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6.3.5 Grounding Design for HT-7U Super-conductive Tokamak
Grounding system is a relevant part of the layout of Tokamak. It is important and
indispensable for the system reliability and safety on the one hand, but on the other hand
it is a difficult subject.
“Grounding” has not been the precise technology yet at present and grounding design
for Tokamak is not perfect up to now. Tokamak is too complicated to be grounded in a
conventional way or according to some existent standards directly because it depends on
too many different sub-systems and too many factors. Further more, ground-fault caused
by lightening or accident is usually a small probability event, the experimental study and
statistics roles are hard to be applied in Tokamak.
1) Grounding objectives
Nevertheless it is obvious that the main objectives of grounding design in Tokamak
here is to construct a common grounding plat-form for all sub-system to realize:
 Reliable operation
 Safety for both of equipment and human body
 Limiting damage in cases of any accident against over-current, over-voltage and
Electro-Magnetic Impulse (in normal operation and fault condition; internal and
 Electro-Magnetic Compatibility (EMC) in a noisy and complicated electromagnetic
2) Two types of grounding
The “grounding” may be divided into two types according to its performance:
 Functional grounding, such as: Working-grounding, Shielding-grounding,
Signal-grounding, and Logic-grounding. The Signal-grounding and Logic-grounding
may be combined together as Low-level DC potential reference grounding.
 Protective grounding, such as: Safety-grounding, Lightning-protective-grounding,
Static-protective grounding, and Erosion-protective grounding.
3) Basis of grounding design
The design work of grounding system is based on:
 The general principle of grounding
 The engineering practice and progress of grounding
 The structure and characteristic of Tokamak
 The present statue and history of grounding in site of ASIPP
 The design and operational experience in different fusion devices (not so much been
published unfortunately)
 The existent standards and regulations (domestic GB China National Standard and
international IEC Standard) about grounding in Architecture, Electrical Engineering
and Electronics, Communication and Information Technology, and etc. are taken as
Technical Requirements
According to the principle and objectives of the grounding design as mentioned above
its technical requirements are as the followings:
1) To keep the operation voltage of living parts under a certain allowed value
(depending on the rating voltage and insulation level).
To limit the voltage differences of idle parts to ground and between each other.
To avoid any possible closed metallic circular circuit that is Electro-Magnetic
coupled with plasma, especially in an air-core Tokamak.
To offer a low impedance branch for any possible fault current to limit its damage
and limit the possible over-voltage as well.
To avoid the interference to low-level electronic system of measuring, diagnostics,
control and data acquisition for EMC.
To protect the lightning and stroke back voltage in any fault conditions.
The layout of experimental architectural complex of HT-7U is shown in Fig. 1. The
grounding system consists of Grounding-electrodes, Grounding-grids, Grounding-buses,
Grounding-objects, and Lightning Protection as shown in Fig.2.
1) Grounding-electrodes and Grounding-grids
In each individual (isolated) building of the experimental architectural complex, the
grounding-electrodes consist of the nature grounding-electrodes, (steel-bar in reinforced
concrete), and the supplementary grounding-electrodes underground.
The grounding-grid is formed by the steel- belt (50mm5mm) in mesh of about
4m6m and in depth of 0.6m to 1.0 m underground.
A copper stub above the ground is connected to the grounding-grid in a proper
position to form the grounding terminal.
Among the experimental architectural complex, all of the grounding-grids in each
individual building shall be connected at each grounding terminal by bus bar to form the
general grounding-grid. The general grounding-grid of HT-7U consists of nearly all the
experimental buildings in site except HV power station.
All of the connections must be welded all together to make the grounding resistance
as small as possible which is less than 0.5  in HT-7U but not necessary in different sites
or cases. .
It should be treated as an individual building if the new buildings and the old
buildings are closely adjacent to each other in site.
In the design process the touch potential difference and the step potential difference,
the thermal and dynamic stability for each sub-system, each equipment and connection
must be checked up at worst fault conditions.
Beside that the tours-base reinforcement (even stainless steel-bar used) should be
electrically insulated with grounding net.
