soft starts tripping main breaker on deceleration 4

[krbernier]

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

 

[krbernier]

attachment did not work...try #2

 

[jraef]

Chances are, your 1600A CB has ground fault built-in as it is a requirement for main circuit breakers at 1000A and above. Sometimes the GF sensing systems used in these breakers can get falsely triggered with non-linear loads with high harmonics, like soft starters. Those that use the Zero Sequence CT method of GF sensing on 4 wire systems can interpret the harmonic current in the neutral conductor as an imbalance in the phase conductors. When a soft starter is in the midst of phase angle firing, there can be high harmonic currents created. Generally these are considered "transient events" with regards to harmonics analysis issues and are ignored, but this is one instance where it can rear it's head. Check the type of GF sensing you have and if possible, you may be able to adjust the sensitivity and or trip delay to prevent this false sensing issue.


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

unfortunately th gf sensing on the trip unit is not adjustable. it is a seimens 545 lig . i have attached the manual for the breaker. maybe you have some advise? page 3 show the adjustments possible.

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

 

[waross]

I would investigate the possibility of shortening the stopping time. I assume that the delay is to allow the check valve to close without slamming shut. It may be that you only need enough controlled stop time to allow the check valve to close gently. Shortening the time from 15 sec. to 12 sec. worked at lower loadings. Perhaps some setting between 12 sec. and zero sec. will do the job.
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

 

[Skogsgurra]

Are you using the SS also for starting the pumps? If so, do you ever get any trips then?

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[krbernier]

shortening the stop time made things worse.increasing the stop time also made things worse. setting the stop time to zero gave us a big water hammer and from the sounds it made cant be good for the pump. the system is set up with a pressure reducing valve. we are pumping the water a good ways uphill and when the pumps stop that water comes back at the pump and when it goes over 65 psi the valve dumps the water back into the clear well.
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.

 

[krbernier]

Gunnar - we have had no issues at motor start just stop

 

[Skogsgurra]

It could be what we observed then.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Didn't find the recordings. But other recordings where gating unit was faulty. You will see similar waveforms if you have DC in the motor.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[krbernier]

just got back from the pump station. the Ii setting on the breaker for instantaneous trip was set at the minimum of 200 x 10 Amps. I adjusted it up to 300 x 10 AMPS and the motors did not trip the breaker when stopped. Hopefully that cures the problem.

i would like to thank all who chimed in with their thoughts!

 

[dpc]

Gunnar,

That was interesting - thanks for sharing that.

Dave

David Castor

http://www.cvoes.com