Case Study:Switching Network Units(20 kA, 5 kV)
for the JT-60SA in Naka, Japan
R1

OCEM Power Electronics’ four switching
network units (SNU) will help induce andsustain the loop voltage needed forplasma initiation in the JT-60SA.
The JT-60SA, or Japan Torus 60 Super
Advanced, is a collaboration between
the Japanese and European fusion
research communities intended tocomplement
theInternational
Thermonuclear Experimental Reactor(ITER) under construction in Cadarache,France. Managed by the Japan Atomic
Energy Agency at the Naka Fusion
Institute, and governed by the “BroaderApproach” agreement between Europeand Japan, the project will be an
upgrade to the JT-60 fusion reactor
built in 1985.
While the JT-60SA and ITER are
independent
research
initiatives,
JT-60SA was designed to support ITER’s
research aims by developing andtesting practical and reliable plasma
control schemes suitable for future
power plants. When JT-60SA goes live in2019, major research ﬁelds will include
operation
regimedevelopment,
stability and control, transport andconﬁnement,
high-energy
particle
behavior, plasma material interaction,fusion engineering, and theoretical

INPUT CONNECTION BARSSCB INPUT/OUTPUT CONNECTION CABLES
BY-PASS SWITCHSIGNALS
TO/FROM LCC
R2AUX SUPPLIES + SIGNALS
TO/FROM LCCLCC

AUX SUPPLIES + SIGNALS
TO/FROM JAEA + SIGNAL
TO/FROM SNUs

STATIC CIRCUITBREAKER
WATER INTERFACE SCB

MAKING SWITCH

SNU ﬁnal conﬁguration

models and simulation codes.
PowerElectronics
was
OCEM
commissioned
toproducefour
Switching Network Units, or SNUs, toinduce and sustain the loop voltage
needed for plasma initiation in the
JT-60SA.
The Switching Network Unit
The JT-60SA employs the mostadvanced fusion reactor available
today, called a “tokamak” — a Russianword for a torus, or doughnut-shaped,magnetic chamber. The plasma is
conﬁned in the chamber using amagnetic ﬁeld generated by a toroidal
ﬁeld coil and heated until fusion occurs.
A central solenoid coil provides theinductive ﬂux to ramp up the plasma
current and contribute to plasma
shaping.

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SNU - Switching Network Units for
the JT-60SA

The central solenoid coil in the JT-60SA
is divided into four superconducting
modules called CS 1-4. Each of the four
superconducting modules is connected
to an independent power supply circuitthat includes a four-quadrant AC/DC
converter and a SNU.
The SNUs are used to produce the highvoltage required for the plasma
initiation by exploiting the magnetic
energy stored in the poloidal coils. Thisis accomplished by inserting resistors
in the CS coils’ circuits. When the main
switch opens it puts the resistors inseries to the coils, and as the current
drops the voltage rises to create
plasma.
Since the JT-60SA central solenoid isdivided into four segments, the switch
time needs to be extremely short toguarantee synchronization of the coils’
rise in voltage. To achieve this, OCEM
developed an innovative hybrid
electromechanical-staticswitch
conﬁguration. The coil current has anominal value of up to ± 20 kA and issustainedbybothanelectromechanical by-pass switch and asolid-state static circuit breaker switch.
Due to the novelty of the system andA detail of Local Control Cubicle

the critical role the SNUs play in plasma
initiation, the ﬁrst SNU was subjected
to rigorous testing before production
began on the additional three systems.
These tests proved highly successful.
SNU Production and TestingOCEM was contracted by the Italian
National Agency for New Technologies,
Energy and Sustainable Development(ENEA) in October 2012 to produce theSNU systems. It completed theapproved designs in July 2013, and theﬁrst full-scale, fully working prototype
was then manufactured in 2013-14. The
components of each SNU are enclosed
insix
cubiclescontaining
anelectromechanical by-pass switch witha super-capacitator auxiliary powersystem,groundingswitchandselectors; an electronic static circuit
breaker with water cooling system; anelectronic make switch with water
Switching Network Unit

