SRT3000XLI Output Overload and Bypass Behavior

[mike11]

Hi, we're occasionally getting the following error on the controller of a Smart-UPS On-Line, model SRT3000XLI: "Output Overload. Backup Not Available". Underneath the message where it shows the operating mode, it shows this as "Online". However, the system is being used by a 3rd party in a remote location on our behalf, so it's not clear if the text changes between the error occurring and the operator spotting it. The unit is not network connected to retrieve logs for further diagnosis.

I suspect the cause of the overload is start-up in-rush current draw by the equipment connected to the UPS. From the error message, is it possible to understand what the UPS did when it experienced the overload? Below are a few possibilities I can think of but I can't work out from the user manual which it is for certain.

1. The UPS continued powering the load during the overload and this was achieved by drawing extra current from the UPS battery

2. The UPS continued powering the load during the overload but went into automatic Bypass mode to do this - i.e. it passed the UPS mains input through to the output for the duration of the overload and then went back to the normal Online mode.

3. The UPS stopped powering the load during the overload/outputted a voltage below the lower acceptable limit

Thanks,

Mike

[TonKei]

Hello Mike

Many units are designed with a bit of overhead in regard to overload, when input power is good and the unit is running normally.

But, for many units, the overhead is not available when running of it's battery and inverter.

So, if indeed you are looking at shorter in-rush currents, you could be in that overhead.

That situation would cause the unit to inform you that it is overloaded, but since you are in the overhead it is still in online mode. But, at the same time, the lack of overhead when running of the battery leads to the "Backup not available" part of the message.

If the unit looses input power at the exact moment of your inrush, the backup is not available and the unit will stop powering the load.

I hope the above makes sense, and if that is the case I would ask, that you mark the post as solved.

Best regards

Tonny

[mike11]

Hello Tonny,
Thanks for your explanation. I have some follow up questions:

1. When the scenario "Output Overload. Backup Not Available. Online" occurs, am I right in saying that all the power for the load comes through the following path: mains AC -> UPS DC -> UPS AC -> load? I.e. not: mains AC -> UPS DC + UPS battery DC -> UPS AC -> load. And not bypassing the AC-DC-AC conversion: mains AC -> (UPS bypass) -> load.

2. When the scenario "Output Overload. Backup Not Available. Online" occurs, I expect the voltage at the UPS output supplying the load will fluctuate a bit. In the manual, the spec given is: AC output dynamic voltage regulation: < ± 2% rms of the initial value. Load Step: 10% to/from 90% resistive load. The in-rush current is caused by input capacitors in the load, so it's not purely resistive and given the overload, the change at turn on will be >100%. So I expect the UPS output voltage could vary more than ± 2%. My question is, would a warning message appear if the output voltage changes by more than a certain amount? Is there a way to see the output voltage value when this happens - from the display or a log? The metering view that shows the output voltage is replaced by the warning message when the scenario occurs.

3. Is there a way to be completely sure the unit did or didn't go into Bypass mode at any point when the in-rush current occurred? E.g. if for most of the time of the in-rush the UPS stayed in Online mode but for a short instant it went into Bypass mode. Would the screen update to show that? Or can it be seen on a log?

Thanks,
Mike

[TonKei]

Hi Mike

We actually have a white paper on the different UPS topologies, and if you look at page 7, the double conversion on-line topology is explained in a bit more detail.

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

As to the output power quality when the unit is running in its overhead window: The UPS is designed based on a hierarchy of priorities. The top one of these being that the unit should at all time deliver good quality power to the load attached. So, if the unit measures that the output power goes beyond the acceptable range, it will seize delivering power. in other words even when you are in the "overhead" you should be receiving the same quality of power as when you are in normal running mode. 

You should be able to see the latest 10 times the unit has changed in and out of online mode in the transfer log in the display.

But, for more detailed logging, a network management card is really helpful. It provides a ongoing log that is stored on the card, so it is available even if the card does not have network connection all the time.

Best regards

Tonny