The ITER toroidal field model coil project

230 A. Ulbricht et al. / Fusion Engineering and Design 73 (2005) 189–327
Fig. 4.31. LCT coil and TFMC safety discharge (I1 and I2 LCT coil
and TFMC winding currents).
the pulse (time t1) with the controlled ramp down of
the current in the TFMC coil at the rate of 70 A/s,
followed almost immediately by the LCT coil with a
ramp down rate of 18 A/s. After 197 s (time t2) a fault
in the 20 kA dump circuit initiated an inverter mode
discharge. This can be seen also in the TFMC current
whose ramp down rate changes suddenly from 70
to 1000 A/s. The ramp rate for the LCT coil remains
unchanged. After 20 s (time t3 = 3147 s) the operator,
having noticed an increase of current in the LCT coil,
induced by TFMC current transient, pressed the emergency
stop thus initiating a safety discharge of the two
coils. If the operator had not pressed the emergency
stop, the overshoot in the current of the LCT coil, which
occurs at time t4 when the current in the TFMC reaches
zero, would have been only 850 A with no problem for
the coil. The following Fig. 4.31 shows a comparison
between computed winding currents and experimental
data during the typical exponential decay of the
safety discharge. In this condition, the power supplies
are disconnected and the coil energy is discharged in the
dumping resistors Rd1 = 125 m� for the LCT coil and
Rd2 = 6.75 m� for TFMC. The current transient in this
case is obtained by simple integration of the circuit Eq.
(4.1) with initial conditions given by the previous run
and zero forcing voltage. Taking into account the resis-
tance of the conventional bus bar system mentioned
before, the resistance R11 in series with the LCT coil
winding in this configuration for the safety discharge
is 125.25 m�. Similarly, summing the resistance of the
dump resistor (Rd2 = 6.75 m�), the resistance of the Al
bus bar and flexible cables mentioned before (0.05 m�)
and the resistance of an additional Cu bus bar system
that connects the dump resistor itself (227 ��) the
resistance R22 of the circuit for the TFMC is equal
to 7.027 m�. The resulting time constants of the two
windings, respectively, of 12.5 s for the LCT coil and
4 s for the TFMC coil, have been validated experimentally
in several safety discharges.
4.5.2. Power losses in steady state, inverter mode
and safety discharges
This paragraph deals with the evaluation of the
power losses due to Joule effect in the bus bar system
and to the eddy currents in the coil passive structures,
induced by time-varying excitation, which represent
almost 90% of the total losses of the coils.
A pulse with flat top current in the LCT coil of 5.7 kA
and in TFMC of 25 kA, referred in Section 6 as “standard
safety discharge”, has been taken as a reference.
The typical ramp up rate used for this type of pulse is
16 A/s for the LCT coil and 70 A/s for the TFMC (see
Fig. 4.32).
In addition to the currents in the LCT coil and the
TFMC windings, Fig. 4.32 shows the following measurements:
heat load on the LCT coil case; heat load
on the TFMC windings and heat load on the TFMC
case. The associated energies, also shown in the figure,
are obtained by signal integration. The heat loads of
the two coil cases, with the exception of a small transient
due to eddy currents induced during the current
rise, remain practically constant. The heat load on the
TFMC windings instead increases by 40 W. This heat
is generated in the TFMC radial plates by the eddy currents
induced by the power supply voltage ripple 1 and
1 A 12-pulse thyristor converter can be represented by a DC controlled
voltage source of amplitude Vd = Vd0 cos(α) with superimposed
a voltage ripple made of sinusoidal contribution of amplitude
√ 2Vn and frequency multiple of 50 Hz (where the harmonic number
n = k × p, with p = 12 and k =1,2,...). At 25 kA flat top the DC voltage
level in the TFMC power circuit is 1.25 V which correspond to
4% of Vd0. Anα-angle close to 90 ◦ is the worse condition from the
point of view of the voltage ripple. For a 12-pulse thyristor converter,