Motor-Driven Compressor

[zdas04]

I'm working on a project where we need to select between a 1,000 hp gas-driven engine and a 1,000 hp motor to drive a reciprocating compressor.

I know a lot about the engine option and can easily estimate the fabrication, installation, and maintenance costs, but electric motors continue to be black-magic to me.

The stuff I think I need for the skid is:
- A motor, from reading threads in this forum I'm thinking 6kV
- A VFD so that I have some amount of compressor capacity control

I'm thinking that the site preparation and plumbing would be the same as the engine-drive (fuel-gas piping on the engine is outside the accuracy of the calculation), I have an estimate from the power company for bringing power to the site, and I've GUESSED that on-site electrical work would be around $50/hp (i.e., $50,000 to connect the motor to the grid).

Back when I was a pup (think vacuum tubes), I was involved in a project where we put in a motor and had to install a motor-controller, is the motor-controller function performed by the VFD these days? I also have read a bit about "soft start" to reduce starting amps, does the VFD provide that also?

Basically I'm trying to figure out if I've missed any major components in the costs that would skew the results inappropriately.

Thank you for your help.

David

 

[jraef]

Not necessarily n order of appearance;

Yes, a VFD will provide the motor controller function AND the soft start functions, as well as motor protection (although I would use an MPR in addition).

At first glance, major components I see missing in your electrical version will be: [ol]
[li] Mitigation of harmonics, which may be possible in the VFD design, but will cost more than those without it.[/li]
[li] Distribution equipment, i.e. substation etc. [/li]
[li] Demand charge factoring from the electric utility if your load demand is not constant. Peak demands on rare occasions can be very costly because they end up as a multiplier on your negotiated best rate. make sure when estimating power costs from the utility that the rate they quote you has that in mind.[/li][/ol]

I would also reconsider your 6kV spec, depending on where it is going. At that HP, 3300 or 4160V VFDs are going to be significantly less expensive than 6kV, and the VFD is going to be a much bigger cost issue than the motor and switchgear.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[zdas04]

jraef,
I got chapter and verse on demand charges from the electric company when they told me the cost to bring in power. I think the cost of the substation was included in their rate ($40,000/mile for 12 kV), I'll confirm that next week.

What is an MPR?

What kind of box mitigates harmonics? I googled "Anti-harmonic" and got some descriptions of the problem, the articles were too problem-specific for me to be able to peal up a corner.

I'll change my target equipment to 4.1 kV.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[waross]

You may want to consider a constant speed drive, if you can modulate the compressor capacity. Your starting could then be a more economical method than a VFD, unless fixing capacity control to the compressor ends up more expensive than the VFD.

 

[jraef]

MPR = Motor Protection Relay. This thread has some links to my favorites in one of my laer posts. thread237-145591

Harmonics is a complicated subject needing more explanation than I can devote to it now. Suffice to say that they are an effect on the power system created by the conversion of line power for use by the VFD, and can affect not only the local installation, but other nearby installations as well.
It's especially important in MV applications, unless your installation is isolated far away from any other electrical loads, and even then the utility may have concerns. You can mitigate harmonics by adding filters, which are expensive, or there are "harmonics-free" VFD designs available that do so by other means. Neither version is free, hence my comment about including the cost in your estimate.

Robicon (now Siemens owned) had some decent white papers on the subject but I'm not sure of how that all survived the Siemens buyout. Even so, they would be a good source for an estimate. That is not an endorsement, and I have no connection to them, they just have a particularly good reputation in this field.

BTW, "Anti-harmonic" would not be a valid term in the industry, so I'm not surprised a Google search on it didn't turn up anything useful. Try "VFD harmonics mitigation".

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[jraef]

Damn, never edit AFTER the spell-check! Should have said "later posts".