An unforeseen peril may be lurking behind the scenes of a data centre: When IT devices exhibit electrical input current with a characteristic known as ‘leading power factor’, generators can become unstable and shut down.
Even a data centre with a long history of reliable operation may suddenly develop a problem, as IT devices are added, or in the case of an unusual electrical event.
Neil Rasmussen, an internationally recognised expert on high efficiency and high density data centres, has published a white paper that explains, in simple terms, the problem, why and how it occurs, and how to detect and correct it. Following are highlights of the guidance he provides for data centre personnel and design engineers.
Defining the problem
Backup generators allow data centres to operate for extended periods during a power loss or some types of maintenance activities. Although generators are rugged machines that are tolerant of load steps, overloads, and other conditions, if the leading reactive current becomes too great, a generator can enter an overvoltage situation that causes it to shut down. In fact, generators must operate with leading reactive current of less than 20% of max rated current in order to avoid instability.
The power factor problem with generators is not caused by harmonics or lagging reactive current, but rather by leading reactive current greater than that threshold value.
This situation can happen when the UPS is in bypass mode or absent, causing the generator to directly supply the IT load.
Assessing the risk of instability
A data centre may never experience a problem if an operating UPS is always in the power path between the generator and the IT load. However, when the UPS is bypassed and the generator must supply the leading power factor of the IT load, it causes a fault condition at the generator and forces a shutdown.
While a superficial analysis of a particular design or installation is likely to overstate the risk of shutdown, it’s possible to make an accurate assessment for both existing and planned data centres.
- Remove load from the generator
- Install an inductive load bank
- Install an electronic power factor correction system
- Identify worst offending IT devices and replace them
- Select IT equipment vendors based on power supply capacitance
- Avoid using very large quantities of small servers
- Design for increased power supply utilisation
- Do not allow two power paths on one generator