Uninterruptible power supply load bus synchronization: Yes or no?

Christopher Johnston, PE, National Critical Facilities Chief Engineer, Syska Hennessy Group, Inc.

Most major uninterruptible power supply (UPS) manufacturers provide a Load Bus Synchronization system (LBS) as a standard option at an additional cost. An LBS system is intended to keep the outputs of two or more UPS systems that can supply the same critical load synchronized with each other, and (hopefully!) provide more reliable operation when switching the critical load between the systems is required.

This switching is normally accomplished by a Static Bus Transfer Switch (SBTS) that provides open transition transfer with a 4-20 millisecond break between the sources. At first glance, LBS sounds like it should be included on every project with multiple UPS systems. Not so quick! As with most things in life, the decision is not as simple as advertised and is not without some potential risk.

Your first consideration should be whether LBS really benefits the project. Here are some thoughts:

1. Modern computer equipment (with switch mode power supplies) experiences no difficulties when switched between sources, regardless of whether the sources are synchronized or not. The entire load is supplied at DC (direct current) from the power supplies.

2. AC (alternating current) motors, such as cabinet cooling fans or coolant pumps, experience mechanical stress and can produce large inrush currents when switched between unsynchronized sources. The inrush currents can produce malfunction of the motor and driven load plus unintended tripping of upstream circuit

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3. A transformer downstream of an SBTS, such as in a Power Distribution Unit (PDU), will experience mechanical stress and produce large inrush currents when switched between unsynchronized sources. The inrush currents can produce unintended tripping of upstream circuit breakers. However, recent SBTS technology advances can effectively eliminate this problem.

Your next consideration should be the potential for unintended consequences of LBS:

1. The outputs of some UPS systems sharing a common LBS can become unsynchronized with their bypass sources. A LBS typically designates one UPS system as the master and the rest as slaves. Each slave UPS system's output is forced to remain synchronized to the master UPS system's output. So, in normal operation the output of all of the UPS systems on the LBS are synchronized to the master UPS system's bypass source. If a slave UPS system's bypass source is unsynchronized with the master UPS bypass source, then the slave UPS system's output is unsynchronized with its bypass source.

2. If a slave UPS system must transfer its output to its bypass under unsynchronized conditions, the resulting transfer can be rough, causing the very problems that LBS seeks to avoid. It can also potentially damage the slave UPS system. In fact, with some manufacturer's products, the slave UPS system will not transfer to bypass under unsynchronized conditions, or will make an open transition transfer of such duration that the critical computer load may be dropped.

Syska Hennessy Group believes that Best Practices in this matter are:

1. If modern computer equipment (with switch mode power supplies) is the only UPS system critical load, then do not provide LBS as it is unnecessary. Simplicity is elegance, and reduced parts count increases simplicity.

2. Provide LBS if the UPS system critical load includes AC motors.

3. All SBTS on the primary side of a PDU or other transformer downstream of the UPS systems should be provided with recent technology that eliminates large transformer inrush currents resulting from switching.

What did you think of this data center tip? Send me your feedback and other data center concerns at mstansberry@techtarget.com.

This was first published in June 2008

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