationary current leads have a reliable electrical contact. When
a command for opening arrives, the drive opens a pneumatic valve of the upper
chamber, bringing the in-chamber pressure down to the ambient one. Under the
action of excessive pressure in the lower chamber, the movable element moves
to the upper fixed position, forming a gap between the current leads. The cur-
rent that earlier flowed through the movable contact goes to the thyristors. The
commutation process takes less than 5 ms.
The FDS is not designed for current commutation. It has no thyristors and
additional elastic ring. The contacts open, when pressure in the upper chamber
is decreased, and close, when pressure is restored after closing of the pneumatic
valve. An increased air pressure constantly maintained in the lower chamber of
all multiacting switches ensures the electrical strength of the gap between the
current leads after breaking.
The FMS consists of a controlled vacuum spark gap with a response time
of ∼10 µs connected in parallel with a proper mechanical MS. The spark gap
is ignited immediately before the mechanical MS is closed, limiting the voltage
in the intercontact to ∼100 V and thereby essentially reducing the energy of arc
appearing, when the contacts are being closed.
The MS has peculiar features associated with the reverse sequence of con-
tact switching. An additional seal inserted into the pneumatic system keeps
the movable contact element in the upper position, even when pressure in the
upper chamber is high. The closing process is triggered by the capacitor bank
discharge through the drive inductor coil. A resultant discharge of the pulse
magnetic field induces an eddy current in the horizontal disc of the contact ele-
ment, which pushes the inductor down by interacting with its field. This process
is facilitated by compressed air coming from the upper chamber into the gap
between the rubber seal and movable element. It takes about 2 ms to form an
electric contact between the stationary electrodes.
Single-Acting Protective Switches. These switches, forming part of FDU and
SNU, operate in case of a failure of multiacting switches. These switches must
be highly reliable. Assuming that a failure of the main switches should be an
exclusively rare event, these switches make use of destroyable elements that
must be replaced after each operation. Unique switching devices actuated by
explosive detonation (PB and EPMS) developed at the Efremov Institute are
used for back-up protection (Table 11.10).
The use of the drives with explosive charge has made it possible to develop
compact devices, which provide arc suppression using a high pressure produced
by an explosion. A number of specific collateral problems, in addition to the
commutation of direct current in inductive circuits, need to be addressed in
the design of such switches. These include the damping of shock wave, main-
tenance of necessary residual pressure inside the switch and prevention of the
coolant contamination by explosion products.