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How to solve uninterruptible power supply and generator incompatibilty

Incompatibility between a uninterruptible power supply and generator is a common problem that is often only discovered at the worst time - when the power goes out.  What often occurs is an oscillation where the ups will accept generator power briefly, but will then switch to battery when the generator frequency or voltage moves out of acceptable range. 

How do we solve incompatibility problems between the uninterruptible power supply and the generator ?

 There are several issues that are typically to blame.

1) What is the load profile on the generator ?

 If a majority of the loads on the generator are high current on / off type loads that will greatly reduce the ability of the generator to maintain stable power.For example, if 80% of the load is electric motors and air conditioning pumps, the 20% left for operation of the ups system will be garbage.  Whenever the high current loads switch on and off the generator will struggle to maintain voltage and frequency.  In many cases the power provided by the generator will fall out of the acceptable range for the ups and it will switch on battery.  Once the ups load is removed, the generator will catch back up and provide clean power.  Now, the ups thinks the power is good again and puts it's load back on the generator, and the cycle continues until the ups batteries are depleted.

2) Does the generator have an electronic governor ?  

A good electronic governor will make managing shifting loads easier and quicker. It also allows for greater adjustability.

3) Is the generator sized correctly ?

   Most ups manufacturers specify that the generator must be at least 1.5 to 2 times the size of the ups system.  When considering this requirement, you must consider not only the size of the ups, but the maximum possible current usage including charger operation and overload capacity.  

What are some possible solutions ?

To solve incompatibility problems, there are several possible solutions.

1) What adjustments does the ups system have ?  Many ups systems can adjust the acceptable power window either through software, digital control panels, or dip switches.  A true online double conversion ups system can accept deep voltage dips without going to battery and is the preferred solution for most mission critical applications.  One other adjustment that can be set on some ups systems is the ramp in rate.  This allows for the load on the ups to be slowly transferred onto the generator.  These settings can greatly increase the likelihood of compatibility.

2)  Involve your generator maintenance engineer.  Have the engineer verify the loads on the generator and the settings to see it there are changes that could improve compatibility.

3)  If none of the above solve the problem, some other things that may help include installing an input isolation transformer for the ups or removing some non-critical loads from the generator to isolate the problem.

If you are working on a new project or consider replacing some older equipment, we recommend the following links:

Generac Generators:

Eaton UPS Systems:

Orion Power Systems UPS:

If you have questions or need help planning your project, please give us a call at 888-330-2538.


Test comment

08/22/2016 at 07:50:19 PM

Test comment


08/29/2019 at 06:40:18 AM

Give me the points on my issue as follows: There is standby DG power 2 X 1250 KVA power provided in facility where the utility power is 2000 KVA. There are many UPS also in the system. Recently when power outage happens, DG gets on through AMF and while this happens, UV coil of DG incomer burns off. Any reason of connected UPS power??