Generator and PLC

[Emadshaaban1987]

Hi folks
Here we have 250kva/CAT generator used as backup for small factory
The main loads are (20kw air compressor/ 10kw machine driven by PLCs and VFDs (4 machines))

Every thing goes normal while powered by national grid .
But When powered by EDG the PLCs got hung each time the air compressor started
The compressor is connected Y/d and the voltage does not seems to got sag in more than 15v , but the generator change it sound severally
What are the diagnostics

 

[itsmoked]

Y/D starting is very likely to be the problem in this case. It's a horrid method to unleash on a generator that isn't as stiff a supply as the POC.

If the transition timer is off by even a little in the wrong direction it will be causing a very large disturbance on the generator supplied network.

Your comment that the generator sound changes during the start correlates well with the Y/D starting being problematic.

You might consider a soft starter and pitching that Y/D in the dumpster.

A network recorder that monitors the current and voltage during a compressor start would show the relationship.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Put an Uninterruptible Power Supply ahead of the PLC.
Set a longer ramp time or lower the current limit on the VFDs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

The VFDs will tend to shave the peaks from the sine waves.
That, coupled with the generator sub-transient reactance and your AVR may not be reacting to the same wave form as your voltmeter is seeing.
The AVR is probably reacting to the average voltage and scaling it by a 110% form factor to get RMS voltage.
With the VFDs distorting the wave-form, the form factor is no longer 110%.
Back in the days of analogue voltmeters, a distorted waveform and a form factor error was readily evident when comparing the line to line voltage with the line to neutral voltage of a wye:wye transformer bank.
With an analogue voltmeter, the ratio was easily recognized as NOT 1.73.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Emadshaaban1987]

Thank you very much (itsmoked & waross )

Based on (waross) analysis ,I may wondered:
Why generator AVR recognize the issue of non-sinusoild waveform in compressor starting only ? Does this issue you are explaining is related to harmonic generated by VFDs ?

 

[waross]

You may be getting a severe current transient when the compressor is connected in wye and again when the motor transitions to the delta connection.
There will be resulting under-voltage transients that are too short to see on your voltmeter.
Re-reading your post I realize that it is the compressor motor, not the VFDs causing the issue.
An additional possibility is that there is an additional voltage drop in the conductors to the machine, if the machine and compressor are on the same feeder.
If you are reading the voltage on the voltmeter on the gen-set, you won't be seeing this additional voltage drop.
Again,feeding the PLC with either a constant voltage transformer or a UPS should solve the issue.
Constant voltage transformer versus a UPS; both have advantages and disadvantages.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I agree 100% with Bill. A CVS (Constant Voltage Source) transformer is a little better than a UPS for things like this because it doesn’t have batteries that eventually die and have to be replaced or maintained. A UPS will be better if you have any longer power interruptions though.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[GroovyGuy]

Howdy Emad,
Need some additional info;
[ul]
[li]What is the size (in hp) of the VFDs?[/li]
[li]What is the voltage rating of the gen-set?[/li]
[li]What is attempting to start when you hear the "severe" sound from the generator?[/li]
[li]Where does the "severe" sound come from (ie the engine or the generator?)[/li]
[/ul]
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)