2) Grounding-buses and Grounding-branches
Above the ground the Grounding-buses are connected from the grounding terminal
and distributed along the building. They can be divided into four types according to its
functions and purpose:
 Working grounding ------G1,
 Shielding grounding-------G2,
 Low-level DC potential reference grounding-------G3,
 Safety grounding, Static-protective grounding and Equal-potential (equipotential)
Bounding--------G4 (B).
And the G1, G2, G3 should be open-circuited and insulated to ground and between
each other. They should be connected to grounding terminal by one point, and only one
On the contrary, the G4 should be close-circuited and re-connected to grounding grid
and the steel-bar in reinforced concrete in multi-point (about in each 5 meter distant), but
not one point.
There are sub-grounding terminals on the grounding-buses in proper positions. And
the grounding-branches are connected to the sub-grounding terminals in a certain area for
grounding connections.
There must be one re-connectable break at least in each grounding-branch at one
sub-system for unwanted ground-loop detection.
The simplified scheme of grounding-buses and its layout are shown in Fig. 3 and
3) Grounding-objects
Grounding-objects are grouped as four types as well.
The working grounding G1:
All electrical circuits should be connected with G1 in only one point directly or
through a current limiting resistor. The magnets and its AC/DC converters are usually
connected with G1 in the central point (or equivalent central point) to reduce the terminal
potential difference. This potential difference should be within the ratings of magnets and
All of the structure components of Tokamak device, such as bases, supports, thermal
shields, cryostats, vacuum vessels, coil cases, etc. should be connected with G1 with one
point through a current limiting resistor as well. The one point grounding is useful for
ground loop detection and to limit ground current in fault conditions. And the current
limiting resistor should be chosen as the potential within a safe value in fault conditions.
Taking into account of the insulation level of sensors and electronic instruments attached
on the structure, this safe potential may be limited as 500V to 1000V.
Shielding grounding G2:
All the screens of electronic devices and signal cables should be connected with G2 in
only one point at transmitting end.
Low-level grounding G3:
It offers a relatively stable DC reference potential for all the electronics of measuring,
diagnostics, control and data acquisition and should be connected with only one point at
receiving end. This reference potential may vary somewhat during Tokamak operation
but need not to be worried if it is used as a reference potential in a common grounding
Safety grounding and Equal-potential Bounding G4 (B):
The metallic enclosure of all equipment, structure, pipes and plat-forms should be
connected with G4.
The TN-S system is applied in the three-phase AC low voltage mains (AC LV). The
neutral and safety grounding in TN-S is separated. The neutral point of the low voltage
transformer should be connected directly with G4 in only one point to limit the grounding
voltage lower than 50 V (AC rms.) or less in case of grounding fault.
All grounding objects should be connected to the grounding buses or ground branches
by grounding conductors as shown in Fig.5. The grounding conductors should be as short
and as firm as possible.
Lightening Protection
The design of lightening protection depends on the local annual average Thunder-day
and the project requirement.
The annual average thunder-day (Td) in Hefei-city area is about 30 d/a (in middle
level), and there is no special requirement for lightening protection in HT-7U. Taking the
importance of the project into consideration, the second-class architecture has been
thought as the design criterion of lightening protection.
The lightening protection system usually consists of Air-termination system,
Down-connector system, and Ground-termination system for each individual building.
The lightening protection system is one of the parts of the common-grounding system
described as above. The ground-termination system is just the grounding-electrodes and
grounding-grids of the same building.
The air-termination system consists of the flashers and the lightning protection grids
(404 steel-belt in 8m12m mesh) on the roof.
The main steel-bars of the pillars of building are used as the down-connector system
except the old buildings where the extra down-conductors were laid before. The
down-conductors of the old building whose along the outer exposed wall should be kept
up, but whose along the inner wall of the building should be cut off.
The main steel-bars of the reinforced concrete including each floor should be
multi-point welded to form a sparse Faraday-cage.
All the electronic equipment should be kept off the outer wall and the
down-connector system more than 1m to 2m.
The Surge Protection Devices (SPD), such as gaps and non-linear resistors should be
applied for the protection of strike-back over-voltage.
Electro-Magnetic Compatibility (EMC)
The EMC problems in fusion technology are becoming more difficult and important
with the increase of size and complexity of the fusion machines itself. It is all because of
the huge amounts of pulsed power required to initiate and heat plasma, the fast variation
of main field, the stray fields produced in large volumes, the high voltage or high current
modulator for additional heating and fast plasma control schemes.
