The Grenoble area electricity supply has a dense aerial network providing customers with electricity. Lengthy power interruptions are rare thanks to the 10 interconnected high voltage aerial transport lines (225 kV and 400 kV). Unfortunately the aerial transport lines located in the mountainous areas are exposed to lightning during storms, which results in voltage and frequency drops of the mains. The statistics over the last 10 years show that 300 lightning strikes per year occur on the high voltage lines surrounding the ESRF. This is four times more frequent than the average for France. Eighty of these events result in a mains drop large enough to trigger an electron beam dump in the storage ring. The biggest drops may induce transients that can damage the electronics boards or leave them in an abnormal state requiring a manual reset.

The sensitivity of the various subsystems driving the accelerators has been observed since commissioning in 1992. At that time, a major storm could result in beam interruptions of several hours, one even resulted in 5 days without beam. To protect against mains drops, a high-quality power supply (HQPS) system was put into operation in 1995. It consisted of 10 rotating electro-mechanical storage systems each coupled to a 800 kW diesel engine via a clutch. The system was operated without problems for seven years. The diesel engines were also run to produce electrical power for the accelerator systems during the 22 so-called EJP days in winter when the cost of electricity is very high. In 2002, a major failure of several of the diesel engines occurred, caused by premature ageing of the bearings supporting the crankshaft. The origin of the problem could not be clearly identified and, although the system continued operation until 2007, it had to be stopped frequently to carry out interventions. The numerous failures provoked premature ageing of a number of components. Therefore, it was decided to replace the HQPS system with a new system without diesel engines, only involves rotating energy accumulators. The new system started operation in July 2008. While several glitches were observed initially, it has protected the accelerator very efficiently, allowing 4 weeks of uninterrupted USM operation in August and September. Due to the absence of diesel engines, the system cannot maintain the full power of 7.8 MW (storage ring with 200 mA and injector in operation) for the accelerator systems for more than 3 seconds. Fortunately, such long interruptions of the mains power are quite rare. During a long power cut, the system is nevertheless designed to switch off the power to the radio frequency system and to the magnets but to maintain power to the critical systems such as computer, vacuum and control, using a single 1 MW diesel engine. This allows a fast restart of the accelerator systems when the power comes back on.

The system is manufactured by the Belgian company KST. It consists of 14 rotating units each providing 800 kVA. A pair of such machines can be seen in the photograph shown in Figure 168. In the centre are alternators rotating at 1500 rpm while the accumulators are located at each extremity, they rotate at a nominal speed of 3000 rpm and are magnetically coupled to the alternator rotating axis via windings. During charging time a special winding couples to the accumulator and converts electrical power to stored energy in the accumulator.

Fig. 168: A pair of rotating units.

Figure 169 presents a schematic of the connection of a pair of rotating machines to the mains and the accelerator system. When a drop occurs on the mains, the system reacts in one of two different ways. If the missing energy involved in the drop is low enough (very large majority of cases), it operates as a network conditioner and maintains a constant supply of voltage and frequency to the accelerator systems. In the case of large amplitude and/or large duration drops (rare events), the system isolates itself from the mains by opening the 20 kV circuit breaker labeled D1 in Figure 169. When isolated, the rotating power stored in the accumulator is transformed into electrical power by the alternator. The accelerator system can run for a few seconds. The system monitors the mains on a permanent basis and tries to reconnect (by closing the D1 circuit breaker) if the mains are back to normal.

Fig. 169: Electrical connection of a pair of rotating machines to the 20 kV electrical grid. The rotating units are operated at 400 V and connected to the 20 kV distribution via a transformer. Seven such pairs of rotating machines operate in parallel with their own transformer and inductor.