Abrupt UPS bypassing

[HRoomba]

Hello,

Thanks in advance for your help, I hope I can gain some insight, as this system is s bit foreign to me.

I have this 30kVA UPS that say takes in a voltage of 226V which rectifies (feeds the batteries) and returns an AC current stabilized to 230V required as it is going to power some very sensitive equipment.
There are some switches (external to the system) that allow the possibility of bypassing the UPS and I was told by the installation crew that under no circumstance should I bypass it manually. In order to do that I should turn to 'bypassing mode' inside the UPS (it internally handles the transition) and then and only then if I wanted to, I could bypass it manually(which would be the same thing)
When I asked about the why, they told me that that way, two different voltages would meet at the exit of the inverter. I mean I get that, but in our case the difference would be of 4V for example, assuming that they are in phase. So I don't see the problematic or I am able to draw what is happening on paper. Could you please shed some light on this? What would it happen if I were bold enough to try it out? Hehehe Thanks!

 

[LittleInch]

I think the issue is more this "assuming that they are in phase".

How can you be sure?

but even 4V is enough to very rapidly exceed the output of the invertor as the grid demand is virtually unlimited.

If you try it and either the phase is out or the voltage is not the same the invertor should instantly trip or blow a fuse.

but you could damage it before those protection devices kick in.

your money - up to you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HRoomba]

Thanks LittleInch,
Goes without saying that wont try anything against what the manufacturer says, my question was purely academic. I was just curious as to how to 'model' this scenario and the end result, from a technical standpoint.
Thanks!
Regards

 

[waross]

First thoughts:
The inverter will do the best that it can to maintain the set voltage.
The impedance of the bypass circuit will be near zero Ohms.
4 Volts will drive a very large current through almost zero Ohms.
In the real world the circulating current will be limited by the inverter's circuits.
What happens next will depend on the inverter circuitry.
The effect may be similar to the effect of a short circuit.
I have installed a couple of those btpass panels.
The ones that I am familiar with are interlocked so that you must interrupt the inverter output before closing the bypass switch.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[HRoomba]

Thank you Bill,

This answer has been very helpful.
In my case, those switches are completely independent, so no possibility of doing that. We'll use a lock on the enclosure to avoid surprises, thank you!
Regards.

 

[waross]

The panels that I installed had the breakers arranged in a horizontal row.
There was a piece of thin metal with slots for the breaker handles installed on the panel cover.
The interlock plate could be slid from side to side.
Step one: The interlock plate all the way to the left. Input breaker on, bypass breaker off. bypass breaker trapped off by the slot in the interlock.
Step two: The input breaker is turned off. Now horizontal slots allow the interlock to be moved one place to the right.

The input breaker is now trapped off and the bypass breaker is free to be turned on.​

A combination of horizontal and vertical slots will force the intended sequence of operation of the isolation breakers.
This is something that you may be able to fabricate and fit to the existing panel.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[HRoomba]

Thank you Bill,
This will come in very handy. Best regards.

 

[thebard3]

Read across the UPS between each phase voltage in and out. A voltage of zero will indicate not only voltage, but phase match as well. I'd be surprised if you don't see a significant voltage that way.

Brad Waybright

The more you know, the more you know you don't know.