Page 349 - Fundamentals of Magnetic Thermonuclear Reactor Design

326 Fundamentals of Magnetic Thermonuclear Reactor Design

The latter consumed about 60 MJ. The energy consumption is a sum of elec-
tromagnetic energy stored in the coil and Joule losses in the conductor. Motor
generators (MG; usually a flywheel synchronous generator) or commercial
network are used as a primary AC PS.
Although MGs have certain advantages, such as self-sufficiency and low
sensitivity to converter loads, their use is limited by technical and economic
factors associated with their size and cost. The kinetic energy gained by an MG
must be at least twice as high as the energy transferred to load. The Japanese
JT-60 is the largest and, obviously, the last tokamak to use these MGs, with only
two out of six diode converters being connected to the generators, while the
other four to the industrial network.
The common and significant feature of controllable AC/DC converters is
that the output voltage can be controlled in a wide range by varying the switch
time of the next valve relative to the AC voltage phase. At the stage of energis-
ing the coils a higher voltage is required than on the current plateau, when it
only compensates for a voltage drop at an active resistance of the coils. As the
relation between these voltages increases, the energy consumption decreases,
but the installed power of the converter rises. This also aggravates the negative
factors associated with the AC/DC converter operation, such as increased reac-
tive power consumption, higher voltage pulsation at the converter output and
distortions of the sinusoidal voltage in the network caused by higher current
harmonics.
To power a converter-type load with a peak power more than 100 MW,
which is characteristic of large tokamaks, a ramified high-voltage electrical sys-
tem is required with a short-circuit power exceeding the installed power of the
converters by a factor of tens. Besides, in most cases additional measures are to
be taken to compensate for an increase in reactive power and filtration of higher
harmonics.
An example to illustrate this is the T-10 tokamak, which was the first to em-
ploy thyristor converters (TCs) connected to the network (Fig. 11.2).
The TF coil is divided into four sections and is powered by four 12-pulse
converters inserted between the sections. Each converter consists of two rectify-
ing bridges with a maximum output voltage of 1.12 kV and maximum current
of 20 kA. The converters are powered by individual three-winding anode trans-
formers, which have secondary coils with a reciprocal phase shift of 30 degrees
(el) thus providing an equivalent 12-phase rectification. The converters have a
total peak power of 140 MW.
They are connected to the 110 kW Mosenergo transmission line, with a
4.5 GW short-circuit power at the connection point. Although the reactive
power consumption can reach 130 Mvar during a pulse, there is no need to
compensate it due to a low PS operation duty factor. Resonant filters tuned to
the 11th and the 13th harmonics and connected to the secondary side of the
step-down transformers are employed to suppress harmonics generated into
the network.