MV Motor Start
MV Motor Start
(OP)
We have multiple motors to start, at 6.3 kV, 50 Hz. The power system is weak. We have looked at the following system:
http://powersystemanalysis.com/EasyMVStarting.pdf
My question is if anyone can see some areas of improvement on the switchgear part.
http://powersystemanalysis.com/EasyMVStarting.pdf
My question is if anyone can see some areas of improvement on the switchgear part.






RE: MV Motor Start
I am right now involved in a similar project where we start four refiner motors from a LV frequency inverter. The motors are 6 kV and somewhere in the 5 MW range. We have done measurements on an older system (with GTO inverters) doing the same thing, but we are replacing the old inverters with PWM. Our measurements look OK and we have no doubts that the system will work as intended.
There are a few caveats, though. You need to start with no or very reduced load. And, the acceleration needs to be kept to low values. Do not be surprised if the starting takes a couple of minutes. Or more.
Gunnar Englund
www.gke.org
RE: MV Motor Start
I think that the switchgear is the least of your problems. First off, in that detailed example they are using a 400HP VFD to start a 1500HP motor and then a 600HP motor. First off you must then assume that the loading of those motors is very light, if not non-existent (especially with the 1500HP). Is your system the same? In other words, that is a tricky endeavor to take on before you even get to the next issues. It can be done, but only if very thoroughly engineered and investigated.
The next issue is closed transition with a VFD (if in fact that is what they are doing). You can ONLY do this if the motor is at exact slip speed when the line power (sinusoidal as they call it) is applied. Any difference in frequency, however slight, will be disastrous for the VFD. Only a few VFD mfrs even attempt synchronous transfer schemes and I have NEVER seen one done with a drastically undersized VFD like this.
If they are instead doing an open transition (as might be indicated by their statement about transitional torque), then again, that will be highly application dependant. That's not to say it too couldn't work under the right circumstances, but you must put a lot of trust in the design to spend that kind of money up front not knowing for absolute sure that it will work or that there will not be costly consequences.
Their statement about Soft Starters and RVATs being in the dark ages is a little bold, because obviously this cannot work for every application. Under the right circumstances though, it has possibilities. I give them credit for marketing it and stating UP FRONT that they have just recently made ONE work! Give it to them for honesty at least!
http:/www.jraef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: MV Motor Start
Also, we take the motors slightly overspeed and then let them coast down to sync speed, where a synchronization device switches them to the 6 kV busbar.
So there is no closed transition. I do not quite see how that would work.
Gunnar Englund
www.gke.org
RE: MV Motor Start