One of the most power-quality aware industries is the financial market, as it is one of the most dependent on the uninterruptible operation of computer and communications technology equipment for their business to generate “positive money.”
It is not surprising that many of the members of the 7x24 Exchange (7x24exchange.org) are from financial institutions. Likewise, facilities like the NYSE, NASDAQ, larger brokerage firms, credit card companies and banks,have installed power quality monitoring systems, despite the fact that they have double or triple redundancy power systems. Downtime cost per hour estimates range from $6.5 million for brokerage firms to $2.6 million for credit card processing facilities, according to figures provided by the 7x24 Exchange.
That kind of “negative”money potential can come from a variety of sources, such as the hands (or mouth) of a squirrel that shorted out a feeder line, which NASDAQ experienced several years ago, and even shut down the exchange for a time. With all of the backup power equipment in place, it would seem unlikely that this could occur. However, sometimes the equipment is not wired together correctly, the equipment itself malfunctions or the power loads change within the facility and the equipment isn’t resized accordingly. Sometimes these problems exist from day one, but remain undetected until a series of events causes the system to fail.
At a facility in New York City, the monitoring system reported a large amount of harmonic distortion out of one of the uninterruptible power supply (UPS) units that was not online carrying the load. Closer examination revealed the significant presence of even harmonics, lots of them. In most systems, even harmonics (2, 4, 6, etc.) are not normally present, unless halfwave rectifier circuits are drawing current from the electrical system. Since UPS units are full-wave, 3-phase circuits, the typical current harmonic pattern would be 5, 7, 11, 13, 17, etc.
The presence of the even harmonics at such high levels indicated that something was broken. When the UPS was taken apart and examined, half of the rectifier circuits were damaged, effectively turning the full wave rectifier into a half wave rectifier. Had this UPS been called on to deliver rated power to the data processing equipment, it would have failed and the system would have come crashing down.
Sometimes the double and triple redundancy backup systems can generate a real learning experience when something does go wrong. Currents from multiple sources can make it difficult to figure out what happened. Did the generator not come on in its rated time and properly synchronize with the output of the UPS before the transfer took place? A phase reversal can cause large current flows in some equipment, which could trip overcurrent protection devices, making it look like the generator wasn’t providing voltage when the transfer was attempted.
Other times, the transfer back from backup power to utility-feed power can cause problems. The chart above shows the output from a UPS when switching back to utility power. Whereas the output was well regulated while running on UPS, there is a significant voltage overshoot or swell when the switching backs off UPS, not exactly what the owners of the UPS were expecting to subject their loads to. This could result in damage to downstream equipment.
Often, HVAC equipment uses adjustable speed drives, which are sources of harmonic currents, and can also be victims of power quality phenomena, such as power capacitor switching transients that can cause overcurrent trips. Not having the ability to regulate the temperature in these server racks can cause premature aging or even thermal-induced shutdowns.
Whereas the financial institutions have been at the leading edge of power monitoring and power conditioning equipment for years, they still experience power quality related problems and need to maintain the awareness of how the systems are designed to operate, and how they are really operating. If one credit card processing facility is down, it only takes a second for a customer at a checkout line to pull outa different credit card and send their money elsewhere.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.