www.ocem.euSNU - Switching Network Units for
the JT-60SA

cooling system; and two breakdown
resistor banks. A single Local Control
Cubicle controls the four SNUs.
In September 2014, the system wastested inside a working Tokamak, theFrascati Tokamak Upgrade (FTU) atENEA. OCEM and ENEA technicians
assembled all the components with alayout identical to the ﬁnal JT-60SA
conﬁguration, emulated the remotecontrol system and synchronization
method, and successfully tested theprototype SNU at nominal current andvoltage.
Primary tests were repeated 10 times
at reduced current and voltage values,
with the operation of a single electronic
static circuit breaker, to validate thecorrect operation and timing sequenceof the current commutation from theelectromechanical by-pass switch to
the electronic static circuit breaker, andthen from the electronic static circuit
breaker to the ﬁrst resistor. The current
was then increased to the nominal
value of 20 kA to simulate the SNU’scomplete operation sequence. Tentests were conducted at 20 kA and 5 kV,with a repetition time shorter than theJT-60SA repetition time (1800 seconds)
and without the water cooling availablein the ﬁnal installation. Diﬀerent faults
were simulated to conﬁrm the SNU
would take the proper protective actionin each case.

Special Features
Each SNU must interrupt a direct
current of up to 20 kA in less than 1
millisecond in order to produce a
voltage of up to 5 kV. Since thecentral solenoid is segmented into
four sections, it is critical that theintervention occur simultaneously
in all four SNUs, meaning themaximum main switch opening andclosing times must be less than 1 mseach. During on-site testing, thehighly critical switch oﬀ-time wasless than 0.1 ms, and the switch
on-time was about 1 ms.

The testing conﬁrmed that the SNU
performed to all speciﬁcations requiredby the JT-60SA, including an oﬀ-switch
time signiﬁcantly below the 1 ms
threshold. Manufacturing of theremaining three SNUs was completed
in 2015, and on-site tests weresuccessfully conducted that fall.

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Thyristor Stack, a component of SNU

SNU - Switching Network Units for
the JT-60SA

Components and Speciﬁcations
SNU Performance Characteristics

JT-60SA Speciﬁcations

Experimental

Nominal Current+/- 20 kA+/- 20 kA

SCB current interruption capability (safety margin)

25 kA

25 kA (3125 A per branch)

Rated voltage (across SNU terminals)

5 kV

5 kV

Maximum voltage (including overshoot and trabsients)

7 kV

5.2 kV

Maximum SNU operating time (including accuracy)

1.5 ms80 us

Maximum MS closing time (including accuracy)

2 ms

140 usMaximum SNU closing time (including accuracy)

1.5 ms1.25 ms

Worst-case SNU operating time in non-nominal conditions

Not considered

700 usMS closing time in worst-case non-nominal conditions

Not considered

40 us

SNU closing time in worst-case non-nominal conditions

Not considered

1 ms

Maximum BPS opening time (only for reference)

15 ms

15 ms

Maximun pulse lenght

250 s> 250 sMinumum repetition (cool-down) time

1800 s< 1800 s

Voltage drop in SNU closed status>10 V allowed< 10 mV

Range of R1 resistance adjustable by selectors

0.25 ÷ 3.75 Ω

0.25 ÷ 3.75 Ω

SNU resistance after successful breakdown

Up to 22 mΩ

Up to 22 mΩ

Tolerance of each breakdown resistors (20°C)+/- 2%

Better than +/- 1.5%

Maximum variation of resistors with temperature

10%

8%

Total energy that can be dissipated in breakdown
resistors R1 and R2

90 MJ>> 90 MJ

Temperature of air to environment

100°C<< 60°C

Maximum current imbalance among parallel branches

20%

11%

Minimum cycles without compressed air2

Unlimited

Self-protection time in any case of fault or alarm

Not speciﬁed< 150 ms

About Us
For more than 70 years, OCEM PowerElectronics has designed, manufactured
and installed power systems for premier
research laboratories around the world. Its
customized power systems are enablingadvances in the ﬁelds of plasma physics,
particle physics and medical research, anddriving advanced industries such astransportation and food processing.

OCEM Power Electronics
a division of Energy Technology Srl
Via della Solidarietà, 2/1
Località Crespellano
40056 Bologna - Italy+39 051 6656611
power@ocem.eu

www.ocem.eu