UPS and Backup Generator Compatibility

[Moto]

Hi All,

I am doing research about compatibility between backup generatro and UPS, for data center, and

I would like to know how sensitive backup generator frequency deviation is for data center UPS.

Below is my understanding, but I am neither generator expert nor UPS expert. So, correct me if I am wrong.

Say, mains fail, and backup generator start up, then suppies power to facility load.

UPS fill the gap from mains failure to generator power supply.

After that, UPS receives incoming power generated by backup generator.

When, large load is instantly applied/or rejected, backup generator frequency could be out of limit that UPS is requesting to incoming power. In that case, UPS will be on backup mode (uses its battery).

However, backup generator frequency should come back to stable shortly, thanks to governor.

I think UPS will go back to normal mode(sync with generator again) from backup mode.

So, even if such frequency deviation occurs several times, it will not affect entire operation. 

Any input/comment/advice will be appreciated.

Have a good day !!

Moto

[Teken]

All parameters relating to power quality is essential when speaking about a UPS and Generator.

All of the enterprise class UPS have some adjustment for frequency deviation. I suggest you contact your local MSP / APC provider to outline the requirements and hardware in place.

They may be able to offer you a demo unit to help validate if the UPS under review is compatible with the generator.

The only true test and validation is having the exact UPS System in place and see how it behaves and performs.

NOTE: It’s imperative to have a maintenance bypass in place. Do not rely on any internal UPS bypass if available!

Questions Ask . . . 👍 

[Moto]

Many thanks for your comment !

So, you suggest we should not rely on battery mode.

Any specific reason ?

[Teken]

I’m not sure where you got that from?!? So I’ll try to offer a little more insight and clarification.

Generator: In almost all cases an existing generator is in place within a building. More often than not this old and existing generator is no longer operating like new and outputs dirty power.

As such you’ll quickly find out upon the monthly generator test or during a grid down event that the *New UPS* does not operate and perform as expected.

The most common behaviours you’ll see is the unit will not switch back to online. Thus draining the internal battery cartridge which quickly degrades the battery health and service life.

Other scenarios are the unit will switch back and forth between online & battery. This causes the internal relays to arc and finally weld together causing a hard fault.

As noted here this is why an external maintenance bypass must be incorporated to allow and ensure the UPS can be bypassed for service or out of band operational behaviour.

You can read in this forum alone how many times a customer started a thread about the internal bypass failing or not working as expected! 🤦‍♂️

As such there was no (Quick Way) or method to route power to the attached loads without doing so manually which is no small feat in a data center or where there isn’t sufficient PDU outlets / cable length etc.

As noted also the usual steps are to define the requirements, environment, and submit a RFP. 

Typically a power quality audit / analysis is conducted and performed to determine the sites true power health. Armed with this critical information the end user will know the power is fine or not.

If not, than this gives the client the time to address those issues prior to the next step. Which would be to request a demo unit to help test, monitor, and validate the target (UPS) device operates as expected within the generator environment that exists.

Almost every APC Enterprise UPS offers the ability to define the sensitivity, upper & lower input transfer voltage, frequency deviation, and some the slew rate.

Other models allow the user to define the bypass threshold, and in older models Power Quality etc.

Regardless of all the above only a live test will allow you and the company any material facts and the data to make an informed decision prior to spending thousands of dollars on any UPS.

Questions Ask . . . 👍

[Moto]

Sorry, I misinterpreted internal maintenance operation. It is not battery operation.

Your further explanation is so helpful for my understanding !

I really appreciate your help.

👍

[Teken · Accepted solution]

You may also review this APC White Paper to understand the different types of UPS systems:

https://download.schneider-electric.com/files?p_Doc_Ref=SPD_SADE-5TNM3Y_EN

You’ll probably find during your research that a Online / Double Conversion UPS is the recommended topology when speaking about mating with a generator.

As stated earlier only a live test and short / long term evaluation will affirm what specific model & topology will work for your specific environment and conditions.

Regardless of all the above an external maintenance bypass must be part of the solution to allow you and the company the ability to perform maintenance and switch over to generator power when necessary.

If any of the replies provided the answers you needed. Please do mark those specific replies as a solution so it may help the next person asking the same! 👍

[Moto]

Thanks !

I also had a chance to talk with UPS user. According to him, voltage variation is more criticial compared with frequency variation. Anyway, I think that I could collect necessary information here.