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Advantages Of Using Ht Motors(6.6kva)/3.3kva Vs Lt Motors(440v)


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#1 dhanvant

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Posted 12 May 2008 - 09:13 AM

I want to know the advantages & disadvantages of using HT motors(6.6KVA)/3.3KVA Vs LT motors(440V).

Our Incommming power Supply from State Electricity is 440VAC.Our technical team suggest to put 6.6KVA motor for Air compressor application.

We have total requirement of 2000NM3/hr.

So we need compressor of @ 1000Kw.

What are the problem we can face if powered through common 33KV -440V Stepdown Transformer assuming I will use soft Starter.

Differentiate in respect of

1) Transformer Losses.

2)Starting current Loss.

3)Cable Size & losses

4)Cost difference involved in HT & LT cable.

5)Effect on Power Factor.

6) Additional Transformer losses if power is sourced from DG in case of power failure.

Can any body elaborate saving by calculations in details by experiance.

Dhanvant




#2 marke

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Posted 14 May 2008 - 02:23 AM

Hello dhanvant

This is a very involved question and needs a full enegineering appraisal. This is beyond the scope of this forum, although there are members who may be prepared to comment on some aspects of your questions.

Part of the issue is related to your local conditions and standard practices. In some terretories, it is common for all motors above 200KW to be connected to MV rather than LV, but in other regions, 1000KW on 400V is not an issue, so much of the consideration is influenced by this before you even begin to look at other factors.

If you can ask specific questions, I am sure you will get a better response.

Best regards,
Mark

#3 jraef

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Posted 15 May 2008 - 03:42 AM

QUOTE (dhanvant @ May 12 2008, 01:13 AM) <{POST_SNAPBACK}>
I want to know the advantages & disadvantages of using HT motors(6.6KVA)/3.3KVA Vs LT motors(440V).
Our Incommming power Supply from State Electricity is 440VAC.Our technical team suggest to put 6.6KVA motor for Air compressor application.
We have total requirement of 2000NM3/hr.
So we need compressor of @ 1000Kw.
What are the problem we can face if powered through common 33KV -440V Stepdown Transformer assuming I will use soft Starter.
Differentiate in respect of
1) Transformer Losses.
2)Starting current Loss.
3)Cable Size & losses
4)Cost difference involved in HT & LT cable.
5)Effect on Power Factor.
6) Additional Transformer losses if power is sourced from DG in case of power failure.
Can any body elaborate saving by calculations in details by experiance.
Dhanvant


I'll take a first stab at some of it anyway, although I agree with Marke that this is something that warrants more attention than you can get here.

1) Transformer Losses.
You have transformer losses somewhere, no matter what. If I read your post correctly, you have 440V coming from the utility already? If so, THEY already have a transformer somewhere, because they will not be transmitting 440V over long distances. So if they are already providing you with 440V, they are already absorbing the transformer losses before billing you. If you add ANOTHER transformer to step it back UP to 6.6kV, then of course you are adding additional losses. Doesn't make sense to me. now IF the utility is willing to provide you with 6.6kV INSTEAD of 440V to feed that new motor, the higher voltage makes sense for everyone.

2)Starting current Loss.
Not much difference. Losses are in kW, not just in current. There would be some additional I2R losses in the motor windings themselves with the LT motors, but you are not talking about a lot of time in starting, so the value is insignificant.

3)Cable Size & losses
Benefit goes to higher voltage. Less cable, less I2R losses.

4)Cost difference involved in HT & LT cable.
Not sure. Less copper in higher voltages may offset the fact that the HT cable is more expensive. Probably less expensive for the HT cable now, although the installation may cost a little more as well. As Marke said, a lot of that depends on your local codes.

5)Effect on Power Factor.
Not likely to be any difference there, although LT capacitors, if necessary, are going to be significantly less expensive.

6) Additional Transformer losses if power is sourced from DG in case of power failure.
Again, if you have a 440V generator feeding the load through a step-up transformer, you will need more generator capacity to make up for the transformer losses.

Not mentioned:
7) Switchgear cost.
HT switchgear is a LOT more expensive. The cost premium is sometimes 2-3X that of LT gear, especially soft starters.

