Start 2.5MW / 6kV Motor with Solid State Soft Starter

[Parkpower]

Hi everyone,

Our team is asked to start a 2.5MW/6kV motor with solid state soft starter. The motor is about 300m away from the electric room. The manufacturer of soft starter, Mototronics suggests that a large reactor shall be connected in the circuit to overcome dV/dt problems. I have concerns about the minimum starting voltage and limited current because of the motor size and distance. Generally, we can start 700HP motor with 60% starting voltage and 300% full load current. However, I do not have an idea how much current needed to start this motor. If starting current exceeds 400%, soft start does not make any sense.

Is there somebody who is experienced in starting Motor of this size? Does solid state starter work or we have to use a VFD as soft starter?

Thanks,

Parkpower

 

[LionelHutz]

You haven't provided enough information to determine how much current is required to start the motor.

 

[jraef]

I have done a lot of large MV soft starter start-ups, you have not divulged anything that would have me concerned at this point, but as Lionel says, you haven't divulged much. While it's true that you will not know (without doing a comprehensive study) what the starting current will end up being, I disagree with your assertion that "If starting current exceeds 400%, soft start does not make any sense." There are numerous reasons why soft starting makes sense, regardless of the level of starting current necessary to accelerate the load. I had several applications where the current needed to be 500% or even more, that was still better than the 600-700% it took to do it Across-the-Line. In addition, your "angle of attack" on the current step can make a big difference in the mechanical shock that your power train components are subject to. A 2 second ramp into a 500% current limit is mechanically a lot softer than a 2 cycle increase.

Is there some other reason why you have trepidation over using a soft starter?


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

LionelHutz and jraef,

Thanks for the reply.

I agree that mechanical and piping system can also benefit from soft start and soft stop. On this project, the main reason for soft starter is to limit voltage dip on the bus during starting the big motor. If we can soft start the motor with a limited current below 300% FLA, voltage level on the bus is acceptable. System study indicates that 300% FLA is the maximum current allowed. We would like to lower the starting current as much as possible. Otherwise, we have to use LCI or split the big 2.5MW motor to two 1.5MW motors and start them in sequence.

It's the first time that I will start motor above 1MW and the feeder is 300m long. LCI is more expensive than solid state soft starter.I am not sure whether we can use solid state starter or not.

Is there any document regarding calculating minimum starting current?

Parkpower

 

[LionelHutz]

Is this a pump?
Does it have a closed valve during start-up?

Motortronics won't help you determine this? If not, maybe you've gone to the wrong soft-start manufacturer...

I'm already impressed with their "we suggest you install a line reactor" line. They should have been able to tell you if it is required or not and if so, provided the proper line reactor.

Have you purchased this soft-starter already?

 

[jraef]

...
Motortronics won't help you determine this? ...

When I worked there, I would have run an TMS analysis on it using SKM Power Tools. I don't know if there is anyone left there who can do that now.

Parkpower, did you ask them if they would do that? They would need all of the motor data, including torque/speed/current curves, load torque/speed curves and a detailed description of your power systems as it is now, including transformer impedances, line lengths, cable sizes, available fault current etc. etc..

I'm already impressed with their "we suggest you install a line reactor" line. They should have been able to tell you if it is required or not and if so, provided the proper line reactor.

It's a bit of a CYA issue. You really need the reactor to protect the SCRs if there is a possibility of high line capacitance, but no other manufacturer says they MUST have them, so if they automatically include them as a precaution, their price looks higher. They used to (again when I worked there) publish a long complicated tech note that described when an where you did or did not need a line reactor, nobody ever paid attention to that and bought on price.



"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

 

[Parkpower]

LionelHutz and jraef,

It is a pump, and the valve can be closed during start-up. We have not bought the soft-starter yet. We need to check feasibility.

We will provide all relevent data and ask motortronics to make a study before we can place an order. I also have concerns about long starting time with limited starting current. If it exceeds motor damage curve, we have to consider other options. We design the power distribution system based on maximum and minimum fault level submitted by others. Now the real minimum fault level is less than the original one. So we have to reduce starting current to regulate bus voltage.

Do you succeed in soft starting the MV motor with 200% FLA with Motortronics starters? I never tried this before.

Parkpower

 

[LionelHutz]

Parkpower - I doubt your motor will start with 200% FLA. More likely would be 250% to 300% FLA assuming the motors have a fairly typical speed/torque curve.

It's a bit of a CYA issue. You really need the reactor to protect the SCRs if there is a possibility of high line capacitance, but no other manufacturer says they MUST have them, so if they automatically include them as a precaution, their price looks higher. They used to (again when I worked there) publish a long complicated tech note that described when an where you did or did not need a line reactor, nobody ever paid attention to that and bought on price.

That's interesting to hear they had some technical documents. The line reactor requirement can be studied and understood. The times we've been involved with customers against Motortronics, the customer has never been provided any detailed technical info from Motortronics which would indicate they really understood the SCR switching issues when building 5kV+ voltage soft-starters. The main issue is dI/dt during SCR turn-on. There is also a secondary dV/dt issue due to ringing which may require more components than just a line reactor to address.

 

[jraef]

Parkpower,
I agree with Lionel, 200% is unrealistic. The only time I ever got anything started at even 250% was when they were unloaded. But I did a lot of MV centrifugal and turbine pumps at 300 - 350%, with starting times between 10 and 30 seconds.

That Motortronics MV soft starter has a built-in Motor Protection Relay (for all intents and purposes) and will not take your motor into the damage curve. But it will trip, which of course means a failed start. That's why a good pre-analysis is helpful. It eliminates that trail and error method; you can get away with that on smaller LV motors but it's not a good idea on expensive MV systems.

Lionel,
In all of the 6kV soft starter systems I worked on (in excess of 20), only two I can remember needed the reactors. Once we got to 10kV and up, we included them as standard.


"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

 

[Parkpower]

LionelHutz & Lionel,

Generally we soft start motor with 300% - 400% FLA to limit voltage dip. This is origial design. However, we have to challenge the limit of 200% FLA for this project. If unrealistic, then we try 250% FLA, but can not be higher than 250%. Even if the study shows that starting with 200% FLA is feasible, we are not sure whether it works on site. It seems nobody did this before.

The distance of 350m is another issue. Maybe we need reactor because of the capacitance of the cable. The 400V VFD vendor also requires that inductor shall be installed when the distance exceeds 150m.

Parkpower

 

[richardgb]

in addition to all the help starting this thing,
it might be useful to mechanically start turning the motor by external means, even if it is ever so little,
like I do on my lathe every time I use it


on a big succer like this, it may make all the difference in the world

in direction of vector.....indirv