In general understanding, the Electro-Magnetic Interference (EMI) problems can be
divided as three parts:
 Interference sources, such as pulse power, switching power supply, RF plant, dirty
mains, some diagnostics, and etc.
 Interference victims, such as measurement & diagnostics, data acquisition &
handling, control & protection, and etc.
 The unwanted and often unexpected coupling paths, including electromagnetic
coupling, electrostatic coupling, magnetic coupling, conductive coupling, and dirty
mains with spikes, dips, surges, and harmonics.
So that the principle against EMI would be:
 Decrease of the intensity of interference sources,
 Increase of the capability of victims against interference,
Cut-off of the coupling paths.
And some measures are well known as the following:
 Isolation
The Electro-Magnetic Isolation and Optic-Electro Isolation are common used;
Separate cable conduit should be designed for power cables and signal cables;
The multi-wire power cables for AC power and the coaxial cable or coaxial bus-bar
for DC power are preferable;
The clean mains with isolated transformer and filter and surge suppressers are
necessary for electronic equipment.
 Shielding
All electronic equipment should be full screened;
All signal cables should be full screened too, The twisted pair cables with individual
screen and over-all screen are preferable;
The metallic conduit or copper tube for signal cables is necessary usually.
The full screen control cubicles or control room is effective.
 Grounding
It is one of the essential parts of EMC just as described above.
 Protection
It is also one of the essential parts of EMC as well. The Filter and SPD with high
rating capability and fast response are necessary and especially for protection against
Lightning Electromagnetic Impulse (LEP).
Remarks in construction
The construction of grounding system should be carried out according to the
blueprints and requirement of technical design. The unmentioned items on the technical
files are treated on conventional regulations.
Besides, some remarks in construction should be emphasized:
All the installations or paths should be isolated if it might lead in or lead out the
potential difference between the general grounding grid and any grounding grid outside.
Any grounding grid out of the general grounding grid must be isolated totally and
SPD should be set up for bonding of potential difference in any cases.
All the metallic structures and pipes under grounding grid should be bonded.
All the metallic structures, cables and pipes that pass through the interface of
grounding grid should be bonded to the bonding bar of G4 (B) at the interface.
The shields and armors of all power cables should be grounded to G4 (B) in both of
two terminals.
All cables should be laid in cable conduits or pipes with screens. All the metallic
structures and pipes in the cable conduits should be bonded too.
The neutral line (N) and the safety grounding Line (G4) should be separated and
insulated, and must not mixed up.
According to the engineering experience and recent progress of grounding practice in
civil engineering, electrical engineering and electronics, communication and information
technology, and etc. the so called BDSGP (Bounding, Dividing, Shielding, Grounding,
and Protection) system design has been proven as the most feasible and successful
approach to realize EMC in a large and complicated environment.
Based on the structure and characteristic of Tokamak, the design and operational
practice of grounding system in the existent HT-7 superconduvtive Tokamak in ASIPP
and different fusion devices world wide, the grounding design for HT-7U has been
carried out and put into the system design of BDSGP.
A common grounding plat-form has been designed and constructed for all
sub-systems. Each sub-system should be responsible for the grounding connections itself
according to the grounding design above. Particular attention should be paid to the
analysis and protection design in fault conditions concerned with grounding in each
sub-system, especially in power supplies, magnets, and AH/CD systems.
As mentioned before, the grounding design for Tokamak has not been yet perfect up
to now. There are still many arguments to be studied and discussed. There are still many
problems to be solved or improved. It needs still great efforts.
Fig. 1 The layout of HT-7U experimental architectural complex
Fig. 2 The structure of common grounding system of HT-7U
Grounding electrods
Experimental Hall
Grounding electrods
Cryogenic Hall
G2 G3
G2 G3
G2 G3
AH/CD Hall
Grounding electrods
Down conducters
lightning-protection gird
Fig. 3 The simplified scheme of grounding-buses
Fig. 4 The layout of the grounding-buses
PS:Power supply
CS:Control system
DS:Diganostic system
LVPS:Low voltage power supply
Fig. 5 The illustration of grounding-objects of HT-7U
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