This entry in a special report series walks readers through a variety of different elements of power quality management (PQM) for today’s data centers, including ground leakage currents, energy efficiency, the evolution of UPS systems, PDU transformers and more.
IT Equipment Ground Leakage Currents
While most ITE PS meet safety approvals, they do generate a small amount of leakage currents into the chassis ground. The current are relatively small, ranging from less than a milliamp up to xx milliamps, from each ITE PS, depending on the PS. However, in a data center with hundreds or thousands of ITE PS the combined ground current can add up to several amps for a typical 200-300 KVA floor level PDU. This is safe as long as the power distribution system is properly grounded. Most large data center floor level PDUs measure ground conductor current, but unless it reaches an alarm set point, it goes unnoticed. Moreover, these ground currents often contain harmonics, which when combined may become significant in the output side neutral leg of a 208/120 volt 3-phase Wye system, where the neutral may be bonded to ground at the PDU.
PUE and the Quest for Energy Efficiency
The Green Grid introduced Power Usage Effectiveness (PUE) in 2007 and since then the quest for data center energy efficiency has influenced many areas of the data center infrastructure. This includes the increase use of variable frequency drives for cooling systems.
If the UPS software detects a problem with the input power, it transfers the load back to the inverter.
Energy Efficient UPS
This has also driven UPS manufacturers to improve the mainstay of the conditioned power; the classic double-conversion online UPS. While there have been many clear improvements in the last decade in the efficiency of the double-conversion electronics (moving the average from the mid-80% range to the low-90% under low loads), another efficiency scheme was developed; the so-called “energy saver” or “eco-mode”. To reduce double-conversion losses, this “eco-mode” of operation monitors the quality of the power input to the UPS and if it is deemed satisfactory, automatically puts the UPS into static bypass of the inverter condition, via the internal static bypass switch (solid state), which routes utility power directly to the ITE Load. If the UPS software detects a problem with the input power, it transfers the load back to the inverter. UPS vendors claim this is safe, since the transfer occurs within 4-8 milliseconds (which is within the CEBMA curve). While this may be true, only a PQ meter with an event recorder could monitor, verify and document if there are any issues.
In the U.S., for most midsize and larger data centers the UPS is typically a 480V system and the voltage to the ITE is stepped down to 208/120 volts at a floor level PDU with an internal transformer. The type and rating (k-factor) of transformer will determine how well it can handle harmonics generated by load, which are primarily ITE switched-mode power supplies (which as noted earlier, have a harmonic currents and a power factor which can vary with load). The higher the k-factor rating specified in the PDU (typically K1, K4, k13, andK20) the better in can tolerate the harmonic content of the load.
However, the cost of the transformer increases with the k-factor and in some cases a lowest k-factor transformer (K-1) was used because the k-factor was not specified in a price based bid, or for other financial reasons. As a result, the transformer will heat-up (or overheat) as well as saturate, introducing waveform distortion, and therefore is a good candidate for PQ metering.
Grounding practices in data centers are a complex and often misunderstood issue. There are many basic electrical code life-safety specifications and requirements for bonding and grounding the electrical system for any building. Data centers have numerous special requirements since data networks and ITE can be impacted by noise. There are many technical whitepapers as well as IEEE and TIA specifications that addresses and differentiates data center and telecommunications system signal grounding from basic life-safety electrical grounding requirements. Standard practice is to have a separate single point telecomm grounding system which only connects with the main electrical ground at a single point.
Sometimes issues occur during initial construction when power distribution equipment in the data center is incorrectly grounded to building steel or if the neutral conductor is incorrectly bonded to ground in multiple places. This can also occur when new equipment is added to the power distribution system. These conditions can go undetected unless an extensive electrical survey is done which examines and measures the ground and neutral currents and voltage differentials. PQMs which can measure and record these ground and neutral data, as well as capture any harmonics can help identify and diagnose problems caused by these issues.
Download the full report, “Understanding the Importance of Power Quality in the Data Center,” to explore the importance of power quality management in the data center.