Derating and Harmonic Contribution

[SilverArc]

Hi Everybody,

I have this thread in two sections. This is related to a mine hoisting application and in some earlier threads I have seeked your suggestions and they were very useful. Now the project is with a company that is involved in mine hoisting applications design. We being the consulting engineers are questioning their design and they are unable to provide satisfactory answers.

Q#1 The application is at 3000 m above sea level. The motor size is 1500 HP. It was advised to us earlier that a VFD for 1500 HP motor is only available in medium voltage specially regenerative and medium volatge drives needs to be derated both for voltage as well as current.

Now, they said we can use a low volatge drive 600 V, VSI,
becuse low voltage drives don't need to be derated for voltage, it is only derated for current.

They could not justify that why VFD needs to be derated for volatge for medium voltage and not for Low voltage.

They meant by derating of voltage means, that a 1000 V VFD will be suupied with 780 V some thing like that.

Could you advise, what is correct here.


Q#2 What would be a differnce in terms of performace issue in comparison of Active front end Current Source VFD and Active front end Voltage Source VFD.

Thanks

 

[itsmoked]

#1 At higher altitude the air is more likely to breakdown electrically allowing corona and arc over. This effects all electrical stuff but the higher the voltage the more serious.

That may be why the medium voltage has more difficulty.

The higher the altitude the less dense the air the less cooling ability. This will effect the current carrying aspects. Which may limit the low voltage.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricuwe]

Regarding Q#2:

I don't know any major supplier favouring the the actvie front-end current source inverter in for low voltage and only one doing so for medium voltage drives.

In your application both topologies should be suitable if properly applied, but I would assume that following the mainstream approach (VSI) is the better way unless there are really good reasons to use a CSI.

 

[SilverArc]

Thanks all for your suggestion.

In regards to Q #1, Because this whole project is on 3000 m altitude, I had to design power system for the installation. So, I am very much familair with this air density issues but it does not answer the question.
Because, how people get around this sitution is by using extra spacing with in live components. For example a 4.16 KV system would use a 8 KV switchgear assembly and it will be powered using a 4.16 KV.
So, I guess that means, to find a logic behind non-derating required for a low volatge VFD I can presume that for a low voltage VFD we can get around situation with out derating for voltage because air density is not bad for low voltage.

Q #2, Rockwell makes Active front Current Source VFD, but for our application we are using Voltage source AFE VFD, the difference current source VFD is that It cannot provide full current at zero speed or full torque at zero speed which is required for Hoisting applications.

This is one of the differences I know.

Any other things, that might apply to this difference.

Thanks for your time.

 

[davidbeach]

It may be that the LV VFD has component spacing driven by considerations other than just the distance for air insulation and that the components are still far enough apart at your altitude that no derating is necessary.

 

[SilverArc]

Thanks all for your suggestions. There is some thing else that came up in regards to this drive issue. I wonder, if henceforth I should beleive these sales guys or even application engineers from these Drive companies. If some thing goes wrong in the end, you cannot hold any thing against them. They just told me that Harmonic contribution from the Active front end VFD will be 12% but earlier on they were stressing that if you buy out Active front end VFD, there will be no harmonic issues.

Now, It is thirty grand more a filter to make the drive package IEEE 519 complaint.

They are again trynig to give a BS(pardon my lanugage)....

They said that... your generator feeding a VFD will have no issues with this much 12-15 % current distortion as as per their experience they have seen Diesel Generators handling close to 30% voltage THD at its terminals in Drilling applications.

I am not sure, how to figure out how much voltage distortion my generator will handle.

This installation is in Africa, and at an isolated location, but there is a mill close to the mine,

What would you say, if the only polluter in my mine is VFD, then it can cause concerns to this Mill facility located a mile away in terms of harmonics. I am not worried about the utility PCC harmonic contribution, but the mill belongs to the same client.

Could you just comment, what could possibly go wrong.

Thanks