2.4 TF Coil Power Supplies

**2.4.1 General**

The toroidal field coil power requirement for the ST reactor (1000 MW
net electrical output) is typically 275 MW, 32 MA towards the end of the
centre-post life. The 32 MA exceeds the highest present day industrial
requirement by a factor of more than 50. The currents used in the
defence research industry for rail guns are the closest to those needed
here but in that application the field only has to be maintained for
fractions of a second. In principle homopolar generators could be
operated continuously but they are not believed to be an economic and
viable option when operated in this mode**.** Compulsators,
superconducting magnetic energy storage and capacitor storage systems
are not suitable for continuous supplies. Previous ST reactor studies
\[4,5 and 6\] included a proposal for a power supply to feed the single
turn toroidal field system, with a current of 6MA. The power supply
design was supported by an industrial study contract \[7\]. The design
drew on the technology of two established application areas: the
electrochemical industry, where rectifier circuit ratings up to 150 kA
were well established, and the transmission industry, where thyristor
valves were increasingly being applied as the switching elements on
static VAR compensators. A degree of development work was assumed in
increasing convertor unit ratings to 250 kA. A 32MA power supply scheme
is now proposed based on an extrapolation of the previous design.

**2.4.2 Power Supply Requirements**

It is assumed here that the power to drive the TF coils will be supplied
from the output of the main turbo-alternators at 22 kV. However, the
power could also be taken after it is transformed to the grid voltage
(400 kV), e.g. for start-up.

The resistance of the copper central conductor increases by \~10% during
its lifetime, due to damage and transmutations caused by neutron
irradiation. The power supply output voltage must increase, as the
resistance rises, to maintain the current at its set value. A current
accuracy requirement of ± 0.1 % is assumed but does not have a
significant impact on the power supply design. The power supply ratings
are shown in Table 2. The dimensions of the reactor and the size of any
power supply equipment capable of providing 32 MA continuously are such
that the connections are necessarily long and therefore extremely lossy.
Reduction of the capitalised cost of this power loss is therefore a
prime driver in the design of the DC circuit components.

  ----------------------------------------------------- -----------------
  Number of turns                                       1

  Continuous DC current                                 32 MA

  Resistance (start of life) Centre rod                 0.1644

  Resistance (end of life) Centre rod                   0.1808

  TF Limbs& busbars                                     0.0832

  Total voltage                                         7.92 V - 8.45 V

  Power dissipation in TF coils                         254 -- 270 MW

  Generator stator voltage                              22 kV

  System coupling voltage                               400 kV
  ----------------------------------------------------- -----------------

Table 2 Power Supply Ratings

**2.4.3 Power System**

A distributed system is proposed, with parallel-connected power modules
positioned as close as possible to the torus. The lowest loss power
circuit is used, with the secondary windings of the convertor
transformers connected in the half-wave double star configuration. The
lowest loss semiconductors are also employed, i.e. large 100 mm diameter
thin slice silicon diodes. Phase angle control is provided by thyristors
in the 10 kV primary circuit, where the losses associated with the
series string of high voltage thyristors is comparatively small.

Power is taken either through a single step-down 400/22 kV, 400 MVA
transformer or directly from the generator terminals at 22 kV. At 22 kV
the power is distributed through 16 feeder breakers to 25 MVA step-down
regulator transformers, with on-load tap changers providing a 20% range
of output voltage variation.

Each regulating transformer supplies four thyristor controllers and each
controller, in turn, feeds a pair of 250kA convertor modules. Each
convertor module is assembled with a 125kA rectifier on each side of the
convertor transformer. Each rectifier comprises two 3 phase diode
modules mounted one above the other forming a 6 pulse group. Plus and
minus 15° phase shift windings, on alternative pairs of convertors,
ensure balanced 12 pulse operation from each regulated 2MA power supply.

The outputs from each group of eight convertor modules are connected in
parallel to the torus, by a single pair of radial 2MA conductors.

The ratings of the power system components are summarised in Table 3.

  ----------------------------------- -----------------------------------
  One Supply Transformer              400 MVA, 3 phase

                                      400/22 kV, connected STAR/DELTA

  16 Regulating Transformers          25 MVA, 3 phase

                                      22/10 kV, connected STAR/DELTA

                                      Voltage Output Range 10 kV to 8 kV

  64 Thyristor Controllers            6 MVA, 3 phase throughput

                                      10 kV input

                                      10 kV to 0 kV output at 350 A

                                      3 phase series connected

                                      3 x 1 phase Protecting Reactors

  128 Convertor Transformers          3 MVA, 3 phase

                                      10 kV/10 V phase

                                      Interstar/Double Star with
                                      Interphase Transformer

  128 Rectifiers                      2 x 125 kA, 8.5 V, 2.125 MW

                                      Open Circuit DC Voltage 11.5 V
  ----------------------------------- -----------------------------------

Table 3 Typical Power System Component Ratings

![](media/image1.wmf){width="8.717361111111112in"
height="5.784722222222222in"}

Figure 4

**2.4.4 Control Scheme**

The output current from each pair of modules (500 kA nominal) is
monitored with a DC current transformer whose output is summed with that
from the other 63 to give a total coil current signal. This signal is
used to control the outer loop servo, which in turn sets the references
for all individual inner loop servos associated with each thyristor
controller.

