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Joined: Jan 2013
Posts: 2
New Member
I have a project where we are providing backup power to a CNG compressor fused for fueling buses. The generator starts normally upon loss of utility power. The manual transfer switch is closed. The generator is powering a motor control center which under normal conditions runs 3 CNG compressors, but is programmed so that, under emergency power, only one compressor will start and run.

The generator is a Stewart Stevenson 750 kW generator that is capable of providing over 2200 starting kVA. The generator circuit breaker is a 1200 ampere GE Power Break II with an GE Entelliguard TU 1200 ampere solid state trip unit. The transfer switch is an ASCO 3000 ampere manual transfer switch (runs 3 motors in normal mode...remember). The Compressor starter is an ABB soft start. The motor is a 500 horsepower compressor that starts unloaded, then starts loading up to FLA.

The problem is that the generator breaker continues to trip after the motor has started and run for over 3 minutes. We have adjusted the circuit breaker trip settings as needed to allow the motor to start. We attached an Fluke recording meter and sur enough, the motor starts (you can see the inrush to 2600 amperes which decays within a few seconds to less than 400 amperes, then loads up to just over 600 amperes) and runs for 3 minutes, then the breaker trips on Phase B overcurrent. Yet the 3-phase trace from the meter shows normal running current on each phase.

Of course GE says that there is nothing wrong with their circuit breaker. I have over 20 years of experience of doing circuit breaker coordination studies so I know that the trip settings I have specified are correct. The Electrical Contractor says that he entered all the settings properly. Yet the 1200 ampere circuit breaker continues to trip at about 600 amperes after about 3 minutes.

I am hoping someone out there has experienced this before and has an idea about what is going on.

Joined: Jul 2007
Posts: 1,337
Welcome to the board. Do you have access to another breaker to swap it out to see if the fixes the problem. Even GE can have a lemon get through QC

"Live Awesome!" - Kevin Carosa
Joined: Jun 2004
Posts: 1,273
Your solid state circuit logic is getting poluted by your SDS power source.

1) Check to see that its path to the neutral is as sweet after the power source is changed.

2) Consider putting a ferro-resonant transformer/ ferrite rings around the power going to the solid state trip unit to stop its logic from being tripped out by spurious harmonic wave action.

( Compared to the loads involved, the actual current needed by the sensors is a joke. -- Meaning that feeble peaks in far harmonics that affect it would never cause distress in any other element -- and never get picked up by the EE's in quality control. )

You should put your mind to rest INRE the actual trip settings. Your problems lie entirely within the trip logic -- the actual engineering of the factory is not too willing to hand out -- trade secrets and all.

Suffice it to say that if the power supplied to the logic board is scratchy, if it's poorly grounded vis a vis the SDS, you've got troubles.

The 180 second delay makes one think that its logic is filling up a cap in an RC timing circuit. Time-linked behavior almost always points to digital logic. Nothing else gives you clock-work failure modes -- and no smoke let out.

Joined: Jan 2013
Posts: 2
New Member
Thanks for the responses. I have been telling the folks on the jobsite that we are probably having harmonics issues and that we need input filters on the feed going to the soft starters. But who listens to the Engineer? Thanks Tesla

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