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Medium Voltage Ac Drive Tripping Overcurrent At Starting


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#1 venuballa

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Posted 10 April 2007 - 05:00 AM

Dear ALL,
I am facing problem of starting the medium voltage AC Drive 6.6kV, 70Amps, 3-phase, 50Hz System for Centrifugal Compressor application.
No load runs smoothly till 38 Hz and creates overvoltage tripping , which may be due to leading current. Hence, it was decided to run the VFD at full load.
Motor is rated for 700kW, 1475 RPM, 76.5Amps with a multiplying gear ratio of 4.6 coupled to a centrifugal compressor load.
Motor at decoupled condition, acceleration time is: 2 sec and whereas in coupled condition, acceleration time is: 4 sec.
Starting torque of motor iss 80% of full load torque.
GD square of motor is 98 Kgmetersquare whereas that of compressor GD square is: 5 kgmetersquare.

Please comment as to how to start the VFD on load ?

While trying to run at set freq. of 50 Hz, VFD got tripped with OverzCurrent ( at 40Amps only) whiel ramping up to 42Hz with setted acceleration time as 120 sec and deceleration time as 180 sec.

Please note : VFD is programmed for 625kW, 70Amps and parameter called - OverCurrent protect is setted as 70. Overload as 120% and overcurrent as 150%.
Adjusting overcurrent limit of 150% to higher values has no effect and drive trips instantly as soon as start command is given.

Please reply me at very urgent basis : balla.venugopal@gmail.com
Mumbai, India. Cell: 9820420980


#2 mariomaggi

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Posted 10 April 2007 - 09:20 AM

Dear Venu,
please indicate right type and manufacturer of your MV drive.

What about lubrication of gearbox, I hope that lubrication motor is active before starting.

My doubt is that there are some resonances at certain speed, in this case the load will ask more current for a short time but your inverter is not able to supply it.

Let us know
Mario

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#3 venuballa

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Posted 10 April 2007 - 09:57 AM

QUOTE(mariomaggi @ Apr 10 2007, 02:50 PM) View Post

Dear Venu,
please indicate right type and manufacturer of your MV drive.

What about lubrication of gearbox, I hope that lubrication motor is active before starting.

My doubt is that there are some resonances at certain speed, in this case the load will ask more current for a short time but your inverter is not able to supply it.

Let us know
Mario


Dear Mario,
Model/Type of AC Drive : HARSVERT-A06/070 open-loop, analogue-given, remote control
Manufacturer : Beijing Leader & Harvest Technologies Co.Ltd., China [ www.Ld-harvest.com ]
The system comprises Transformer Cabinet, Power Cell Cabinet & Controller Cabinet.

The drive is consisting of a Dry type Transformer takes in 6.6kV input and gives around 635VAC 1-phase input to each of Power Cells connected in Star - Phase A ( 6 Nos. in series), Phase B (6 Nos. in series) & Phase C ( 6 Nos. in series) and produce a voltage of 635 x 6 = 3810 Volts w.r.t. start (neutral point and a phase to phase output voltage of 3810 x 1.732 = 6.6kV of variable voltage variable frequency output.
The power cell conisist of Rectifier, DC bus capacitor bank and Inverter IGBT section - producing a single phase output of 635Volts at rated 50 Hz operating frequency.

The Controller cabinet consists of fiber optic communication cards, power card, control interface card and Signal conditioning cards and PLC with analog I/O expansion modules and UPS.

We have rechecked by applying power from a dummy MOCB breaker and VFD is found working satisfactorily at various freqeuncy references upto 50 Hz.
Also, without VFD, Bypass system of DOL starter is running perfectly OK.
But as soon as load is coupled to gearbox+compressor, the VFD trips instantly with overCurrent even at 0.5 Hz.
We have setted torque boost = 0 to 5 % but no effect on such settings.

Kindly reply as to what could be the problem and what settings need to be altered to make the VFD run smoothly on actual load.

Lubrication of gearbox is okay.

Why the VFD should trip at start command immediately ? No-load uncoupled motor was run smoothly by VFD till 38 Hz without any tripping. Where does the resonance speed come into picture ?


Regards,
Venu


#4 mariomaggi

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Posted 10 April 2007 - 02:52 PM

Dear Venu,
this VSD is not so common, as far as I know.
This application seems to be new, I think. Probably this unit is still under warranty period.
What is the opinion of the chinese manufacturer? I think that no one will buy such big and expensive unit without having the assurance to have the necessary technical support.

Has this unit some alarm on certain power cells?

Don't wait an help from my side, I've not enough info on this unit.
Regards
Mario




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#5 marke

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Posted 10 April 2007 - 07:43 PM

Hello Venu

I agree with Mario, you need to involve the suppliers of the equipment as they should know far more about it than we do.

From you description, is sounds as though there is insufficient flux in the iron of the motor at very low frequencies to develop the torque required to break the load away. This will result in a stall current as the frequency of the drive gets beyond the full load slip of the motor.
This can often be corrected by increasing the voltage boost at start (assuming that it is a V/Hz drive system).

I strongly recommend talking to the suppliers of the equipment rather than playing with settings incase there is an actual problem that you make worse!!

Best regards

#6 jraef

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Posted 11 April 2007 - 05:20 AM

Either something is not set up correctly or is not functioning correctly if it is tripping out that quickly. Looking at the information on that drive and your description, it appears to be a Siemens / Robicon drive, or a Chinese copy of one. If the Chinese "manufacturer" cannot provide support, I would guess it is a copy and maybe they don't really know how it is supposed to work properly to begin with.
"He's not dead, he's just pinin' for the fjords!"

#7 cnavlekar

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Posted 11 April 2007 - 09:40 AM

QUOTE(jraef @ Apr 11 2007, 10:50 AM) View Post

Either something is not set up correctly or is not functioning correctly if it is tripping out that quickly. Looking at the information on that drive and your description, it appears to be a Siemens / Robicon drive, or a Chinese copy of one. If the Chinese "manufacturer" cannot provide support, I would guess it is a copy and maybe they don't really know how it is supposed to work properly to begin with.



I think if you have instrumentation on panel or hook one a fast recorder like fluke 1735 will be able to record actual event and help to resilve the issue may be by involving supplier.

#8 venuballa

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Posted 12 April 2007 - 04:44 AM

Dear ALL,
Thanks for your valued support and feedback.

During major shutdown activities, we were given a chance to hook up the VFD concept to exisitng Gas Booster and were not gettign proper support from the supplier.

Now, I have got a reply that setting of parameter called " Overcurrent protect = must be 680 corresponding to 200% trip limit instead of 70. Max. range is 1024 for this parameter.
There are two more parameters which governt he current limits and starting torque/current demands, that is :
overload = 120% and over current = 150%. These are different parameters than that of a parameter "overcurrent protect".

We have not changed this parameter during our commissioning activity assuming that this is the factory default value.

During our next visiit to end-customer site, we will do the necessary parameter changes and will updatre to you with the results.
Regards,
Venu




#9 marke

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Posted 12 April 2007 - 08:05 PM

Hello Venu

That certainly makes sense, good luck with sorting the problem out.
If the current limit and trips are set too low on that application I would expect to see the problems that you have.

Thank you for keeping us informed.

Best regards,




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