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  • Avoiding a sinister failure mode of data centre backup generators

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Sample of a generator capability curve showing acceptable operating conditions for load and power factor.

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.

For an existing data centre, it is necessary to determine the leading out-of-phase current that the generator may be called on to supply, and then to compare that to the current limit. Accurate measurement instrumentation may be unavailable, so it’s best to try to measure the out-of-phase current of the IT load at the UPS output, checking to make sure it’s less than the generator limit. The reactive currents can be measured separately on different parts of the data centre and added.
For data centres still in the planning stage, design choices can greatly reduce or eliminate the risk of instability. For example, if a dual path system is designed so that all the IT supplies on both paths cannot end up connected to and fully loading a generator, the risk of instability is much lower. If UPS systems on both paths are bypassed, the system could be interlocked so that an upstream maintenance cross-tie cannot connect both paths to a single generator.
Solving the problem
Mitigation is only necessary when a risk analysis finds a current or impending situation requiring correction. The following are options for correcting a situation of excessive leading out-of-phase current:
  • 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
To help ensure generator backup systems perform without fail, analyzing potential generator instability should be a part of any effective data centre design and management plan.

Download the white paper Impact of Leading Power Factor on Data Center Generator Systems for advice from a recognized technology expert.
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