3.7 Grounding Design for EAST Superconducting Tokamak

Transcription

1 3.7 Design for EAST Superconducting Tokamak LIU Zhengzhi Introduction 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. 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 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 external) Electro-Magnetic Compatibility (EMC) in a noisy and complicated electromagnetic environment Two types of grounding The grounding may be divided into two types according to its performance: Functional grounding, such as: Working-grounding, Shielding-grounding, Signalgrounding, 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 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 193

2 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 reference 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); 2) To limit the voltage differences of idle parts to ground and between each other; 3) To avoid any possible closed metallic circular circuit that is Electro-Magnetic coupled with plasma, especially in an air-core Tokamak; 4) To offer a low impedance branch for any possible fault current to limit its damage and limit the possible over-voltage as well; 5) To avoid the interference to low-level electronic system of measuring, diagnostics, control and data acquisition for EMC; 6) To protect the lightning and stroke back voltage in any fault conditions Structure The layout of experimental architectural complex of EAST is shown in Fig.1. The grounding system consists of -electrodes, -grids, -buses, -objects, and Lightning Protection as shown in Fig electrodes and -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 EAST 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 Ω in EAST but not necessary in different sites or cases. 194

3 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 buses and -branches Above the ground the -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 point. 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 subsystem for unwanted ground-loop detection. The simplified scheme of grounding-buses and its layout are shown in Fig. 3 and Fig objects -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 converters. 195

4 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 plat-form. 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 (T d ) in Hefei city area is about 30 d/a (in middle level), and there is no special requirement for lightening protection in EAST. 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, Downconnector system, and Ground-termination system for each individual building. 196

5 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 downconductors of the old building, which along the outer exposed wall should be kept up, but which 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: 197

6 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. 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 198

7 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 Summary 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,, 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 superconducting Tokamak in ASIPP and different fusion devices world wide, the grounding design for EAST 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. 199

8 Fig.1 The layout of EAST experimental architectural complex 200

9 objects objects objects Branches Sub- Terminals G1 G2 G3 Bus B Branches Sub- Terminals G1 G2 G3 Bus B Branches B Sub- Terminals G1 G2 G3 Bus Terminal Terminal Terminal Grid Grid Grid electrods electrods electrods Cryogenic Hall Experimental Hall AH/CD Hall Flashers-and lightning-protection gird Down conducters Fig.2 The structure of common grounding system of EAST 201

10 G3 G2 G1 B ACLV Fig.3 The simplified scheme of grounding-buses Fig.4 The layout of the grounding-buses HT-7U LVPS PS a DS O/E CS b c n o Rg G2 G1 G3 G4(B) PS:Power supply CS:Control system DS:Diganostic system LVPS:Low voltage power supply Fig.5 The illustration of grounding-objects of EAST 202