The SLAC National Accelerator Laboratory, Menlo Park, Calif., is home to a 2-mile-long linear accelerator — the longest linear accelerator in the world. This multipurpose laboratory for astrophysics, photon science, accelerator, and particle physics research is operated by Stanford University researchers, but used by scientists from many countries. In fact, a few scientists have been awarded the Nobel Prize for work carried out at SLAC. With that kind of pedigree, it’s no surprise that data storage needs are taken very seriously on this campus. So when the facility was experiencing problems with harmonics a few years ago that affected the servers’ power supplies, Boris Ilinets, P.E., electrical engineer assigned to the data center, knew it was time for action.
“We were having problems with harmonics from the computer load in our data center,” says Ilinets. “Our 100% non-linear load is actually inside the building, but outside the building we have another non-linear load — for example medium voltage rectifiers. Sometimes, we have voltage harmonics originated at remotely located loads that could affect some server rows, but current harmonics from computing loads also create voltage harmonics. That’s why we decided to stay on the safe side, especially considering we had some unpleasant incidents with high level of harmonics. We actually lost some very sensitive disc arrays.”
While Ilinets looked at both passive and active harmonic filters, he opted for active filtering in this instance (10 50A and one 100A StacoSine active harmonic filters from Staco Energy Products Co.) due to the wide range of harmonics present from the servers, disc arrays and switches in the SLAC facility — plus installation was simpler and more cost-efficient. Most importantly, according to Ilinets, they automatically corrected the multiple harmonic orders that were present. Since installation, he’s seen no problems from harmonic distortion. “In a new facility, you can design passive filters on the front end — in an existing building with all equipment in place, however, it is not as easy,” explains Ilinets.
His first step was to perform analysis to determine what exactly he was dealing with. “In my case, I had prevalent 11th, 13th, 23rd, and 25th harmonics” reports Ilinets. “In other instances, you might have more 5th and 7th order harmonics — it depends upon what type of load and power supply you have.”
To understand the approach in this case, it’s important to look at how the system works to correct power quality problems. Harmonics are currents and voltages that have multiplied within an electrical system. Commonplace linear loads become more non-linear due to the electronic and digital devices. This causes the traditional sine wave to change shape, reflecting these many current and voltage distortions. Harmonic currents due to non-linear loads generally flow from the load to the utility source. The presence of harmonics may impede equipment from functioning properly. In the SLAC instance, this was manifested in crashing a pair of disc arrays.
To mitigate the problems associated with harmonics, it’s necessary to filter out at the source the problem harmonic orders, and thus clean up the sine wave. If the specific harmonic order is known (which can be determined through on-site analysis), passive filters can mitigate the problem. Tuning to a specific harmonic frequency (for example the 5th order) allows the other currents to flow into and out of the filter, while essentially eliminating problems from the 5th harmonic order. Other (orders) harmonics (for example the 7th and 11th) can be reduced as well. The key to successful implementation of passive technology is determining which orders are present, and installing filters specifically designed to those harmonic orders. If, however, equipment is installed in the future with a harmonic order that differs from the ones corrected (the 23rd order, for example), the solution will be ineffective for the new harmonic orders present.
All of the filters are installed between the distribution transformers and the panel main breaker. Four of the 50A filters are installed on the first floor of a computer room, while the remaining six 50A filters and the 100A filter are located on the second floor of a different computer room. Two of the 50A filters on the second floor installation are connected in parallel to the same panel, because this was easier than using a single 100A filter. According to Ilinets, installation was not difficult; however, the 100A filter was more challenging, given the larger size, heavier weight, and the fact that the floor was raised. Overall, the most difficult part of the installation was getting approved power outages to connect the filters to the panels, says Ilinets.
StacoSine active harmonic filters use power electronic technology to monitor the non-linear loads and dynamically correct every harmonic order from the 3rd through the 51st. Through an injection and cancellation process, the sine wave is restored and distortion is reduced to less than 5% total harmonic distortion (THD), meeting the stringent IEEE 519 Standard. By injecting a compensating current into the load, the waveform is restored, dramatically reducing distortion to below 5% THD. Power is moved from the AC source to the DC electronic platform, then back to the AC. This occurs at a very rapid rate, allowing for cancellation of the high frequency output current, followed by determining the precise value of the injected load current. The power electronics platform continuously adapts to rapid load conditions, yet maintains a small physical footprint. The 50A filters installed at SLAC are wall mounted to further minimize floor space requirements. In addition to the elimination of harmonics, installation of the filters also allows SLAC to use smaller than K-13 rated transformers because there’s much less heat impact on the equipment and the cables, says Ilinets.
While the SLAC researchers and other scientists who use the SLAC facilities for their varied research projects probably have no idea that the StacoSine filters are on-site, they can rest assured that their data is safe and secure.