soft starter still tripping main breaker

[krbernier]

i have 2 smc-flex soft starters running 350 hp motors at a water lift station. the main 1600a incoming breaker is tripping when one of the motors is shut off. sometimes it trips sometimes it doesn't. i finally got the use of a fluke 435 power quality meter and was able to record the main when it tripped.

i have a recording of a normal shutdown and a shut down with a trip.

when this happened both motors were running and only one was shut down. stop time is 15 seconds. im going to attach both recordings hoping someone with more knowledge can help!

 

[electricpete]

which may or may not be enough to trip it

actually for some reason if there is individual pump protection, it seems that it is not coordinated with the main breaker.

Attached I have put 2 of the pictures into powerpoint and resized...may be easier to read

=====================================
(2B)+(2B)' ?

 

[krbernier]

the current increase is the motor stopping. each motor pulls 400A running normally. the softstarts do not fault. the fuses in the soft starts dont blow and the 600a breakers feeding the softstarts dont trip. the 1600A main trips. the instantaneous trip setting is set to 4600A. so the breaker is not tripping on overcurrent.

im thinking of changing the trip unit in the main breaker to one that has an adjustable gfi.

i am also thinking about line reactors on the drive outputs but i am still unsure what is the best option.

 

[electricpete]

I see a dramatic increase in current when the motor is shut off.

It suggests either:
1 - the remaining pump is overloaded... either due to system requirements or maybe backflow through the secured pump (discharge check valve malfucntion?)
or else
2 - the pump being secured is ramped down in voltage slowly and so draws increasing current during shutdown?

=====================================
(2B)+(2B)' ?

 

[krbernier]

i have alot more readings from these motors...would you like to see the 2 sets of power harmonics recordings?


i also have the complete recordings in fluke power log if you have that installed..actually i have a copy of power log you can have if it will help

 

[krbernier]

number two would be correct...the softstart reduces voltage to slow the pump before dropping it out completely...this is set at 15 seconds stop time to avoid water hammer

 

[krbernier]

here is the fluke logger program

 

[electricpete]

So, an increase in current is expected during shutdown. Do you have a long-time overcurrent trip on your main breaker that is tripping? How close does it come to the current excursion seen during normal shutdown?

=====================================
(2B)+(2B)' ?

 

[electricpete]

What I meant is: if you plotted time vs current during shutdown, how close does it come to the time/current curve of the main breaker time overcurrent trip?

=====================================
(2B)+(2B)' ?

 

[krbernier]

i am way below all trip settings on the breaker...

 

[electricpete]

then... the breaker is malfunctioning due to trip with current far below all settings?

=====================================
(2B)+(2B)' ?

 

[krbernier]

max current is 1500 a trip unit is set at 1600. long time delay doesnt activate until 6x trip unit setting. instantaneous trip is set at 4600A.

attached is both recordings in power log format incase you decide to install it.

 

[krbernier]

i believe the breaker is tripping because of the gfi. the harmonics are "fooling the gfi trip unit because they are in the neutral.

 

[jraef]

When you soft stop using a solid state starter, you are doing phase-angle voltage reduction and that is inherently harmonic rich, especially as the phase angle is at the lowest levels. So in this case that would be right when the soft stop function is almost complete. You can technically mitigate it somewhat with load reactors, but that will also cost you in added permanent losses in the motor circuit. Most likely though, you are probably right that the GFP on the breaker is picking up some of the harmonics as a current imbalance and falsely tripping. Can you increase the delay time on the GFP? It might be one of those issues that can be easily solved by understanding and adjusting to the reality rather than chasing a cure.

If I remember correctly, the older A-B SMC soft starters had an issue with how they did the Decel function and as a result, they had to disable the OL protection in the starter while soft stopping. Their original Pump Stop system was closed loop with a PF feedback but if the pump response took too long, it stayed in the voltage control mode and caused an OL trip before it detected that the valve had closed. This came to my attention because of a spectacular (and very expensive) series of large pump failures I heard about at the Las Vegas Valley Water District during an IEEE meeting I presented at down there. But to be fair, I was told they fixed that when they released the SMC Flex a few years ago.


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[krbernier]

unfortunately the gfp is non adjustable on this trip unit.

i am leaning towards replacing the trip unit with one that has an adjustable gfp.

 

[jraef]

Might be the least expensive solution. Or disable it in the breaker (i.e. un-wire the shunt trip from the sensor) and use an external GF relay wired to the shunt trip. What breaker is it?


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[krbernier]

seimens type vl breaker

pg frame 1600 amp

right now it has a 545 lig trip unit with fixed gpf sensing and delay.

im going to replace it with a 576 lsig trip unit that is fully adjustable

 

[jraef]

Oh yeah, that would be easier. The 545 trip unit is referred to as "out of the box GF coordination" but in reality it's just the least expensive way out of the requirement, so nothing is adjustable.


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[krbernier]

after further scrutiny of the logs. there is a huge amperage imbalance happening during motor stop. at the point of trip the amps are L1- 4000 A L2 4500A L3 1500 A.

the 4500a is causing the breaker to trip on the instantaneous setting.

 

[jraef]

4500A instantaneous trip setting on a 1600A breaker is pretty low in my opinion, that's only 281%. For example on the 525 trip unit on a 1600A VL breaker you can't set the IT below 7000A. The 545 trip unit allows you to set it as low as 1.5X In, but that doesn't mean that's a good idea. Has anyone ever done a Fault Coordination Study at your facility? Sounds as though you need one. At the very least your IT setting of the main should be higher than any down stream device, unless you like having a complete shutdown.




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[krbernier]

this facility was engineered and installed by the electrical contractor. the soft starts have 800 amp time delay fuses in them. they are fed by 600 amp circuit breakers. other that a lighting transformer these are the only loads on the switchgear. i will try adjusting the IT setting to 5600a (350%) hopefully this will allow it to ride through the shutdown. per nec code i cant adjust any higher correct?