11kv motor vs 3.3 kv motor in a 33 kv system

[7465]

hi all,
I would like to know your views on this subjcet which will be a better option with the incomming supply to the plant is 33 kv moderately stable voltage

thanks in advance
7465

 

[waross]

It depends.


How large is the motor?
How firm is the supply?
Dedicated transformer or added to a partially loaded transformer?
Starting method?
What is the nature of the load?
respectfully

 

[ScottyUK]

Is this considering only one motor, or many motors? If the latter, what size range do they cover? 3.3kV motors are economically viable from about 200kW extending up to about 5MW before the size of switchgear and cable becomes impractical. 11kV motors are priced out below about 2MW unless there are significant other factors which justify their use. 11kV motors are custom built and are available from fewer suppliers compared to a 3.3kV machine. They are therefore more expensive than an equivalent 3.3kV machine. 11kV machines are both expensive and time-consuming to repair, and the choice of repairers is restricted.

11kV switchgear and cabling is more expensive and requires a different set of skills to install and maintain it compared to a 3.3kV installation. 3.3kV cabling is little different to LV cable in construction except the XLPE is thicker; 11kV cable is much more complex in construction and requires purpose-made terminations and joints which in turn need an increasingly rare set of skills to install correctly. Maybe this isn't an issue if there is already 33kV gear on site.

If you need to feed the motor from a remote substation then 11kV will allow you to use cheaper and reasonably sized cable whereas 3.3kV would require multiple conductors. Cable cost is getting increasingly significant - burying copper in the ground is an expensive proposition these days - so if the installation requires a long cable run to the motor the 11kV cable will likely have a lower installed cost than the 3.3kV cable.

11kV equipment will almost certainly see PD activity at some level. 3.3kV gear essentially does not suffer from PD. You would be prudent to consider PD monitoring if this is an important installation.


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Sometimes I only open my mouth to swap feet...

 

[7465]

ok.waross and scooty uk,thanks.This is basically for a a 1200 kw motor.The supply will be better at 33kv.the idea will be hook up at the 11 kv bus in case of 11 kv motor ( the bus is supplied from a 33/11 kv tx)and put an additional transformer of 11/3.3 kv in case of 3.3 kv. soft start starting method.Nature of load is a mill load with varying load condition.
7465

 

[ScottyUK]

1.2MW is already small for 11kV, even with the cost of the additional transformer factored in. The costs of an 11kV soft start will be huge - does the load require soft start? Most 33kV systems will be stiff enough to withstand a DOL start of a 1.2MW drive without serious disturbance, so the soft start is probably not necessary. I suggest you do a load flow study including motors starting to confirm this. If you absolutely need the soft start then economics will force you to go with 3.3kV.

Have you considered using (say) 690V for this drive? LV soft starters are available in this size range - you open up a lot more possibilities for the switchgear, at the expense of an unusual motor and some very heavy conductors. You will need to keep the LV substation and soft start close to the drive.


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Sometimes I only open my mouth to swap feet...