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.

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.

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. lightning is through a 240/120v transformer. are harmonics still an issue? unfortunately my knowledge of power quality and harmonics is practically none.

MORE NEWS