What the best system to start large synchonous motors driving pumps?

[Bronzeado]

Hi folks,

We need to specify a system to start large synchonous motors driving water pumps. The motors are between 5 to 13 MW at 6.9kV. They are feed by a transformer 230/6.9kV, two motors per transformer.

Is the SCR soft-starter the best system for this application?

Thank you in advance for your reply.

Best regards,

Herivelto S. Bronzeado
Brasília, Brazil

 

[dpc]

The cheapest way is to start them across the line. The captive transformer will act as a reduced-voltage starter. But if you could have a running motor on the same transformer as the motor you are starting, then you will need to do a voltage drop/transient starting analysis to see if the running motor can live with the voltage dip during starting. Assuming the pumps are centrifugal and started against a closed valve, things may be as bad as you might think.

A solid-state reduced-voltage for a 13,000 kVA motor is going to be expensive. LCI adjustable frequency drives are probably still a viable option for motors this size.

 

[LionelHutz]

You really didn't give enough information to determine this. However, a full-voltage start would be the simplest. A SCR based starter will be very large and expensive. I believe your 13MW motor will be in the 1200A range, and this is quite a high current for a 6.9kV SCR based starter.

 

[jraef]

I know that Solcon makes 6.9kV solid state starters that large. They are going to be significantly smaller and less expensive than an LCI drive. But that said, it really comes down to that TMS (Transient Motor Starting) analysis that dpc mentioned, an application like this all but requires it before deciding on a technology up front. In that process your engineer will investigate and therefore be able to SHOW all of the important factors affect of your decisions, i.e. not only voltage drop on the secondary of that transformer, but also on the primary (which at 230kV, has very wide reaching consequences). So even though an RVSS starter is cheaper and smaller, an LCI might end up being the only viable solution for soft starting a 13MW pump.

Then if you start to consider the possible benefits of varying the speed to match flow requirements, the next level up, a VFD, might be worth considering as well. LCIs are of course VFDs, but tnot the most efficient ones if running below full speed, so a CSI or VSI might be a better choice.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
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[oldfieldguy]

We are currently installing a couple of 15,000 HP (11MW) synchronous motors with softstarts provided by Phasetronics, operating at the 13,800 volt level. There are a few manufacturers in the US who provide these. Being all provincial and all that, I don't know of non-US providers, but I would imagine that there are several more.

we balanced the cost of the solid-state soft start versus the cost of an autotransformer-based system.

I am expecting good results.

old field guy

 

[jraef]

I used to work there OFG.

Good luck with that...



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Bronzeado]

Thank you guys for the replies.

Cheers!

Herivelto

Best Regards,

Herivelto S. Bronzeado
Ministério da Integração Nacional - MI
Brasília, Brazil

 

[ScottyUK]

Hi jraef,

Curious about your distinction between LCIs and CSIs. Always believed one was a subset of the other.