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.