THDi Current Harmonic distortion could cause over voltage?

[Freshsurfer]

Hi, after some overvoltages trips on a VFD our client did some measurements on the LV MCC bar and found a high level of current harmonic distortion.

It could cause the overvoltage on this VFD and other components?

Note: No voltage harmonic distortion was found.

Thanks!

 

[RedSnake]

Yes, if it is because the motor starts working as a generator.
If the converter controls something with a large mass such as a flywheel or a fan, the moment of inertia can cause the frequency converter not to brake the load properly and the motor starts to supply power to the converter and it will trip on overvoltage.
There are usually settings to choose for this in the frekvence converter "torque drive" or something like that.

We hade this problem with a excenterpress motor when the stamp was on its way down, it tripped.

More tips here.

https://voltage-disturbance.com/var...fd-overvoltage-fault-a-troubleshooting-guide/

Best regards A.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[waross]

VFD overvoltage trips are caused by overvoltage on the DC bus, not the MCC bus.
It's pretty hard for a drive to alter the grid voltage on the grid bus.
This is evidenced by the lack of voltage harmonic distortion on the MCC bus.
Forget the bus reading for now and look at Anna's (RedSnake) post again.

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

 

[RedSnake]

I have such a poor grasp of all these abbreviations, especially in English.
Most of the machines I work with are German.
I'm just a little better at them.
I just saw overvoltage trip I didn't get the rest obviously. LOL
But that might be enough

Best Regards Anna

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[FacEngrPE]

Redsnake nailed this one.
If changing the settings does not completely eliminate DC Buss Link over voltage trip, add a breaking resistor. The VFD manual will explain sizing and how to connect.

Fred

 

[jraef]

If it is determined to NOT be related to the load (which is the most likely cause), there IS a rare phenomenon called "DC Bus Pump Up" that can cause Over Voltage trips on VFDs as a result of other conditions of the installation, but not the current harmonics that they have observed. Current harmonics are the result of using a VFD in the first place, i.e. you are always going to see that when you have a VFD.

Before taking you down the road of DC Bus Pump Up phenomenon, report back on the load conditions, that is FAR FAR more common of a cause for DC bus Over Voltage trips.


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

 

[Freshsurfer]

Thanks all for your reply!

@RedSnake, I'm totally agree with you if we are talking about a high moment of inertia load. But in this case is a conveyor without inclination, It's totally flat and working without material cause its on commissioning.

So, maybe there was another kind of disturbances on the grid that could performed this over voltage on this MCC bar? or could be possible this THDi generates this failure?

 

[RedSnake]

Is this a disturbances constant or intermittent?
And are the overvoltage tripping in at a specific point?
Starting, breaking, at the end of acceleration?

Best regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[Freshsurfer]

Is just during the starting, we suspect that is during the VFD precharge process, before the motor start to move.

But is possible that a THDi can cause an over voltage?

 

[RedSnake]

English is't my first language all these abbreviations makes it a bit confusing. sigh winky smile
THDI ????
I assume you have tried to increase the allowable value for max overload?
An motor always has a higher starting current at start up than during normal operation after ramping up.
Normally, it is not a problem to take out 150% of the motor's maximum current for short periods if it is allowed to cool down afterwards.

Best regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[Freshsurfer]

Sorry, THDi = is the amount in % of the current harmonics generated by the drive. We are assuming that this condition is happening during the pre-charge process before the motor start to run.

 

[RedSnake]

Well that should only be a problem if the frequency converter has been without incoming voltage.
It can take a while to charge the capacitors and so on, and it should preferably be done in stages.
But if it has been connected to the grid for a long time, it should not be a problem.
Of course, sometimes new products can be broken, it often happens in the first 3 months, otherwise it can take 30 years but it is not common.
May I ask exactly which frequency converter we are talking about here, maybe I can help better.
What is yor parameter for overload or max current ?


BR A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[Freshsurfer]

Is a regular conveyor with a 150% max overload. And yes, its a new installation and we are on commissioning, I believe that maybe it was a grid issue, but our client is associating this concern to the high level of harmonics due the drive pre-charge.

My argumentation for is that a high level of current harmonics could not generate an over voltage, high level of voltage harmonics could be, but is not the case. I`m not sure that my statement is correct.

 

[RedSnake]

Okey. Just so we are on the same page, since english is't my first language.

Is the current harmonics generated or the measurement done on the incoming feed to the drive ?

Frequency converters can cause a lot of disturbances to the electrical network especially the motor cable, if it is not the correct cable or the shielding isn't correct or if it is routed with other electrical cables incorrectly.
But if it is the frequency converter that triggers overvoltage, then this has nothing to do with the current disturbances that the frequency converter can create on the powergrid as waross pointed out.
They may cause problems in other parts of the system but not to the frequency converter it self.

The only way the frequency converter kan trip för overvoltage is if the motor draws too much current or generates current.
U=RxI since R the resistance in the motors and cables is fairly constant it can vary a bit with temperature and rotor / stator angles etc.
But the only thing that can trip the overvoltage is the current going up.

Do this motor have a brake ?

Since I do not know what you already have done or how well you are acquainted with this equipment,
I will certainly point out things that you have already looked up.
But it is also my way of working because of my experience after many years of troubleshooting.
I am by no means a frequency converter specialist.
But my motto is,
Always start from the beginning.
Assume nothing.
If something can bee checked, check it even if it seems improbable, especially if it is easy to do.
Troubleshoot logically
and methodically.

Best regards A


“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[waross]

Are your harmonics different in any way from the harmonics normally generated by a rectifier circuit?
You keep saying "Overvoltage trip".
We generally associate this with a DC bus overvoltage.
Just what is tripping on "Overvoltage"?
What is the grid voltage, the actual voltage as well as the nominal voltage?
What is the voltage rating of the VFD?

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

 

[jraef]

THDI created by the drive would not affect the DC bus of the VFD. You can get off of that train of thought.

What CAN and DOES happen on conveyors is a well know phenomenon commonly referred to as the "rubber band effect". The conveyor is like a giant rubber band and if you try to accelerate it too aggressively, the belt stretches, then snaps back and over speeds the motor, which in turn can become regenerative energy into the VFD and cause it to trip on OV. There are entire companies that exist to overcome this on large overload conveyors where the effect can be so great as to destroy equipment, but on smaller systems it's generally a matter of employing an "S curve" acceleration profile in the VFD (if it has that feature) and/or extending the acceleration time so as to not stretch the belt. Production managers sometimes don't like that, but I will tell them "How much extra time is lost when it trips?"


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

 

[FacEngrPE]

I am going to change my recommendation a bit - Breaking resistors are more expensive than fixing your rubber band energy regeneration problem by changing to an S curve and slowing down the acceleration.
If the drive program change works, and the production manager is unhappy, dynamic breaking resistors can be installed later.

 

[RedSnake]

I hope it solves the problem for you
If breaking resistors are expensiv it is olny a one time cost, cutting down on cykel times can cost a lot in the end
An argument if needed ..

Anna

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein