Bubbles for POPS
At the beginning of May, the new power system of the PS (POPS) got its official send-off. Built around an array of static power converters, POPS is undergoing testing while waiting to be connected to the accelerator.
The team from the Electrical Power Converters Group (TE/EPC) is joined by the Director of Accelerators, the heads of the BE, TE and FI departments, CERN managers and Converteam representatives in a group portrait in front of three of the containers that house the capacitor banks of the PS's new power supply system, POPS.
The inauguration of POPS was well worth popping the champagne corks for. The replacement of the old power system was a long-standing concern for Frédérick Bordry, head of the Technology Department, who was head of the Power Converters Group at the time of the project launch, and who notes wryly, "I got so worried about it that people started referring to it as 'Freddy's nightmare!'" That is because practically the entire accelerator complex of CERN would be paralysed in the event of a power supply failure of the PS, which is a crucial link in the chain. And in addition it will have taken seven long years of development and manufacturing work before the successor to the PS's venerable motor-generator set was installed and ready to go into operation.
The main problem is that you can’t power a pulsed particle accelerator by simply connecting it directly to the electricity grid. That is because the accelerator magnets have to be repeatedly and rapidly energised and de-energised. In the case of the PS, that means producing very high-power—60 megawatt—pulses of electricity, so as to deliver an energy of 14 megajoules to the magnet circuits—and then to reabsorb the same amount of energy, less than two seconds later; and to do so cycle after cycle. As an added challenge, the power supply system must withstand this punishing regime for millions of cycles every year.
The 'pacemaker’ that has performed this task since 1968 is a giant piece of rotating machinery, a motor-generator set with an 80-tonne rotor turning at 1 000 revolutions per minute and performing ten million braking cycles per year. A record to be proud of! Still, 40 years is a long time, and so, inevitably, the PS’s motor-generator has begun to show signs of fatigue. Thus, in 2006 the rotor broke down and had to be replaced, in a feverish race against the clock (see Bulletin 27/2006).
Magid-Michel Saikaly, energy and infrastructure director at Converteam, receives a prize from Steve Myers, Director of Accelerators at CERN, for the development and fabrication of the new electrical power system for the PS, called POPS.
The solution adopted was to construct an enormous array of static power converters and capacitor banks, housed in a new building and six large containers. The power converters are used to convert the AC power supplied by the electricity grid into DC, so that it can be stored in the 60-tonne capacitor banks. The energy stored in the capacitors is transferred to the magnets—and then back again—through a system of 14 power converters arranged in an unusual architecture. "The converters we use are basically of the same type as those used in cruise ships such as the Queen Mary 2," explains Jean-Paul Burnet, group leader for the Electrical Power Converters Group, who has been in charge of managing the project from the start. “What’s completely new is the circuit architecture.” The topology, developed jointly with EPFL, has been patented. It is this architecture that makes it possible to meet the PS’s peak power requirements.
Another difficulty was finding a manufacturer capable of producing such an unusual system. “The design required a lot of development work, with all the industrial risk that entails,” stressed Jean-Paul Burnet. Converteam, a leading manufacturer of power converters, especially for maritime use, took up the challenge. By 2009 the system had been installed and connected to a test line consisting of ten SPS magnets. The results of the testing done since then have validated the solution adopted. Freddy is emerging from his nightmare. But POPS will have to wait for the next extended shutdown of the accelerators, in 2012, to be connected to the PS and then the old rotating machinery can finally be retired.
Did you know?
Finding an efficient way of storing energy is one of the ultimate challenges for electrical engineers. Batteries and capacitors have very limited storage capacity. That is why, among other things, electric cars have failed to enter the mainstream, their bulky batteries providing limited range before a recharge becomes necessary. “To give you an idea, the 14 megajoules of energy needed to supply the PS's magnets is equivalent to the energy of a third of a litre of petrol,” observes Burnet, the Electrical Power Converters Group Leader. So, 60 tonnes of capacitors, distributed in six containers, are needed to store the energy that is contained in… the fuel tank of a small lawnmower! Still, capacitors have improved considerably: in the last fifteen years, their energy density—the amount of energy they can hold per unit volume—has increased fivefold. New systems, more efficient than conventional capacitors have also appeared, such as the aptly named supercapacitors. A tonne of supercapacitors would have sufficed to store the energy needed for the PS. However, their lifetime is counted in thousands of charge/discharge cycles, whereas, as Burnet points out, “we needed a system that could handle 200 million cycles over 20 years of operation." As space is not a major issue on the PS site, a solution using conventional capacitors was therefore the logical choice.
