Download Poloidal Field Power Supply Systems for the HT

 Poloidal Field Power Supply Systems for the HT-7U
Steady State Superconducting Tokamak
P.Fu, Z.Z.Liu, J.Z.Xu, G.Gao, J.L.Wen, Y.Cao, Z.Q.Song, L.J.Tang, L.S.Wang, and X.Y.Liang
Institute of Plasma Physics, Chinese Academy of Science
Box. 1126, Hefei 230031, China. Email: [email protected]
Abstract—The paper gives a description of the poloidal
As an electrical device, a tokamak is made up of four
field power supplies and the control system of the HT-7U
main electrical loads: (a) the toroidal coils, which establish
superconducting tokamak required to energize the magnetic
the stabilizing toroidal magnetic field; (b) the central
field coils for plasma excitation and confinement. A
soloiend coils, which induce the voltage for gas breakdown
original configuration of AC/DC converter, thyristor DC
and build up, maintain and ohmically heat the plasma; (c)
circuit breaker （ TDCB ） , and power supply control
the plasma equilibrium coils, which control the position
system are introduced in detail.
and the shape of the plasma; and (d) the additional heating
devices, which increase the plasma temperatures and drive
Index Terms— tokamak, power supply, AC/DC converter,
the plasma current. All these loads require alternating-
TDCB, control, and quench protection.
current/direct-current (AC/DC) power conversion and
voltage, current, and power profiles suitable to meet the
tokamak operational requirements.
I. INTRODUCTION
In the HT-7U tokamak, the main loads are the PF
The HT-7U superconducting tokamak is an advanced
(including the central solenoid coils) power supply, the TF
steady state experimental device being built at ASIPP in
power supply, the plasma position control power supply,
China from 1998 to around 2004. The device consists of
the LHCD power supply, the ICRH power supply, the
toroidal field (TF) and poloidal field (PF) superconducting
ECRH power supply, the NBI power supply, and the
coil systems, a vacuum system, a power supply (PS)
auxiliary load, including the cryostat system. With the PF
system, a cryogenic system, a data acquisition and plasma
power supply system working in sequential control mode
control system, a microwave heat system, and a plasma
to minimize the reactive power taken from the grid, the
diagnosis system. Its main parameters are listed in Table 1:
active and reactive power that the HT-7U system requires
are shown in Table 2 and Fig.1.
Table 1：HT-7U tokamak parameters
Major radius R0
1.7m
Plasma current
1.0MA
Minor radius a
0.4m
Toroidal field
3.5T
Elongation Kx
1.6-2
Pulse length
1-1000s
Triangularity dx
0.6-0.8
Volt second
8～10vs
Table 2 Power required by HT-7U equipment
Phase
Magnetization
PF
(MW)
25
TF
(MW)
0.2
Heating
(MW)
0.8
Other
(MW)
4
Sum
(MW)
30
Initiation
-106
0.2
0.8
4
-101
Slow ramp up
-35
0.2
0.8
4
-30
Burn
15
0.2
15.8
4
35
Ramp down
-15
0.2
0.8
4
-10
1
All the TF and PF coils in the HT-7U tokamak are
installed, they are supplied by a double busbar and distant
superconducting. Therefore, compared with a normal
power connection points 14 km away from the 110 kV
tokamak, the power required by the magnets will be
power grid in eastern China. The short-circuit capacity of
considerable smaller, especially to the TF system: its power
each 110 kV interconnecting point is 1980 MVA, the short
decreases from hundreds of megawatts to hundreds of
circuit impedance of each transformer between primary and
kilowatts. In HT-7U, the TF power supply is realized by a
secondary is 10.5%. A schematic of the power distribution
30V/16KA and 12-pulse converter, which takes the electric
system in the HT-7U tokamak is shown in Fig. 2, [1].
power directly from the local grid.
