The ITER toroidal field model coil project

232 A. Ulbricht et al. / Fusion Engineering and Design 73 (2005) 189–327
Table 4.7
Computed energy losses (grouped) for two inverter mode and a standard
safety discharge
Run �Ei, i =1,...,5 E1 + E2 E3 + E4 + E5
[kJ] [%] [kJ] [%] [kJ] [%]
IMD1 1490 3.46 1360 3.16 130.5 0.30
IMD2 4337 10.08 4300 10.00 37.2 0.08
SSD 42964 99.88 42310 98.36 654.3 1.52
�Ei, total energy losses; E1 + E2, energy losses outside the cryostat;
E3 + E4 + E5, energy losses inside the cryostat.
for the safety discharge is 654.3 kJ and this is in very
good agreement with the experimental values of the
integrated heat load shown in Fig. 4.33. Also for the
safety discharge, the biggest contribution to the energy
losses inside the vacuum vessel is generated in the
TFMC radial plates (i.e., 265.5 kJ) and it is transferred
to the conductor by the heat diffusion process [63]
described in Section 6.
The first two columns of Table 4.7 show the total
energy losses for the three cases—in kJ and in % of the
total energy stored in the coil. At first sight, the inverter
mode discharge (IMD1), where the total energy losses
are at the minimum, appears as the most convenient.
But, if one analyses separately the losses in the external
bus bar system and the losses in the cryostat, the simultaneous
ramping down strategy (IMD2) appears to be
more convenient for the cryogenic plant (i.e., energy
losses of 0.08% of the total stored magnetic energy in
the coil against 0.30% of IMD1). The safety discharge
is the more severe for the operation of the cryogenic
plant (i.e., total energy losses 1.52%), but has to be
retained as backup protection in case of failure of the
inverter mode discharge.
An additional advantage of simultaneous ramping
down of the currents in magnetically coupled circuits
has to do also with the behaviour of the thyristor converter
during switching off of the firing pulses. Usually
this action is performed by the thyristor firing pulses
controller when the current in the circuit is below a
minimum threshold. When the thyristor firing pulses
are turned off, induced currents from magnetically
coupled circuit, although of modest amplitude, might
inhibit the last active pair of thyristor to switch off.
In this case the AC input voltage (i.e., 50 V peak at
50 Hz) is applied directly to the load present at the DC
side.
Fig. 4.34. TFMC AC/DC converter failure to switch off during an
inverter mode discharge in combined operation of LCT coil and
TFMC.
The fault just described occurred a few times
during the power system set up Phase 2 (see
Fig. 4.34). As mentioned in Section 4.3.2.1, during
an inverter mode discharge (i.e., dI1/dt = −18 A/s and
dI2/dt = −1000 A/s), with initial current flows in the
LCT coil at 2.3 kA and in TFMC at 10 kA, the integrated
heat load for the TFMC windings, shown in
Fig. 4.10, reached about 600 kJ which is of the same
order of magnitude as the energy losses of the standard
safety discharge. The computed current in the TFMC
windings with a sinusoidal voltage source of 50 V and
50 Hz, as shown in Fig. 4.35, is 123 A (rms) which produces,
for 2 min, 5.13 kW (rms) of losses in the radial
plates due to the eddy currents. The quench detectors,
based on compensated voltages, and integration over
0.5 s period, did not intervene. Unfortunately, due to
the tight time schedule of the tests, it was not possible
to change the control software of the 30 kA and 50 kA
power supplies to allow a simultaneous ramp down during
an inverter mode discharge and therefore most of
the protective actions had to be implemented with the
safety discharge.