8) Service and maintenance workers.
In some areas, you may also have a problem with service worker qualifications for working on HT equipment. I had that happen to me once. The end user had no technicians qualified to work on 5kV equipment, so they had to pay extra for training, then the guy demanded more pay!
"He's not dead, he's just pinin' for the fjords!"

#4 dhanvant

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Posted 15 May 2008 - 11:38 AM

QUOTE (jraef @ May 15 2008, 09:12 AM) <{POST_SNAPBACK}>
I'll take a first stab at some of it anyway, although I agree with Marke that this is something that warrants more attention than you can get here.

1) Transformer Losses.
You have transformer losses somewhere, no matter what. If I read your post correctly, you have 440V coming from the utility already? If so, THEY already have a transformer somewhere, because they will not be transmitting 440V over long distances. So if they are already providing you with 440V, they are already absorbing the transformer losses before billing you. If you add ANOTHER transformer to step it back UP to 6.6kV, then of course you are adding additional losses. Doesn't make sense to me. now IF the utility is willing to provide you with 6.6kV INSTEAD of 440V to feed that new motor, the higher voltage makes sense for everyone.

Actually I belongs to India.In India we are getting 33KV supply from utility at our factory gate.We are stepping down it further as per requirement say 6.6KV/3.3KV/1100V/440V AC by using stepdown Transformer.

So whatever Transformer losses are there company has to bear since metering is at input of transformer.

Now 1)for my LT motor I need 440VAC --Required Stepdown transformer 33KV-440VAC.( Loss No1)
2)For HT Motor I need either 6.6 or 3.3 KV---Required Stepdown transformer 33KV-6.6/3.3 KV.( Loss No2)

So if I use only LT motor --Loss no 1 will be in Picture.
If I use both LT & HT motor ---Loss No 1 + Loss No 2 will be in picture.

I mean to ask for 1000KW (600KW HT Motor + 400KW LT motor) configuration whether it is benefitted to use LT motors instead of going to HT?(600KW)

Or

Go for 600KW HT motor + 400KW LT motor.




2)Starting current Loss.
Not much difference. Losses are in kW, not just in current. There would be some additional I2R losses in the motor windings themselves with the LT motors, but you are not talking about a lot of time in starting, so the value is insignificant.

Starting KW loss of LT motor (total 600KW) is > Starting KW loss of HT motor (total 600KW) Is it right? What i want to ask u.

3)Cable Size & losses
Benefit goes to higher voltage. Less cable, less I2R losses.

Rep:Thanks for reply.

4)Cost difference involved in HT & LT cable.
Not sure. Less copper in higher voltages may offset the fact that the HT cable is more expensive. Probably less expensive for the HT cable now, although the installation may cost a little more as well. As Marke said, a lot of that depends on your local codes.

5)Effect on Power Factor.
Not likely to be any difference there, although LT capacitors, if necessary, are going to be significantly less expensive.

Rep:As per my knowledge HT motors are Synchronous motor where Losses due to slip is less.That's why P.F will not hamper by using HT motor? Is it right?(Pl.correct me if I am wrong)

6) Additional Transformer losses if power is sourced from DG in case of power failure.
Again, if you have a 440V generator feeding the load through a step-up transformer, you will need more generator capacity to make up for the transformer losses.

Rep:Suppose I will use LT & HT motors then in case of power failure I need to backup by Generator having either 440V or 6.6KV output.
So any means I need eithr Step Up or Step down transformer.
Naturally losses @ 5% will add to my billing.


Not mentioned:
7) Switchgear cost.
HT switchgear is a LOT more expensive. The cost premium is sometimes 2-3X that of LT gear, especially soft starters.
Rep:Thanks a lot for Important info.Skipped by me.

8) Service and maintenance workers.
In some areas, you may also have a problem with service worker qualifications for working on HT equipment. I had that happen to me once. The end user had no technicians qualified to work on 5kV equipment, so they had to pay extra for training, then the guy demanded more pay!

Rep:Thanks a lot for Important info.Iwas not think of it.

Dhanvant

#5 Bysikmode

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Posted 20 April 2014 - 01:15 PM

Thank you for good communication.



 


dubli

 





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