Help for critical issues

[thanhvn]

Dear Experts !

I have got here an critical issue that messing me around day by day:

We have 6 tetrapak filling machine TBA/19 with GE-Fanuc 90-30 series, CPU 351.

The issues are : Machines will down randomly cause of following

- Grid line is not stable, sometimes power down in such time as flash, very fast, maybe just 1 or 2 second.
- We have test to transfer from grid line to generator. If from grid to generator, every thing is OK, but if from generator to grid, above symptom occur.
- If we just use CB to turn on and off and n machine, nothing happen, no error occured.

This kind of machine have a rung to check if power is loss for less than 4 second, it will recover its status, otherwise it will go back to the first step. The problem is it will take 4 hours to come from the first step to the production step.

The question here is:

- If tranfer from grid to generator, how much time does it take to reach regular voltage. Working voltage is 380 V 3 phase, total load is 450 KVA, generator is 600 KVA, transformer is 750 KVA, 50 HZ.
- How much voltage go down in starting period.


Thanks for your attention.

 

[dV8r]

I do not know the answer to your question, but I would recommend a UPS at each PLC.

 

[NWBeaver]

If I understand correctly, you are saying that the problem occurs when switching from the generator back to the utility grid?

If so, I would have a look at the transfer switch. If it is a closed transition switch, the generator and grid need to be in phase with each other, before the switch transitions.

 

[danw2]

Is the real problem that your PLC's lose power and the PLC doesn't know where to start from or restart from?

Or that the equipment the PLC's drive/run/control lose power and that the loss of power causes restart difficulties?

 

[thanhvn]

To NWBeaver

1. Sorry i have made a typing mistake, problem occur when transfer from grid to generator.

2. We use ATS to transfer between grid and generator. I do not understand " in phase"". ?????

To danw2 :

Actually, if PLC know that power go down, it will recover all parameter before. But here PLC did not realize any lost of power, but some sensor send back the info. that parameters were not enough to continue,so it turn machine back to the starting point.

 

[NWBeaver]

The transfer switch will typically have circuitry that can compare the phase angles of the generator and the grid, and attempts to reconnect the load to the grid at the precise instant that the phase angles are the same. or "in phase" with each other. This is important in the case of a closed transition switch, because for a brief period, the generator and grid are connected in parallel with each other, allowing the load to be transferred back to the grid without interruption.

It sounds like you need a UPS though, as danw2 suggested.

Depending upon the type of generator you have and how much voltage sag is allowed before it starts, it could be as much as 10 seconds before the generator output stabilizes and it picks up the load.

Although you may have a battery back-up to prevent program loss, if the program execution is interrupted the PLC isn't likely to know how long the power was off. The logic may be able to tell how long the machine power was off, but probably not if the PLC power is off too.

 

[djs]

You need to have a UPS connected to the PLC power supply at least. Then the PLC can ride through the power transitions. The second item is someone needs to decide how long of a power interruption is necessary to start re-initization of the machine. Then sense a power failure with on of the inputs to the PLC, and run that through a timer. If the timer times out, then re-start the machine. Else when power is restored, continue as normal.

 

[NWBeaver]

One other comment on this subject, is that this type of problem can appear to be random due to the hold up capability of the PLCs' DC power supply. If the power outage event is brief, it may take the machine down, but the PLC may ride through because the DC power does not drop out instantly, but rather decays over a period of time.

So it could be possible for the AC power to be cycled and not lose the DC power to the PLC.