Generator Sizing 6

[SilverArc]

Hi Everybody..

I have to size a generator for a mine hoist. The mine hoist is 600 HP that means 600 * 746= 447600 W= 447 .6 KW
For some moments of time, the load even becomes 847 HP during acceleration period of the hoist. But this period is in msecs.

The calculations involved come with a -200 HP power fed back to the motor during hoist going down the shaft.

Now what I understood that, if I size a emergency generator for this application, It should be capable of acting as a sink of -200 HP power which means:

200 * 746= 149 KW now I have to convert this in to KVAR.

The motor is fed with a VFD, some of you who have mine hoist experience can comment on this; Power factor with VFD assumed: .8(Please advise, if this PF is not real when it comes to hoisting application)
149 KW= 186 KVA=1111.75 KVAR

Could you advise me, normally to what percent of its size a generator can act as a sink and source of VARS ?

I heard that to sort out this regenerative power issue with mine hoists: Load banks are used. COuld you advise me, where I can find some information about this.

I have given a best shot of my engineering judgement... An advise from some of you who are experts in this area might give me a better direction.

Thanks

 

[SilverArc]

Any comments Please ?

 

[waross]

You've left us speachless.
A drive that requires a combination of wye and delta transformers would be designed as such and may not be capable of operating without the transformers.
On paper it will be easy to feed generator power to 6 pulses of the drive and back feed one of the transformers, tie the transformer primaries together and pull the other 6 pulses off of the second transformer.
In practice the protection and switching would be problematic to the point that I doubt that anyone would recommend doing it.
It may be best to go with a drive that can accept a three phase 480 volt supply.
If you go with a 12 pulse drive, the transformers should be part of the drive package so that the input to the drive package may be 480 volts either from the main 4160:480 volt transformer or the 480 volt generator.
I would check with the vendors for their recommendations concerning reactors. It would be well also to make the generator supplier aware of the duty to which the generator will be put. They may have some requirements for drive service.
There are a couple of drive experts (skogsgurra, jraef and marke if he's following this) here who will know better than I how a generator handles the unique current waveforms inherent with drives.
respectfully

 

[davidbeach]

There are a couple of drive experts (skogsgurra, jraef and marke if he's following this) here who will know better than I how a generator handles the unique current waveforms inherent with drives.

Badly. Actually it is more a question as to how the AVR on the generator handles the voltage distortion that results from the current waveforms. You will probably need at least 100% additional capacity in the generator to deal with the drive; maybe 200% or 300% additional capacity.

 

[waross]

Sorry to have missed you David. Thanks for the valuable input.
Respectfully

 

[SilverArc]

Thanks Guys. You have always been more than helpful in giving me suggestion. Here is what I came out from my research and ofcourse your advise was of great help.

I was planning to size reactor my self.But I will leave it to the manufacturer now.
As the case is an emergency generator feeding a DC Drive and Motor, I was advised that I need reactors.

But reactors will cause voltage drop.. but I am not sure that will be an issue or not.

I read all the posts of harmonics reduction by using reactors and a lot were by DavidBeach, which talk about concentrating harmonics at one place by using reactors which bring to my mind that using reactors in an emergency generator case, which is a finite source will be an issue or not.

Davidbeach, your argument about using reactors in front of Drives holds even when it is fed from emergency generator. ?

Thanks

 

[davidbeach]

SilverArc, you really need to hire someone locally who understands these types of installations. There are too many elements working with/against each other here for you to be able to get a good solution. You may need to be working with more than one person, you need someone who understands the drive and what it does and you need someone who understands the generator system, including the AVR, and how it responds to the drive.

Current into a reactor is equal to current out of the reactor and the reactor will have a much lower impedance than the internal impedance of the generator, so you will likely still have problems. The reactor is a good place to bury voltage distortion in a normal system, but it might cause more problems than it solves in a generator system. I don't know though, it depends on too many things.

 

[waross]

I suspect that Cat has built sets that were optimized for SCR drives for the oilfield industry but I do not know any details. It is probably worth while calling a Cat sales engineer. You may also start a new thread with a specific query to catch the attention of anyone who may not be following this thread.
Something like "Generators optimized for SCR?".I agree with David that you need an expert. I also understand that you need as much general information as possible so you can talk intelligently with the expert.
respectfully

 

[SilverArc]

H All,

I thought of starting a new thread but as this question is related... I will ask here:

After going the IEEE recommendation of THD(Current Distortion), Isc/I(Load) determines the distortion allowed.

I am just guessing that, this is the requirements of industrial and commercial facilities limits at PCC.

I am trying to do a simulation an ETAP with a emergency generator feeding a DC Drive and A 1000 HP Dc motor.


The TH D at the bus before drive is 26 %.

After that I used a .012 ohm, 5% current limiting reactor, the distortion became 46 %. I am wondering, what could possibly go wrong.

Any comments plz?

 

[peebee]

Where are you measuring THD, and are you measuring THDv or THDi?

As I previously mentioned, "Beware -- the inclusion of reactors in front of a harmonic load can result in very high voltages on the load side of the reactor."

And as davidbeach previously mentioned, "SilverArc, you really need to hire someone locally who understands these types of installations." I'm going to give db another red star for that one.

 

[peebee]

PS -- you would gain a much better understanding of what's happening if you did some quickie calcs by hand. Model the drive as a harmonic current source. Model the reactor and generator/system as impedances (not voltage sources). The calcs are simple, it's straight Ohm's Law, V=IZ, where I = your harmonic current source and Z is a voltage divider network consisting of your reactor and generator/system impedance. V is then the harmonic voltage developed.