Electrical networks
Don't mess with AC current
5 Oct 2020
Operating the ITER magnetic system requires the same kind of current that batteries deliver to flashlights, laptop computers and smartphones. This current, which flows in one direction only, is called "direct current" (DC), as opposed to the "alternating current" (AC) that powers most appliances and industrial machines. The behaviour of the AC current can be represented by a sine wave, also called a sinusoid, and it is very important that this wave remain regular. The problem is: the conversion process from AC to DC "pollutes" the AC current and disturbs its distribution. It is therefore imperative that operators, for the sake of their own equipment and for preserving the quality of distribution throughout the grid, implement corrective actions to protect the regularity of the AC current's sine wave.
What can be contained in a medium-size building in a steelworks or another heavy industrial facility requires a one-hectare yard at ITER. Adjacent to the Reactive Power Control Building, the reactive power compensators area accommodates reactors, capacitors, resistors and sensors that aim to smooth the flow of AC current both inside the ITER installation and in the immediate vicinity.
In most heavy industry zones, such as steelworks or shipyards, the equipment for "reducing the harmonic distortion" in the AC current fits into a medium-size building. In ITER, which requires up to 500 MW of DC power, it occupies not only a building—the Reactive Power Control Building—but also the adjacent one-hectare "reactive power compensators area."
What happens within this dense arrangement of thyristor-controlled valves*, reactors, capacitors, resistors and sensors is a complex process aimed at smoothing the flow of AC current both inside the ITER installation and in the immediate vicinity—namely the neighboring CEA-Cadarache research centre and the villages closest to the ITER site.
The process is about compensating the "reactive power" that is generated in the AC current by the presence of "charges," such as the AC/DC converters in the ITER Magnet Power Conversion Buildings.
Contrary to "active power," which lights lamps and operates machines, this ghost-like current does not perform any useful work—it simply moves back and forth within the AC flow.
Any AC/DC conversion installation tends to intensify the reactive power in the AC network and to create harmonic distortions in the sine wave. Because the equipment inside the ITER Magnet Power Conversion Buildings is exceptionally powerful, the distortions, if not compensated, would deeply affect the local distribution of AC current.
It could also be quite costly in terms of electricity bills: transporting the useless reactive power has a cost, which the utility provider would end up billing the customer for. In this perspective, the construction of an efficient reactive power compensation system is a sound investment.
The ITER reactive power compensation and harmonic filtering system is among the largest in the world. In the Reactive Power Control Building, which hosts nine 3-metre-tall thyristors, equipment installation is now 80 percent complete; in the reactive power compensators area, it has passed the 75 percent mark.
Provided by China and manufactured by Rongxin Power Electronic Co. Ltd (RXPE), the equipment is being installed by the Italian Consortium Fincantieri SI and its partner SAET.
*Thyristors are powerful electronic switch controllers.
Made in China
Procured by China, the equipment for the Reactive Power Control Building and the reactive power compensators area is manufactured by Rongxin Power Electronic Co. Ltd (RXPE) and installed by the Italian Consortium Fincantieri SI and its partner SAET under supervision from the system's technical responsible officer David Stachler (first one on the right).
Nine thyristors, 3 metres tall and weighing over two tonnes each, are hosted inside the Reactive Power Control Building on the ITER site. Thyristors are power electronic devices that act like switches. ITER Organization contractors Consortium Fincantieri SI and SAET have now finalized 80% of the installation.
These 8-metre-tall tower-like structures are thyristor-controlled reactors. Directly connected to the thyristors inside the Reactive Power Control Building to the right, their function is to smooth the sine wave of the AC current.
The inside of a thyristor-controlled reactor is almost empty- essentially, it is a simple coil.
Arranged in 3 sets, there are 39 of these "current transformers" in the one-hectare Reactive Power Compensators Area (A39). Despite their name, they can be described as sensors that monitor the variations of the current.