FLA on a medium voltage motor

[Tyal51]

We have a reconditioned NEMA motor - rated at 1750 HP 6000V 3 Ph 50 Hz (overseas application). The motor is about 730 ft from the MCC which is fitted with a soft start. The electrical system is supplied by 4 Hyundai generators rated at 6550 V 3 Ph 50 Hz.
The elelctric motor drives a large gas compressor which has the valves removed to allow it to operate without loaded condition.
When the site starts the motor, the SCR in the soft start burns out - we have replaced the softstart several times.The softstart is rated at 150 amps per microsecond, at the cutoff point before damaging the SCR's as advised by the equipment supplier, Motortronics.

Any advise on what may be happening in this system or method of corection appreciated. We are looking at installing "inductors" in series with the softstarts, but these will not be online when the softstart is bypassed.

Second question - the site are considering starting the motors "line to line" - is this recommended with a generator supplied power source.

 

[jraef]

Motortronics had lots of problems with the SCRs they were using on 6600V systems when I worked there a few years ago. I don't know if they have solved the problems by now, but at the time we would insist on line reactors and that seemed to fix it.

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

SCR switching seems to become very problematic with any soft-starter that uses 6kV or higher rated SCR's. The make of devices can make a difference too, since different manufacturers structure their SCR's differently. Motortronics commonly sells units without reactors to have the lowest possible sell price. The line reactors will probably fix it. I can't tell you the details, but it's probably not the di/dt like they are suggesting. Just saying, they likely don't fully understand the fix they are suggesting even though it generally does work.

 

[7anoter4]

30 years ago, ABB provided a synchronous compensator with 6 kV ignitors [instead of thyristors].I don't know if there is a further development of ignitors but they are able to work up to 20 kV and 100 kA. See:

http://www.electricstuff.co.uk/pulse.html#Ignitrons

One could check GaAs rectifiers also.