50
Q
40
30
Ip
20
10
P
0
P--Active power (MW)
Q--Reactive power(MVR)
Ip--Plasma current(50kA)
-10
-20
0
5
Fig. 2 The power distribution system of HT-7U tokamak
10
15
TIME( S )
Fig.1 The active and reactive power required by HT-7U tokamak
Basically, the load of the tokamak consists of AC/DC
converters, which are pulsed. Therefore the load will have
The PF coils in the HT-7U require a large amount of
a large harmonic content and high reactive pulse power.
power during the plasma initiation and fast ramp up phase,
These factors will produce a large effect on the power
which lasts for about 1s. Such a high pulsed electric power,
system. Due to excessive repetitive mechanical and thermal
having a peak value of more than 100MW, can not be taken
stresses on the local turbogenerators in the grid, a
directly from the grid because of unacceptable electric
maximum level for the power step is required. The overall
disturbances to the local utility network. To reduce the
power derivative must also be limited, due to the time
disturbance within an allowable limit, two options can be
response capability of the power-frequency regulation
chosen. One is to use an AC flywheel generator and
system. Since there is a maximum voltage excursion
converter to supply the power required during plasma
guaranteed to customers, a maximum ΔV/V is permitted
initiation and ramp up; another is to use a switch and
resistor network to produce the high power required. IPP,
Chinese Academy of Science, now has a 120MVA/400MJ
AC flywheel generator, but considering its operating and
maintenance cost, the commutation switch with resistor
network is preferred to realize the plasma initiation and fast
ramp up, and the converter is used to achieve the slow
plasma ramp up. Therefore a new 81.5 MVA substation is
being built. Two transformers, whose rated continuous
power are respectively 50 MVA and 31.5 MVA, will be
to the pulse, leading to a limitation in the reactive power
swing, while the maximum active power is limited by the
system power capability at the time of the pulse. Finally,
when making substantial use of rectifiers, such as for a
tokamak power supply system, the voltage and current
harmonic level must be minimized. In the HT-7U tokamak
system, a reactive compensation of the 30MVA and a
harmonic filter have been included. And the negative active
power is limited to not more than
–5MW by using a
suitable operation mode of the PF power supply [2].
2
(2) provide a fast discharge for the CS and PF coils in case
II. OUTLINE OF THE HT-7U PF POWER
of quench of the CS, PF or TF coils. (3) protect the PF coils
SUPPLY
against overvoltage and overcurrent due to abnormal or
faulted operation of the power supplies. (4) measure the
The poloidal field system consists of 14 upper and
lower superconducting coils arranged around the plasma in
the HT-7U tokamak. The configuration is shown in Fig. 3.
During the first few years, HT-7U will operate in doublenull mode, the PF power supply system is designed
voltage and current in the PF circuits. (5) provide
grounding and ground leakage current sensing in the PF
circuits. (6) isolate or ground the PF coils and the electrical
equipment of the PF power systems for safe access to the
areas where the both the loads and the power supplies are
according to this operation mode; and in the same time the
power supply can also operate in signal–null operation of
PF9
tokamak. Therefore the PF coils set is reduced to 6 coil
PF11
PF7
pairs by suitable series combination. The equivalent
PF5
inductance parameters are shown in Table 3 and the
PF13
PF3
configuration of the PF system and its power supply is
PF1
shown in Fig.4. Because of the special characteristics of
PF2
the superconducting PF coils, their operating requirements
PF4
are listed in Table 4.
PF14
PF6
PF8
Table 3. The equivalent inductance parameters
of the PF coil pairs
L(mH)
Coil1
Coil 1
69.06
Coil2
Coil3
Coil4
Coil5
Coil6
Coil2
22.58
52.40
Coil3
8.240
19.90
50.90
Coil4
12.30
17.75
32.03
358.3
Coil5
6.020
6.060
6.024
32.99
110.0
Coil 6
3.806
3.644
3.326
16.10
30.82
41.85
Plasma
0.122
0.095
0.065
0.197
0.170
0.119
PF12
PF10
Plasma
installed by personnel, as required, during a maintenance
period.