Controller out-of-range and low power factor signals are derived to
initiate appropriate tap changes simultaneously on all transformer
regulators.

In this way the total DC load is maintained balanced on all connected
convertors and phase delay angle excursions are limited to within that
necessary to bridge a tap voltage of the regulating transformers.

**2.4.5 Circuit Topography**

Both the installation and capitalised power loss costs depend strongly
on the physical arrangement and location of the power supply equipment,
owing to the very high DC current, 32 MA, required for the single turn
coil. The DC bus bar circuit is therefore kept as short as the equipment
size will allow, by installing the high current rectifiers on two levels
and coaxial with the torus, as described in section 2.2.2. Each of the
"trapezoidal" blocks shown in this figure contains four 250kA rectifier
modules.

The DC terminals of the 4 rectifier modules in each block are connected
in parallel with one another and with the 4 modules in the block
immediately above/below. The combined output of the 8 modules (2MA) is
then connected, by one of the 16 pairs of radial spoke conductors, to
the magnet assembly.

Access to any 250 kA module, for maintenance or repair, is achieved by
disconnecting the DC and AC terminals and removing the module, as
described in section 2.2.2.

The 10 kV thyristor controllers, each serving a pair of power modules,
and the sixteen HV step-down regulating transformers are positioned
outside the outer biological shield wall.

**2.4.6 Equipment Protection**

2.4.6.1 Over Voltage Protection

Equipment over voltage protection is provided at both the 400 kV and 22
kV voltage levels by metal oxide surge arrestors. The power factor and
harmonic reduction filter banks, which employ a "double tuned" circuit,
have arrestors connected as shown in Fig. 4.

The thyristor controllers are fitted with voltage grading networks to
ensure good surge voltage distribution across all thyristor levels.

2.4.6.2 Over Current Protection

Conventional over current protection is provided by relays operating on
each 22 kV feeder breaker and on each 10 kV convertor group breaker. The
voltage controllers have series connected reactors to limit the maximum
rate of change of current through the thyristors to within the
capability of the devices. They also have electronic protection,
described later, which includes a fast acting servo circuit with current
limiting capability.

The diode convertors are protected against forward overload by a three
tier scheme: (i) current is limited in the controllers by phase angle
control, (ii) firing pulses are suppressed and group breakers are
tripped, and (iii) diode fuses operate.

2.4.6.3 High Fault Current Forces

The principle concern is the possible occurrence of very high currents
and hence forces, during fault conditions, both of which may be
particularly severe in such high current systems. Whilst all convertor
modules are operating normally and contributing equal currents, symmetry
ensures that the forces between adjacent DC bus bars are largely
self-canceling and hence, acceptably low. With any one module switched
out of circuit, however, the symmetrical balance is lost. Under fault
conditions the forces become significantly larger and the bus support
system must be designed for these conditions.

The system could not withstand a DC pole to pole low resistance bus
fault and the mechanical design of both the buses and the coils must be
such that its likelyhood of occurence is negligible. Diode failure is
possible, however, and would produce high forces on conductors within
the equipment. Reverse current in the faulted phase is supplied by each
healthy phase in turn, together with a slow moving DC component from
each of the other convertor modules. The maximum possible value of this
current is extremely high and it must therefore be limited and
interrupted as rapidly as possible to prevent equipment damage. Hence on
the detection of reverse current, the gate firing pulses to the
thyristor controller serving the affected unit are inhibited and the
fault current is interrupted by a fast acting diode fuse with DC
clearance capability.

**2.4.7 Power Supply Performance**

2.4.7.1 Availability and Reliability

The loss of individual power supply components necessitates substantial
reductions in the total current delivered, despite the large number of
parallel-connected power supply circuits, because the reactor physics
requires the currents in the 16 coil return limbs, and hence in the
radial feeder busbars, to be equal. The effects of typical equipment
losses are:

87.5% current available with a single convertor outage

75% current available with a single thyristor controller outage

0% current available with loss of a single radial feeder network, e.g.
regulating transformer outage

In the convertors proposed, the power diodes are rated to permit
continuous full load operation with a single failure per phase, whilst
the power thyristor strings in the controllers can tolerate a single
level failure. Only certain comparatively low cost components, such as
cooling pumps, have been duplicated in the designs. Unless the
requirement for current symmetry can be relaxed, it may be necessary to
either (i) accept operation at reduced current for short periods of time
(e.g. 87.5% of full current if one convertor is out of service) or (ii)
under-rate or duplicate major components, which will make the convertors
larger and hence increase the overall system losses.

**2.4.8 Equipment Losses**

The electrical efficiency of the complete power system, up to the busbar
terminals of the magnet, will be much lower than is achieved for the
majority of convertor applications, owing to the very low DC voltages
required for the single turn coil system. Scaling of the losses from the
previous low current study gives the power losses shown in table 4. This
gives a relatively poor efficiency of about 66% at full load current (32
MA) and shows that the convertor module losses form the main power loss
which must be reduced to give a more efficient power supply system.

