Soft starter and Ground

[SilverArc]

Hi,
I read possibly all the threads on soft starts but my questions about soft starters still remain unanswered. In this part of the thread, I will just explain the background and a small question and as the project will progress I might have some more questions to ask depending upon the dilemma I encounter.

For now only: I have a 300 HP twin-screw compressor (I have no clue about the mechanical loading char. of this load)

A 250 HP axial vent fan (which might be reversed) for a mining application,

I was asked to specify type of motor and starters for this application:
I called the sales engineer for both manf. and the compressor guy advised me a star-delta starter and the fans guy advised me a soft start, as the price is competitive.

I read the FAQ section and some other literature from ABB about selection criteria of starter methods but I guess it helped a little bit. Anyways,
I went through the manual of a soft starter and it has so many settings to be done and being a consulting engineer, I have to advise the contractor about it. I have to still work on settings my self and after my homework, I will come back to this part in near future.
For now
Q # 1 When I was reading a readymade specs from a soft-starter manfu, It talks about the term” High resistance ground" with the starter. How grounding has anything to do with type of grounding of the system? Kindly advise

JRAEF: I apologize, I am sneaking another question:
Could you advise me some grounding literature in IEEE or any part of web.. where I could find out of a situation where I might have two substations in vicinity or a hydroelectric substation and a step up transformer station. I am not, sure what distance makes it mandatory for two sites to be connected with each others grounding grid. If you have some distance that would help.

Regards,

 

[TestBeforeTouch]

A few years ago I tried to put in a GE ASTAT 700HP 480 volt soft starter on a B phase corner grounded delta system and it did not work. GE told me that their design was incompatible with B Phase grounded because of the way they calculate voltages inside the controller.
I am not sure but it may be the same problem for a high resistance grounded system, because if a ground fault occurs on a HRG system all the phase to ground voltages (e.g 277) shift to line to line voltages (480 volts) except the faulted phase which is at zero. This essentially acts like a b phase grounded system during the fault. Therefore during a sustained fault somewhere on the HRG system the soft starter may cease to work.

 

[jraef]

Well of course if you have a fault, you WANT the device to shut off I would think.

I can't speak for ABB, but generally the Ground Fault issue is that many soft starters have Ground Fault protection built-in, but use the Residual Current method of detecting it. In a resistance grounded system, the ground fault current may be so limited by the resistor that this method may not detect the GF and NOT shut down. Usually for resistance grounded systems then, they will recommend using a supplemental GF relay that uses a core balanced Zero Sequence CT. Some soft starters have that ZSCT option available in their catalog for this reason.

As to the the above issue, that is in fact another problem in design. Many soft starters are based upon the original Saftronics design where they calculate the zero-cross points to be used in the firing scheme by monitoring only 2 of the line voltages. If they become severely asymmetrical, the gate firing circuit begins misfiring, which tends to exacerbate whatever other problem was going on at the time. Not all soft starters are designed this way however, so if you have some sort of unusual system, discuss it with your supplier before implementing it, then try another source. If they are not able to get an answer to your question, that's usually a good indicator that they are not a good supplier.

JRaef.com
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