Fig. 3 The configuration of the PF coils in HT-7U tokamk
0.004
Table 4. The parameters required by PF coils
PF rated current
15 kA
Maximum voltage between terminal and terminal
10000 V
Maximum voltage to ground
10000 V
Delay time of quench protection
1s
2
8
2
Max. I dt in quench
5.110 A s
Maximum PF field ramp rate
7 T/s
The power supply system that supplies the CS and PF
Fig.4 PF coils and their power supply system in HT-7U
coils shall provide the following functions: (1) provide
controlled current in the CS and PF coils for plasma
In addition, provision shall be made to reverse the
initiation and plasma current, shape, and position control.
direction of the current in all the coils. The required current
3
the reference plasma scenario; (b) other PF current
Table 6. Maximum PF voltage required for HT-7U
operation
distributions due to plasma equilibrium conditions different
Phase
from the nominal plasma scenario; (c) the current
rating of the PF circuits has been determined based on (a)
Coil pair
Magnetization
1
(kV)
0.25
2
(kV)
0.27
3
(kV)
0.24
4
(kV)
0.6
5
(kV)
0.15
6
(kV)
0.1
variations due to control; (d) the current variations due to
Current initiation
2.0
2.1
1.9
4.7
1.2
0.6
plasma disruption. In addition, the design of the PF power
Other phase
0.46
0.49
0.42
0.98
0.39
0.18
supply must satisfy the performance requirements given in
Table 7. Maximum PF current and current ramp rate
provided by converter in HT-7U operation
Table 5.
Table5. PF power supply system performance requirements
Parameter
Value
Nominal cycle period
2800 s
PF magnetization
20 s
PF dwell time to keep magnetization
20 s
Plasma initiation
0.06 s
Plasma current ramp up
4s
Maximum plasma flat top
1000 s
Plasma current ramp down
4s
Coil pair
1
2
Rated current (kA)
12
Max. ramp rate (kA/s)
18.2
3
4
5
6
13
12
13
14.2
14.1
20.3
18.6
9.2
4.9
3.5
Sensor
The PF system consists of 6 coil pairs, with each pair
connected to its own power supply system. The required
Fig.5 PF power supply circuit in HT-7U
voltage and current to be applied to the CS and PF coils has
been determined by consideration of three nominal plasma
shape (elongated, round and large volume), along with
III. THE HT-7U AC/DC CONVERTER SYSTEM
voltage required for control of the plasma current, position
and shape. The maximum voltage and current required for
The HT-7U PF magnets are supplied by thyristor
plasma current initiation and control are given in Table 6
AC/DC converters which provide the current necessary to
and Table 7.
produce the scenario and to control the plasma shape and
Each HT-7U PF power supply is composed of
position. This PF converter of the power supply is rated at
rectifier transformers, converters which provide the slow
a total installed power of about 210MVA. The key issues in
ramp up and control of plasma, thyristor DC circuit
the design of the AC/DC conversion system are low cost,
breakert (TDCB) which are used both to initiate the plasma
high availability and reliability, along with reactive power
and as the first action for quench protection, and explosive
reduction.
breakers (EB) that are used as a backup for quench
The 110kV substation is located at about 200m from
protection. A distributed control system (DCS) is used for
the PF converter building. A main transformer, 110 kV/10
the control and measurement of all PF power supply
kV, 50 MVA continuous rating, supplies the additional
system. A typical PF power supply circuit is shown in
heating power supply, the TF power supply, and PF power
Fig.5.
supply, at 10kV intermediate voltage. Inside the PF power
supply building, 12 dry rectifier transformers associated
with the converters and other switches are installed. The
4
typical circuit diagram for the PF power supply converter is
save half of the capacity of the required transformer, and
shown in Fig.6.
also ensure that the load current crosses zero smoothly
Each converter operates in four quadrants with
circulating current between the head and tail sets through
from positive current direction to negative current
direction.
