The ITER toroidal field model coil project

226 A. Ulbricht et al. / Fusion Engineering and Design 73 (2005) 189–327
in series as shown in Fig. 4.26, thus reducing the total
resistance of the short circuit from 327 to 227 ��.
A second problem, which was solved also during
Phase 1, was the shift of current from the 30 kA power
supply to the 50 kA power supply during fast ramp
down. This abnormal behaviour, which was due to
an error in the current sharing controller, would have
been particularly dangerous during the early phase of a
safety discharge at 80 kA when the two power supplies
operate in inverter mode [61].
Like the 30 and 50 kA power supplies, the 20 kA
power supply is a thyristor rectifier, made of two 6pulse
bridge converters (� and Y) of 10 kA each connected
in parallel to yield a 12-pulse converter. Also,
this power supply had a specific problem in the balance
control of the two 10 kA rectifiers, which was
implemented with a �-processor operating at the fixed
sampling rate of 300 Hz. The problem was solved using
an analogue circuit for the current sharing controller
and leaving the dump resistor (125 m�) permanently
connected in parallel with the LCT coil.
The circuit analysis presented in Section 4.5.1 deals
with the normal ramp up and ramp down of the currents
Fig. 4.26. TOSKA power supply system functional diagram.
in the two circuits, with the inverter mode discharge and
with the safety discharge in the configuration for Phase
2. The high voltage discharge of the TFMC coil, performed
with the POLO switching circuit, is described
in detail in Section 9.
4.5.1. Circuit analysis and comparison with
measurement
The analysis presented hereafter, performed with a
computer model, will be limited to the DC side of the
LCT coil and the TFMC power circuits. A plan view
showing the position of the two coils in the vacuum
vessel is given in Fig. 4.27 (dimensions are given in
millimeters). The horizontal axes of the two coils are
not parallel to each other but they form an angle of
4.5 ◦ . The magnetic coupling between the two coils
is not too high (e.g., only 20% between coil windings).
The absence of ferromagnetic materials in close
proximity and the hypothesis that the two coils do not
move justify the use of a linear time-invariant model
for the two coils. The two power circuits, magnetically
coupled through the LCT coil and the TFMC
themselves, are shown in Fig. 4.28. Denoting with: