Power conversion
Alien structures and strange contraptions
20 Jan 2020
There are places in ITER that seem to belong to another world, places full of alien structures and strange contraptions. The feeling—a mixture of awe and puzzlement—is especially acute inside the Magnet Power Converter buildings, two 150-metre-long structures located between the 400 kV electrical switchyard and the Tokamak Complex. In these vast and, for the moment, half-empty spaces, size and complexity challenge every familiar object and notion: wall sockets resemble rocket exhaust nozzles, extension cords are as thick as an overhead crane beam, switches approach the size of a small truck, and electrical cabinets have the complexity of a giant microprocessors.
Thicker than railroad rails, the DC busbars are steel-jacketed aluminium bars actively cooled by a constant flow of pressurized water. There will be no less than 5 kilometres of busbars at ITER, including those that connect electrical equipment and run half the length of each of the Magnet Power Conversion buildings on elevated supports.
The twin Magnet Power Converter buildings are the ITER equivalent of a smartphone or laptop charger: their function is to convert alternative current (AC) into direct current (DC) and to feed it to the Tokamak's magnetic system. In a phone charger, however, the intensity of the current is of the order of one amp; in the Magnet Power Converter buildings it reaches 65,000 amps for the largest of the ITER magnets.
Powering 10,000 tonnes of superconducting magnets requires a massive and extensive industrial infrastructure. Current from the grid must undergo a cascade of transformations before reaching the individual magnet feeders: first, the voltage is stepped down from 400 kV to 66 kV or 22 kV by the three very large transformers of the pulsed power electrical network; then, depending on which magnet the current is destined for, the voltage is transformed from 66 kV or 22 kV to approximately 1 kV by a set of 29 transformers located in concrete bays along the outside walls of the buildings.
The 1 kV AC current reaches the converters inside of the buildings by way of thick metallic bars, called "busbars," that pass through the interior/exterior barrier through large "wall sockets" that look like a rocket's exhaust nozzle ... Each converter is connected to a strange contraption made of stacked coils and cables—a "reactor," which acts like as a filter to smooth the spikes in the DC current's waveform. There are 12 such pairs in one of the building, 17 in the other, each dedicated to a specific circuit, coil, or coil module (in the case of the central solenoid).
Once the DC current has been "cleaned" by the reactor it can be fed to the magnets. However, the current intensity is such that it cannot be transported by conventional cables. Here again busbars (slightly different from the AC busbar that connect the outside transformers to the converters) will do the job.
Thicker than railroad rails, the DC busbars are steel-jacketed aluminium bars insulated in epoxy wrapping and actively cooled by a constant flow of pressurized water. There are no less than 5 kilometres of busbars planned at ITER to connect electrical equipment and running half the length of each building on elevated supports.
In order to reach their assigned clients (toroidal and poloidal field coils, central solenoid modules, correction coils) busbars will travel from the Magnet Power Conversion buildings to the Tokamak Building along twin 50-metre-long bridges. On the last leg of the journey, the busbar network, along with the fast discharge and switching network units, will almost fully occupy two levels of the Diagnostics Building.
The busbar network installation required for First Plasma is now 92 percent complete in the first of the twin conversion buildings, and 72 percent in the other. All 32 converter units should be installed by 2021. The science-fiction décor, half-empty for the moment, will soon be packed to the brim.
Transformer alley
Two transformers procured by Korea and China are shown outside of the Magnet Power Conversion buildings. Depending on which "client" they serve, the transformers step down AC power from 66 kV or 22 kV to 1 kV AC. Passing through the building walls, massive AC busbars connect the transformers to AC/DC converters.
A five-kilometre network
Five kilometres of actively cooled aluminium busbars, procured by Russia, run through the twin Magnet Power Conversion buildings. Busbars carry the high-intensity DC current that is fed to the ITER magnetic system.
Fresh from Russia
Workers unpack a section of DC busbar that has just been delivered from Russia. The contorted shape reflects the complexity of connections in some areas of the buildings. This section is a vertical interface for a correction coil converter. Busbar installation is 92 percent complete in the first Magnet Power Conversion building, and 72 percent complete in the other.
The first of a set of four converter modules for poloidal field coil #6 has just been installed. Connected to an outdoor transformer by way of three AC busbars, the converters are tasked with converting the 1 kV power from AC to DC. There are 12 such converters in one of the buildings, 17 in the other. The converters serving the poloidal field coils are procured by China; all others are procured by Korea.
Smoothing the current
This strange contraption that looks like a giant vintage seismograph is a "reactor." Its function is to create a magnetic field that filters the small spikes in DC waveform before it is fed to the magnetic system. Korea and China are sharing procurement of the 31 reactors, each coupled with a converter or set of converters, that the system requires.
High-intensity current (65,000 amps) is fed to the toroidal field coils through a double set of busbars (right). The simple set of busbars to the left is for a poloidal field coil that requires "only" 55,000 amps. Just like in daily life, DC outlet busbars have a positive and a negative polarity.
Utmost precision
In the densely packed environment of the Magnet Power Conversion buildings, laser measurement ensures that each component—whether busbar, converter or reactor—is set into place with utmost precision.