Big motor starting 2

[victa1288]

I want to start a 5000HP motor andthe system is too weak. I thinking of using Auto-tranformer and softstarting methods to start it. But whenever I tried it by ETAP, it always caused the bus Voltage drop to exceed the requirements. Does anybody here have better solution for this system?

 

[Skogsgurra]

Is it an induction motor or a synchronous one?

Gunnar Englund

http://www.gke.org

 

[waross]

What is the load?
yours

 

[BJC]

Do a search on Pony motors. There's at least one thread on it somewhere hereabouts.
A Pony motor is using a smaller motor to spin the big motor so it's moving when the starter is activated.
Lots of factors determine wheater it will work for you. What are the details on the motor, the load?

 

[jraef]

Usually if a solid state starter or an autotransformer will not work, the only remaining solutions are a pony motor (if it can start with no load) or a VFD. You still need a lot more information posted if you want help.

http:/

http://www.jraef.com

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

 

[itsmoked]

4MW?! I could easily see your plant NOT starting an additional 4MW load that is probably going to suck down 15MW on starting! Maybe you need a new MV drop brought in?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[victa1288]

It is a induction motor. I tried softstarter and Autotransformer. Both of them don't work very well. So I am thinking of reducing motor starting current, plus Load tap changer and Auto Tap changer.

 

[electricpete]

Just to throw out a question, would a vfd be an option? There may be other process or efficiency benefits derived from a vfd as well. But in this hp range I'm guessing vfd's don't come cheap. What is the voltage?

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[BJC]

I did a study recently on a VFD for a 2,500 HP motor and the cost was in the 1/4 million dollar range.
victa 1288 hasn't said what his application is but his options may be moving the plant or doing nothing.

 

[dewone]

Can the motor be Wye, Delta connected. Six leads in the condulet.

 

[Laplacian]

As others have said, you need to provide more information:
Detailed utility information; voltage, transformer(s), fault MVA, etc.
Load information; inertia, torque/speed, etc.
Motor voltage and your main distribution voltage

If you need to start a load, you could use a combination of pony motor and fluid clutch - start with pony motor and clutch disengaged, once main drive is up to speed, slowly engage the clutch to turn the load.

Note that at this power range, there are no cheap options to make this work. You need to provide much more information before the members here can give you the most feasible and reliable starting method for this machine.

 

[DanDel]

In ETAP, there are an almost infinite amount of adjustments for the starting method, including multiple autotransformer taps and softstart options. Did you make any adjustments, or just use the default settings?

 

[victa1288]

The utility S-C is 100MVA. 35kV-10kV, 10kV motor. Transformer is 15MVA.
As you guys have said, VFD could be an option.

In ETAP, I used Aoto transformer, the combination of fixed tap and LTC, I started the motor without causing unacceptable voltage drop on the buses (20%, which is still too low.

 

[DanDel]

You can adjust the tap in the Autotransformer to any percentage (the default is 65%). Try it at 10% or so, just to see what the voltage drop is there.
You can pick 'Current Limit' to model a basic softstart, and start by limiting it to the running current. Then adjust up from there to get the voltage drop below 10%, which is typical for acceptable limits. (If you still have an unacceptable voltage drop at running current, then you can't even run the motor in your system, never mind start it.)
If you model the motor and load correctly, and use the Dynamic Start module, you can see if and how long it will take to come up to speed at the starting settings you have set.
You can also model a VFD or a ramp softstart by using a Current, Voltage, or Torque Control model. ETAP alows you to practically draw the start curve from there.

 

[jraef]

5000HP on a 15MVA transformer is not going to be a running load problem, it is starting that is causing the voltage drop for sure. Autotransformer would have been the best choice in this situation because you get more motor current per line amp than any other fixed frequency method. If that didn't work, your choices are really limited now. A VFD or a pony motor as I said earlier (and even a pony motor is going to be suspect) are your only purely electrical options. The good thing about a VFD will be the ability to accelerate the load with 1x FLC because you can get full torque that way. As skogsgurra said, you can then also look at possible process cost reductions as a way to help offset the capital outlay, which will be big as BJC mentioned.

A magnetic or hydraulic clutch may also work if you can start the motor unloaded. Try running your ETAP with no load on the motor first before investing time on a mechanical solution. Remember for any mechanical solution you need to start the motor first!

If you were close with the Autotransformer you might want to consider changing motors. I had several projects in that size arena where motor performance made a big difference. I use SKM instead of ETAP but I suspect they are similar. The software takes the motor and load inertia data, motor performance data such as the torque-speed, power factor and current curves, then plots that against the load torque-speed requirement curve to determine the interaction of all of those elements. I found that since motors in that size range are all engineered differently they tend to have very different curves and sometimes you can find one that gives you what you need for that particular load profile.

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

 

[apowerengr]

Try reducing the impedance on your 15 MVA power transformer to 4% or so. In addition, if you have a round-rotor induction motor a liquid-rheostat can be used to vary the rotor resistance thus varying the starting current while maintaining output torque.

 

[electricpete]

I think you meant "wound rotor"

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[DanDel]

And reducing the impedance of a transformer is easy in the software, but impossible in the actual system without changing out the transformer, which is probably not an option in this case.

 

[swn1]

One option you appear to have not yet evaluated is using a wound-rotor motor.

Another would be to make sure you are specifying the smallest starting current (highest code letter) compatible with your application.

If you can unload the motor for starting you don't need a 5000HP drive, you can spin it up with a smaller VFD and then bypass the drive before loading the motor.

Easier with a synchronous motor, you can do it with a low-voltage drive if you are very very careful.

Yet another option: add stiffness locally with another motor (or generator) with a large inertial load.

I've seen remote pumping stations that start their emergency generators during plant startup to add stiffness.

 

[RRaghunath]

I work in Oman. In this part of the world the grid is weak in many places. Whenever a large motor such as a 4.5MW motor is to be supplied, the practice to connect the motor to the network through a two-winding trafo (33/11kV or 132/11kV as the case may be). The trafo + motor come as one package and the motor is designed to start at low voltage (as low as 38% rated at times) and the impedance selected for trafo ensures the bus voltage upstream is maintained as per stipulations. This arrangement must be costly but cheaper than VFD and more rugged.

Trust the above is helpful.