Disable PSU cold redundancy for power from overhead lines without UPS

Server with redundant power supply unit can operate power supply units active/active or sometimes also active/standby (Cold Redundancy). Since power supply units generally have a higher efficiency at higher loads, the active/standby operation can reduce the energy demand of the server. During the direct use of mains power from overhead lines (without USV), however, active/standby mode should be avoided, as the switchover to the cold standby power supply may fail due to the automatic reclosing of the network operator.

Operating mode of redundant server power supplies

Redundant power supplies can either be operated active/active or active/standby. The 80 PLUS initiative stipulates for certain certificates minimum efficiencies. The minimum efficieny of redundant power supplies with 80 PLUS platinum certification is for example:

• 90 % minimum efficieny at 20 % load
• 94 % minimum efficieny at 50 % load

Active/Active

IF only two 1.000 watts power supply units are used active/active for the use of a 500 watts performance, this results in:

• power supply unit 1: 250 watts required -> approx. 278 watts power drawn at 90% efficiency
• power supply unit 2: 250 watts required -> approx. 278 watts power drawn at 90% efficiency

Consumed total performance: 556 watts

Active/Standby - Cold Redundancy

In active/standby mode, the total power consumed is reduced because the active power supply unit is utilized at 50%, which achieves a higher efficiency:

• power supply unit 1: 500 watts required -> approx. ‘’'532 watts'‘’ of power drawn at 94% efficiency

Problems with cold redundancy with automatic restart

In active/standby (cold redundancy) operation, failure tests in which the power supply of the active power supply unit is simply removed usually work reliably.

However, if such a power supply unit is operated directly (without UPS) from the mains, problems can occur when switching to the cold standby power supply unit, e.g. during thunderstorms. These switching problems are caused by the attempts to automatically restart the grid operator. Specifically, we have observed the following case:

• Power supply unit 1 (active) was operated directly from the mains. A power failure occurred here during a thunderstorm. After the first outage, there was a brief power spike again during an attempted automatic restart. As a result, there was no correct switchover to power supply unit 2.
• Power supply unit 2 (cold standby) was operated behind a photovoltaic system with integrated battery storage. Although there was no impairment of the power supply here, the server failed.

Solution

In environments in which a power supply unit is operated directly without a UPS, we recommend, for reliable operation, not using the Cold Redundancy function, even in cases of automatic restarting by the grid operator. If this function is still desired, we recommend to secure both power supply units with different USVs (that are connected to different power circuits)

Example configuration ASUS RS700A-E11-RS12U server

The following example demonstrates the deactivation of the PSU Cold Standby function on an ASUS RS700A-E11-RS12U server.

• 
  Dashboard view of the server administration (PSU Cold Redundancy activated). Click on Settings.
• 
  In the configuration view, click on PSU Redundancy.
• 
  The function is deactivated at the moment.
• 
  Click on the dropdown menu and select Disable and click on Save.
• 
  By clicking on the ? symbol, the help text quoted appears.
• 
  After deactivation, the display in the dashboard shows Cold Redundancy inactive. The power supply units are now operated active/active.
• 
  Click on Sensor in the left gap and scroll down.
• 
  The PSU1 Power In as well as the PSU2 Power In should now show a power consumption similar to this example.

Please verify at the end if the LEDs of both power supply units light up green on the backside of the server.

References

Author: Werner Fischer Werner Fischer, working in the Knowledge Transfer team at Thomas-Krenn, completed his studies of Computer and Media Security at FH Hagenberg in Austria. He is a regular speaker at many conferences like LinuxTag, OSMC, OSDC, LinuxCon, and author for various IT magazines. In his spare time he enjoys playing the piano and training for a good result at the annual Linz marathon relay.

Translator: Alina Ranzinger Alina has been working at Thomas-Krenn.AG since 2024. After her training as multilingual business assistant, she got her job as assistant of the Product Management and is responsible for the translation of texts and for the organisation of the department.