soft starts tripping main breaker on deceleration
soft starts tripping main breaker on deceleration
(OP)
I have 2 350 hp 480v motors running vertical turbine pumps.
they are controlled by allen-bradley SMC -FLEX soft starters.
The station has a 1600a main breaker and each soft start is fed by a 600a breaker.
The soft starts were set for a 15 second stop time. When a pump is stopped it decelerates for a period of time and then trips the 1600A main breaker. Allen bradley were called in and ultimately set the stop time to 12 seconds. The motors were stopped a few times that day without problem. Now that we are starting and stopping the pumps daily instead of letting them run for weeks on end our tripping problem has resurfaced.
it would seem that vfds should have been used for this project. i am going to try and adjust the breakers trip setting to allow more time before trip. if that works am i causing the system damage by doing this?
The easy solution is to set the stop time to 0, but the engineer who designed the system was not willing to do that the last time we looked at this issue.
Any input is appreciated. attached is the drive configuration file
they are controlled by allen-bradley SMC -FLEX soft starters.
The station has a 1600a main breaker and each soft start is fed by a 600a breaker.
The soft starts were set for a 15 second stop time. When a pump is stopped it decelerates for a period of time and then trips the 1600A main breaker. Allen bradley were called in and ultimately set the stop time to 12 seconds. The motors were stopped a few times that day without problem. Now that we are starting and stopping the pumps daily instead of letting them run for weeks on end our tripping problem has resurfaced.
it would seem that vfds should have been used for this project. i am going to try and adjust the breakers trip setting to allow more time before trip. if that works am i causing the system damage by doing this?
The easy solution is to set the stop time to 0, but the engineer who designed the system was not willing to do that the last time we looked at this issue.
Any input is appreciated. attached is the drive configuration file





RE: soft starts tripping main breaker on deceleration
RE: soft starts tripping main breaker on deceleration
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RE: soft starts tripping main breaker on deceleration
Ir is set to 1600a.
Tr setting is unknown. it says the timer starts at 6 x Ir so i dont think this is coming into play as it would be 9,600a to activate the timer.
Ii setting is also unknown any advise? the trips are intermittent on motor stop. sometimes trip sometimes not.
if this is a harmonics issue causing the gf to trip what are some possible cures?
thanks for your time!
Kyle Bernier
Lead Electrician
Pennichuck Water Works
RE: soft starts tripping main breaker on deceleration
It may be that a long stop time may be needed to allow the check valve to close gently, but once the check valve has closed, the soft start may be turned off and the motor allowed to coast to a stop.
What I mean is a timed deceleration for a 15 second or 12 second stop but the soft start turned off by a timer and the motor allowed to coast after the check valve has closed, possibly in 5 or so seconds.
Once the check valve has closed the remainder of the stop time may not be needed.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: soft starts tripping main breaker on deceleration
I have had peculiar effects with soft starters where the thyristors stayed on during start but were switched off during deceleration because counter-EMF made voltage across them negative prematurely. This resulted in DC in the motor and tripped the CB because of overcurrent.
The SS in this case was a large one - 1200 kW - and more or less custom-built with a separate triggering unit. We could not make it work and had to put a VFD there instead.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: soft starts tripping main breaker on deceleration
i am going down this morning and checking the setting for Ii on the trip unit and will turn this setting up if i can.
to jraef -- there are no neutral loads on the 480v system. all loads are straight 480. lighting is through a 240/120v transformer. are harmonics still an issue? unfortunately my knowledge of power quality and harmonics is practically none.
RE: soft starts tripping main breaker on deceleration
RE: soft starts tripping main breaker on deceleration
Connect a set of DC clamps and record current as the motor ramps down. You should see terrible looking, but symmetrical, waveforms.
If you see waveforms that are unsymmetrical, then you have the DC problem. Will look for recordings from that occasion. Stand by.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: soft starts tripping main breaker on deceleration
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: soft starts tripping main breaker on deceleration
i would like to thank all who chimed in with their thoughts!
RE: soft starts tripping main breaker on deceleration
That was interesting - thanks for sharing that.
Dave
David Castor
www.cvoes.com