VFD versus Soft Starter for ship application 2

[eljas]

Currently in our ship project has VFD (Variable Frequency Drive) been used to start and ramp up 3.1kV/60Hz/2-pole/5400kW electric motor for Compander.

Ship power network have 4 pcs 6,6kV/3800kW/60Hz Diesel Generators whereof 3 is running when motor is started. Sufficient capacity to start the motor with VFD.

VFD Data (IN):
Voltage 2x1800V eff
Current 970A eff
Frequency 60Hz

VFD Data (OUT):
Voltage 3150A rms
Current 1170A rms
Frequency 60Hz
V range 0-3150V
F range 0-60Hz

Q1: Can Soft Starter do the same job?

Q2: Has been proven earlier in other shipbuilding projects and what is requested from the power network?

If this could be possible, a significant cost saving is possible.

 

[X49]

First, check with the manufacturer of the Compander to see if it's been done.

If you want to determine whether the genset will be able to start the load on a soft start, you need to find out what the starting torque requirements are from the manufacturer (required torque vs motor speed). You should also get the torque and current vs speed characteristics for the motor.

Once you get this information, determine both the minimum current and minimum power required to start the load in a reasonable amount of time. Use these to determine voltage and frequency dip levels on your generators and see if they are acceptable.

The disadvantage to using a soft start is that it will limit how often you can start the motor since the motor will be much hotter after each start.

 

[jraef]

If this is for N2 compression/expansion on an LNG ship, I did some work on that concept once and we proved that soft starters could work, but the points that X49 made are the main issues to consider. For sure the norm was to use a VFD or LCI as a glorified soft starter because of their ability to start with low current, but the size and cost was extreme compared to a soft starter, which is what lead to the project I was involved in.

Before making that final decision, take all that data he mentioned and have someone run a TMS (Transient Motor Starting) analysis on it using SKM or ETAP software to determine viability. the software is expensive if you don't have it, but it can be less expensive for a one-shot project to hire someone who has it to run the numbers for you.

X49, if it is an N2 compressor, the duty cycle is fairly low, 3-4 times per day maybe?

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[eljas]

Dear colleagues,

Thank's for your valuable replies.
You listed pretty much the key items on the issue.

Again, only reason to go this path is to save some money (if possible) and still keep the needed performance of the System.

It looks that technically it should be possible taken in consideration that all parameters are well known (compander torque, start/stop ramping time, reduction of starting current in suitable level for Generators to cope with it etc.).

IgelElectronic in Germany seems to have full range of MV softstarters (ISA-HD) for the purpose.

My next step will be to contact them to have information of similar projects, if possible.

Further study is needed to check the other items one-by-one for comperensive overlook of the 'problem'.

Thank's for your input.

 

[jraef]

If IGL doesn't have specifics, that soft starter is actually made by another company called Solcon. They have done thousands of shipboard installations and are who I did the study on that I mentioned above.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[eljas]

Dear jraef,

Yes, these soft starters have been used in my previous LNG Projects for Cargo Pump and LD/HD Compressors as well.

The key issue will be the current and torque control capabilies of the drives at motor start as well as start/stop ramping of the current.

 

[GreenDriv]

eljas, I think you need to know the purpose why apply VFD.

If just for soft starting electric motor, I think you may try to apply soft starter. That will be much cheaper.

But if there's other purpose, such as energy saving on that electric motor, VFD is needed.

And I noticed some message "shipbuilding projects". So I think VFD may be used for process control in those projects. Is that right? Motor soft starter can't achieve that same goal for process control.

World class variable speed drives

 

[eljas]

Dear GreenDriv,

The Compander Maker is stating, that it is only high start inrush current which is problem, and it is not problem for the Compander itself but for the ship power network of stability point of view.

When compander is running, no control from the drive is needed, only protection.

Starting current with ramp functions are essential to provide smooth start/stop of the motor/compander.

If the soft starter have reasonable low inrush current and slow (adjustable) start/stop ramps it may be suitable to do the job.

VFD can ramp up the current from 0-100% with full torque when softstarter is giving some inrush current (2-4 times nominal?) with low voltage and torque. This needs to be clarified now with the Soft Starter maker in relation to the Compander data.

I am still working to get all necessary information..motor curves etc.

Energy saving is one important issue when evaluating the systems, but that will be checked after the starting issue.

Thank's for you input.

 

[itsmoked]

What is a compander?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Keith,

It's a hybrid machine used in the process industry where a turbo-expander on one process stream is used as a source of motive power to drive a compressor on another process stream. It's a lot more efficient than a let-down valve and allows useful work to be extracted from the expanding stream.


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If we learn from our mistakes I'm getting a great education!

 

[eljas]

Dear colleagues,

Before closing this thred, I'd like to present my result of evaluation of Soft Starter vs. VFD for Compander in ship use.

Without going into detailed descriptions, I'd like to make summary of some main features when starting the motor (main issue):

A. VFD:
1. (+) Voltage constant during start sequence.
2. (+) Current control (linear ramp) 0-100% with full torque. No inrush current when starting the motor.
3. (+) Can be used for starting up heavy loads (big motors).
4. (+) Smooth start do not stress mechanically the motor or the load on the motor (compander).
5. (+) Tuning of the motor/compander speed to achieve optimum efficiency is possible.
6. (+) Easy to implement as part of the IAS.
7. (+) No changes needed on the upstream power network (Switchboards, Generators).
8. (-) System is more expensive than Soft Starter applicaction.

B. Soft Starter:
1. (+) Much cheaper than VFD application.
2. (+) More simple concept. Less risk for failure.
3. (+) Less space for System Units needed.
4. (+) Widely used in diverse applications with good reputation, but not necessary suitable for big size of motors as used for N2 Companders in Ships.
5. (-) Low voltage and torque values at the starting of the motor.
6. (-) Big inrush current (4-4,5xIn, In=401A) at the starting of the motor.
7. (-) The inrush current is difficult to handle by 3x3380kW Generator's with base load.
8. (-) Modifications needed on the Power Network (HV SWBD's and Generators) if Soft Starters are be planned to use. Big capital cost.

CONCLUSION:
As a result of the items listed above and attached documents, I would like to state that VFD is only proper method for starting N2 Compander Motor.

It is evident that Soft Starters can not be used for starting big motor loads without unreasonable cost impact to handle the big starting inrush current.

Waiting for your comments, is any..Thank's for your input.

 

[itsmoked]

Scotty thanks for defining "compander" makes sense compressor expander. Doesn't make complete sense to me that something using an expander as a motive force would still need a large motor.

eljas; I'd change A3 to "Allows full torque starting."

I'd add B9 (-)Can cause large harmonics during starting.

And because of B9 I think B2 is not true. I'd expect that statement to be on the VFD side.


Since I still don't understand the point of a big motor on something that's supposed to be extracting useful work from a stream that was just going to be lost otherwise the following comment might be off-the-rails.

If the large motor is just to supply the needed starting torque could a VFD use a much smaller motor able to provide its full torque from just a few rpm instead? That would save, more yet, as the switch gear, cabling, motor, and VFD could all be, perhaps, half or 1/3 the SS rating/size?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[eljas]

itsmoked,

Thank you for your additional comments.

It looks that even the VFD may not be necessary for the process point of view, it have several features which can be used for finetuning the process and thereby increase the efficiency of the project.

I have to trust on the Compander, Motor and SS Maker's data and attached calculation on the issue.

It looks that the high price is justified technically for this application..and used VFD's (Siemens and Converteam) are proven and improved based on the experiences on operation.

In my opinion the thread can be closed now..thank's for everybody.

eljas