+--------------------------------------------------+-------------------+
|                                                  | *Losses (MW)*     |
+--------------------------------------------------+-------------------+
| Convertor module losses (128)                    | 122               |
|                                                  |                   |
| Thyristor controller losses (64)                 | 5.4               |
|                                                  |                   |
| Regulating transformer losses (16)               | 3.6               |
|                                                  |                   |
| Total losses\*                                   | 131               |
|                                                  |                   |
| Total TF coil power                              | 254-270           |
|                                                  |                   |
| TF Coils AC Input (22 kV AC)                     | 385-401           |
|                                                  |                   |
| Efficiency                                       | 66-67 %           |
+--------------------------------------------------+-------------------+

Table 4 Electrical Losses (MW) and Efficiency

\* Excluding 400/22 kV transformers

Some degree of improvement could be achieved by using more active
material in the DC bus feeder; in the transformers, especially the high
current convertor transformers, and to a lesser extent in the diode
convertors and the thyristor regulators.

**2.4.9 Power Factor**

Without correction equipment, the power factor at the 22 kV bus will be
low, particularly during the early "life" of the column. This is
inherent for convertors working at high current and low voltage. The
power factor becomes worse when the voltage is reduced by thyristor
phase angle delay.

Partly for power factor improvement, the 22/10 kV step-down transformers
are provided with a 20% range of voltage control. This enables the
thyristor controllers to be operated at low delay angles for most normal
reactor operating conditions. Any required fast load correction or
transient can be accommodated by phase control to provide system
stability; any long term or follow-up adjustment can be made by the
regulating transformers to improve the power factor.

The two power factor correction banks are dimensioned to correct the
power factor to 0.95, when the thyristor controllers are operating with
a delay angle of 10°.

Each power factor bank is "identical" and is designed to also act as an
AC filter, to attenuate the highest amplitude current harmonics produced
by the convertor system. The worst case situation occurs when one
6-pulse convertor group is switched out of circuit.

The filters are designed as double tuned circuits, able to
simultaneously attenuate the 5, 7, 11 and 13th system current harmonics
to values normally permitted, for supply systems with a high voltage
point of common coupling, such as the 400 kV assumed here.

**2.4.10 Cost Considerations**

2.4.10.1 Component Capital Costs

The highest cost segment of the power supply system is necessarily that
which is directly associated with the high current output circuit.

At these low voltage levels, the component costs for the high current
circuits are virtually linearly proportional to the required DC current
and almost independent of the voltage. Altering the number and ratings
of the convertor modules, whilst fixing the total current, has a
negligible effect on the total cost.

The number of modules in the arrangement proposed is a compromise to
avoid too large an increase in current from the maximum found in
existing convertors, whilst attempting to minimize the equipment space
requirement and hence the length of the DC bus circuit.

2.4.10.2 DC Busbar Costs

Any possible reduction in the length of the DC busbar circuit saves
money simultaneously in three areas: the capital cost of the bus
structures; the capital cost of the building shell, and the capitalised
cost of the power losses. Any possible method of shortening the busbar
system therefore warrants careful consideration.

In the scheme proposed the DC equipment has been made as small as is
possible, using "normal" oil immersed convertor transformers. Any
further significant reduction could probably only be achieved by using
transformers with more intensively cooled windings.

2.4.10.3 Transformer Considerations

The transformer windings could certainly be shrunk by using hollow
conductors cooled by injected water. Such an approach is currently used
at similar DC voltages for a number of industrial applications, albeit
at much lower unit current ratings. Examples from the fusion field
include DC inductors in the JET poloidal and toroidal field
flywheel-generator-convertor circuits. However, the winding of larger
section hollow copper conductors on relatively small diameters would
require the development of new winding machines.

There would also be the need for the development of a coil design which
did not allow appreciable increases in the electrical losses due to
fundamental and harmonic induced eddy current effects. Such losses are
traditionally reduced in oil immersed designs by the use of separately
insulated multi-filament, continuously transposed conductor ropes.

Five limb transformer cores are proposed with the smallest core
dimension, ie. the core lamination stack depth, lying on the radial
plane. The transformer cores are positioned in their tanks in the
conventional manner for large units, resting on their bottom yokes. The
alternative possibility sometimes employed on water cooled industrial
units, with the core resting on one outer limb, does not appear to give
any dimensional advantage for this application.

2.4.10.4 Cost Optimisation

A capitalised loss value of £1000/kW was assumed, for determining the
quantities of active materials in the components \[7\]. Any significant
change in this capitalised value would have principally affected the
design of the convertor transformers. For example, an increase to
£2000/kW would have encouraged lower loss designs and consequently an
increase of copper in the windings, particularly the secondary windings,
and an increase in core steel. Still higher capitalisation would have
also encouraged loss savings in the convertor, with the use of more
derated silicon diodes and more silver elements in the diode fuses.
Lower capitalisation on the other hand would have lead to a decrease in
active materials, principally in the convertor transformers.