the inductance L1, L2, L3 in series. Converter G1 consists of
The synchronous operation of the two series power
two half bridges G11and G12 in parallel, G11 and G12’s
bridges of each converter permits a twelve-pulse waveform
angle difference is 180 degrees, and they share one phase
on the AC network. At low load voltage, the reactive power
angle controller. As do the converters G2, G3, and G4.
consumption is minimized in the converters by a firing
For G1 and G2, G3 and G4 upper and lower, each set head or
offset between the two series power bridges. In this case,
tail is made of two series identical Graetz bridges with a 30
the twelve pulse reaction can approximately decrease the
degree angle difference between the two transformer
maximum reactive power 50 MVAR to 25 MVR in the PF
outputs, and the 12-pulse output voltage. G1 and G2, G3 and
power supply system of HT-7U [3].
G4, which are respectively paralleled for one tail and head
sets by inductance L1 and L2, make use of the same
IV. DIRECT-CURRENT CIRCUIT BREAKER
transformer secondary. The operating principle is based on
the simultaneous conduction of only two of the four
The function of the DC circuit breaker is to insert, in
bridges selected automatically according to the amplitude
the tokamak coil circuit, discharge resistors, which are used
and the direction of the total load current. An integrated
to absorb the stored magnetic energy from the coils. In
cooling water exchanger extracts the heat energy produced
each PF power supply of HT-7U, DC breakers are used
in the bridges.
during the plasma initiation and current ramp-up, its
parameters are shown in Table 8.
Since this DC circuit breaker system is subjected to
very frequent operation, both the electrical and mechanical
life of the system are of prime importance. In the plasma
initiation phase, each PF circuit must contribute initiating
plasma. Therefore each of the DC breakers in each power
supply must be well synchronized. In the HT-7U power
supply, the required current and voltage parameters of the
DC circuit breaker are not extremely high, therefore a
TDCB associated with resistor network is chosen, which
Fig.6 Simplified diagram of PF converter
can also switch bi-directional current and act as the main
switch for quench protection. The TDCB consists of
When the coil current is in the range from 10% to
100% of the maximum operating current, the main AC/DC
converters in the PF coil circuit operate in two quadrants
mode, in which G1, G3 drive the positive load current and
G2, G4 drive the negative load current. When the coil
current is below the 10% level, the converters operate in
the four-quadrant mode, in which only G11, G31, G22, and
G42 operate. This operation mode of the PF converters can
thyristor, diode, capacitor, resistor and inductor. Its
parameters are given in Table 7,and the circuit is shown in
Fig. 7. The two thyristor sets and two diode sets (Th1, Th2,
D1, and D2) permit bi-directional current and voltage.
The fast discharge circuit is composed of capacitor
C1, L1, th3 and th4, C1 is precharged to 2.4kV. When the
positive current of power supply flows through Th1 and
D1, while Th3 is triggered to discharge the capacitor C1
5
through the series circuit including Th1, D1 and L1. The
A RC snubber circuit is employed to absorb the
discharge produces an artificial current zero of current in
instantaneous overvoltage during TDCB operation. When
Th1, which turns off Th1. The full current of the power
TDCB opens a small current load, it makes the coil current
supply is transferred into the C1 branch. When C1 is
to increase, because of excess energy in capacitor C1.
recharged in the opposite direction, the current is
Therefore D1 and D2 are respectively paralleled with Th1
progressively transferred into the discharge resistor
and Th2. A dynamic and static current distribution in the
connected in parallel with the switch. When the opposite
parallel thyrisor is also ensured by careful layout of the
current flows through Th2 and D2, the commutation is
structure.
realized by triggering Th4.
Quench protection for the superconducting magnets
is of critical importance because of the large amount of
Table 8. The rated parameters of TDCB in HT-7U
stored energy. In each power supply, one TDCS is used as
Rated current
15 kA
the main quenching protection, and two explosive breakers
Rated voltage
2.4 kV
(EB) are as stand-by switches.
