Using a softstart on long cables

[hph]

Hello

We are wanting to start a subsea electric motor through 3000m of cable with a softstart. This is a marine application so it is ungrounded. The boost transformer (Delta to ungrounded wye) 460:3000 is after the softstart.

Is there a rule of thumb for sizing a reactor that will counter the capacitance of the cable? Would a VFD be a better approach for this? Any problems with a VFD on this length of cable?

thx

 

[magoo2]

I wouldn't worry about the capacitance for 3 km of 3 kV cable. It's just one more component to worry about.

We have some 34.5 kV cable that is about 8 km long. The voltage is more pronounced at this higher voltage but shouldn't be an issue at your lower voltage. We installed a voltage regulator at the load end, but ours is not a subsea application.

Is 3 kV your phase-to-phase voltage? Have you calculated the expected voltage rise?

 

[LionelHutz]

I can't see you having an issue. The transformer will work like a line reactor.

A VFD would work too. The transformer will filter out most of the PWM voltage coming from the VFD.

 

[hph]

We've actually tried a softstart and it doesn't work very well. System acts like it is two phasing while the Softstart is ramping up.

 

[ScottyUK]

If the soft start doesn't produce an output with zero DC component then the transformer core will saturate and you will hear noise as the transformer protests. It doesn't take much DC to saturate a transformer without an airgap in the core. The output of a saturated transformer will be heavily distorted and reduced in amplitude, and the primary current will be abnormally high (unless the soft start limits it).

 

[hph]

The transformer certainly does make an awful noise and the primary current is higher than expected. I don't know where a DC component would be coming from as it is a 3 phase system.

 

[ScottyUK]

The soft start uses thyristor switches. If the switching is not perfectly symmetrical then you'll end up with a resultant DC component. Some soft starts are better than others in this respect, but few are perfect.

 

[itsmoked]

Scotty do some just switch two of the phases? That would probably do it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Good point Keith, some soft starts don't even pretend to give a symmetrical output.

 

[jraef]

I agree, it's a common "trick of the trade" going on now in efforts to reduce costs and keep smaller soft starters competitive against VFDs. Some make it clear and give you warnings, others are less forthright about it. If you post the make and model, we can tell you if you can't find out on your own.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[controlsdude]

Most vfds need a load reactor or some type of impedence matching device especially when the distances are long. Most manufacturers have recommendations for the load reactor.

Remember most vfds has parameters that if your using a long leads to the motor these need to set on. Maybe also do a static tune at commissioning would also help.

 

[hph]

Yes we've tried a couple of softstarts. The original system acted like it was two phasing so when I was poking around I thought I found a short. To my chagrin I found it was designed like that. Then tried a softstart that chopped all three phases with the same result. Both worked beautifully on short cables.

Then added a load reactor that seemed to work initially but is now back to scary, noisy, light dimming starting. Haven't found any guidelines, formulas or help on sizing a reactor for a softstart - just warnings on lengths of cables over ~700m.

Controlsdude - Do you think the result would be the same with a VFD? I think there would be less harmonics so wouldn't be as susceptible to cable capacitance.

 

[controlsdude]

VFD - same result? no
I have not used a submerged motor like your using but the distance was up to 800 feet. But it was a dry environment. I know cable selection is critical in wet locations and would think a VFD would probably get you out of your problem with soft start. But 700 meters is long distance (~2200 feet)

I know in some applications vfd manufacturers they recommend load reactor + some type of filter.

I would go here and read up some of there recommendations. Maybe give you some idea what the solution could be?

http://www.transcoil.com/Support/Documentlibrary.htm

 

[LionelHutz]

The cable length issues normally associated with a VFD are mostly irrelevant in this case. The transformer filters the VFD voltage waveform so it will closely resemble a sinewave at the transformer secondary. The transformer also electrically isolates the VFD from the motor and cables breaking any potential capacitance to earth coupling issues. The transformer works much like a VFD sinewave output filter, but better.

I do find it odd that soft-starters won't work on your system. We have supplied a number of applications over the years where a soft-starter powered a transformer and they all worked fine. Giving the name brands you tried might help determine what is going on.

You have tested the transformer and motor and cables for failures, right?

 

[jraef]

A cheap soft starter that only has 4 SCRs would cause havoc in a transformer fed application like that. So we seem to be getting closer to that being the issue, but again, a make and model of the soft starter would confirm or teject that speculation.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington