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Autotranfomer starting Compressor

Autotranfomer starting Compressor

Autotranfomer starting Compressor

(OP)
Hi, Guys,

I need the inputs from you.
Our client has an existing compressor, which is aerial. It is running now. And they are planning to relocate this compressor to a new location with a weaker power system. The existing compressor motor has an auto transformer starter with 50% tap setting.  It was successfully started in the past. Study shows that the motor terminal voltage at the motor terminal is 48%nomianl based on the existing system. The new system will bring the voltage down by 2% at the motor terminal during motor starting.  I am thinking of increasing the tap setting to 60% and that will bring the voltage up to 52.88% nominal at motor terminal. Theoretically, it should be working because the starting torque will be increased proportionally by Square of voltage increase as well and the motor should have enough torque to start up the load. The only problem is, the flicker limit on the primary of the 2MVA transformer (25KV/4.16KV) will be exceeded, so we would have to get permission from Utility to do so. This is the only load on this line.   

I used the same system data and simulated another pump by the  auto transformer starting method and motor/pump data. But motor/pump data are different. However the motor failed to be started.

I am assuming, the compressor motor and its load data could be different than the one that failed.


The client really doesn't want to spend money on VFD. So we have to find an alternative way to start the compressor.

Thanks a lot in advance.
 

 

RE: Autotranfomer starting Compressor

Take a look at your system and decide if it is feasible to go down (up? down? usage varies) one step on the main transformer taps and raise the plant voltage slightly. If the system is weak, this may result in the plant voltage being closer to nominal under load, as a side benefit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Autotranfomer starting Compressor

Would the flicker limit be exceeded with the 48%?

Can any changes be made like opening the compressor outlet on starting to allow successful 48% starts?

How about not a VFD but a soft starter?  Probably 1/3 the cost of a VFD.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Autotranfomer starting Compressor

(OP)
Based on my experience, softstarter woon't work as long as Autot-tranformer starter doesn't work.

We will try  to reduce the load torque during starting by any means.

Raising one step on the tap of the main transformer should help improve the 4160V voltage as well as the motor terminal voltage. But my concern is the flicker limits on the primary side of the main transformer,whcih may affect other customers on the line.  

RE: Autotranfomer starting Compressor

The purpose of changing the taps and raising the 4160 system voltage is so that you may stay on the 50% starting tap. Does this still give too much flicker?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Autotranfomer starting Compressor

(OP)
yes we may be able to stay at 50%. But since the system is so weak that the flicker is way too much.

RE: Autotranfomer starting Compressor

Now comes the creative part. What do you have in your boneyard?
If you have a smaller, unused motor and control gear, you may try using it to run the compressor up unloaded and applying the 4160 when the compressor is close to speed. This is usually only feasible if a suitable motor is available. It is probably not economically feasible to purchase a motor for such an application. Other than that look at a soft start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Autotranfomer starting Compressor

(OP)
All the motor and compressor will be reused in a new location.

RE: Autotranfomer starting Compressor

To clarify my suggestion, use a second, smaller motor with less starting surge to spin up your compressor and 4160 v motor before energizing to avoid voltage dip. If you have a suitable motor on hand, and if your compressor is belt driven rather than close coupled and if you can rig a suitable drive. It's a long shot, but it may be cheaper than a soft start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Autotranfomer starting Compressor

(OP)
This is something new to me. I was thinking of adding a reactor between the transformer secondary and the 4160V SWGR. The drawback of this method is, it will bring the motor terminal voltage down  abit lower.But it would boost the voltage on the primary of the transformer.

RE: Autotranfomer starting Compressor

(OP)
Waross,

Did you mean Jackng motor start method? Our application is axial compressor.  

RE: Autotranfomer starting Compressor

Hamid,

The usual term for the smaller motor is 'pony motor'. They are normally used when it is possible to decouple the load from the motor using a clutch or similar arrangement such as a torque converter.
  

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

RE: Autotranfomer starting Compressor

Hello HamidEle
1> A VFD will work but it will be expensive.
2> A soft start will probably work and will be less expensive than a VFD.
The following methods may work in one instance and not in another instance. After consideration they may have to be rejected.
3> Adjust voltages and taps. Try to find a compromise between suitable voltages and acceptable voltage dip.
4> Use a pony motor to spin up the compressor. This depends on the availability of a suitable surplus motor and switchgear. It also depends on the feasibility of connecting a second motor to the line up. In one instance it may be easy, but in the next instance it may be virtually impossible.
There has been a discussion in another thread concerning the use of capacitors to reduce motor starting current.
You may want to look at the maximum KVARs that your PF correction gear has available and the maximum that may be temporarily connected to your system without over voltage issues. I have the impression that we are only looking for a few percent improvement to bring the voltage dip to an allowable amount. Try adding in a lot of capacitive KVARs to  your simulation software. If you can get your voltage dip to an allowable value, evaluate the effect the extra KVARs will have on your system voltage before during and after starting. Over correcting the power factor for a few seconds will have no effect on your PF penalties on the metering schemes and tariffs that I am familiar with. Please let us know how your simulation results are with added capacitors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Autotranfomer starting Compressor

(OP)
Thanks, Waross,

I tried Capacitor and even VFD starting this morning. Those can not help a lot on 25KV level. As matter of fact, voltage drop on the pimary is almost 9%. It is  almost impossible to boost the voltage level on 25kV side with such big portion of load and high impedance of power supply. The only way is, the utility to add a volage regualtor to boost voltage at 25kv tie in point by 5%. Then even the  initial auto-transformer will work.

RE: Autotranfomer starting Compressor

Since this is a compressor and not some high inertial load how can a VFD not make this happen?

Keith Cress
kcress - http://www.flaminsystems.com

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