Rated switching time
2 ms
Dwell time before repeatable switching
200 ms
Energy dissipated in plasma initiation
8 MJ
Energy dissipated in quench protection
20 MJ
V. PF POWER SUPPLY CONTROL SYSTEM
The local control system of PF power supply
provides
the
routine
control,
real-time
control,
communication of control data, a timing system for precise
control of event initiation, and data acquisition. The PF
power supply system has more than 140 analog signals and
600 digital signals to be measured, supervised and
controlled. A distributive control system (DCS) consisting
of industrial personal computer (IPC) computers has been
used, and all computers are connected as real time Ethernet
network by the QNX real time operation system. In
addition, a fieldbus network is also used on the spot. The
NET:TCP/IP INTERNET
overall structure of the PF power supply control system is
shown in Fig. 8.
Fig.7 the TDCB circuit diagram
During the commutation,
System State
theDisplay
resistor
QNX is a real time operating system, developed by
QNX Software System Limited (QSSL) in Canada, and is
Fault
network
Diagnosis
Waveform
Supervisory
SystemPC’s.
Analysis
widely
used in Control
industrial
Besides its high
produces a high voltage to initiate the plasma discharge,
performance in real time, reliability, and embedded
and dissipate the energy stored in superconducting coil in
characteristics, the unique feature of the QNX real time
quench protection. The resistor is made from stainless steel
Center Contol system
QNX Server
PF Current
controller
operating systemFeedback
is its network
technology. Its FLEET is
NET:FLEET
tube in a tight serpentine pattern to minimize the
an
inductance, and it is cooled by water. Each resistor unit
innovative and feature-rich design turns isolated machines
consists of several modules connected AD,DI&
in parallel or in
subsystem
controller
into aPF1
single
logical supercomputer.
Because FLEET
is
controller
controller
Database
CAN
Controller
ultralight,
high-speed
networking
PF2 subsystem
series. This arrangement allows the resistor value to be
protocol.
Its
PF6 subsystem
NET:CANBUS
adjusted according to the requirement.
AC
components
PF1
components
PF2
components
PF6
components
6
[2] R.Shimada, et all, “JT-60 power supplies”, Fusion
Engineering and Design, pp47-68, May, 1987
built on the message passing architecture of the QNX OS,
[3] Benfatto I. et al., “AC/DC Converters for the ITER
it offers the very high flexibility. Its features are fault-
poloidal system” in Proc. of the 16th SOFE, 1995.
tolerant networking, load-balancing on the fly, efficient
Champaign, IL, USA, pp658-661.
performance, extensible architecture, and transparent
[4] C.Sihler, P.Fu, M.Huart, B.streibl and W.Treutterer
distributed processing. Via the QNX all IPC computers in
“
PF power supply control system can be connected as a
Optimization Of the ASDEX upgrade Power Supply”,
single large computer, and all IPC share the data and signal
21st Symposium on Fusion Technology, Madrid, Spain,
in a high speed and transparent network.
Oct., 2000.
The distance between the power supply building and
its control room is 40m. To decrease the transmitting cable
Paralleling Of two large Flywheel Generation for the
[5] SIMEC GmbH, “Simplorer”, Version 4.2, Chemnitz,
2000.
and disturbance, A network consisted of CANBUS has
been built. Many CAN modules work at the location of the
power supply. All supervised signals, a majority of control
and protection signals, and a majority of analog signals are
transmitted by CAN BUS. Other critical signals required
for high real time performance are transmitted directly by
wires.
All logical signals and A/D conversion of analog
signals measured by the IPC and Fieldbus modules is
transmitted to a database computer [4].
VI. PF power supply R&D progress summary in
HT-7U
Simulation studies by Simplorer software and
laboratory tests of PF power supply in HT-7U have been
completed. They show that the design of the PF power
supply system is feasible and reliable. All of the equipment
of the PF power supply is being fabricated by industry.
Now one set of PF power supply has arrived at the institute
and been installed [5].
VII. Reference
[1] E.Bertolini, et all., “The JET magnet power supplies
and plasma control systems”, Fusion Technology,
vol.11, pp71-119, Jan.1987.
7