A customer contacted AEC to investigate potential power disturbance on the control power of an automatic transfer switch (ATS) or backup generator during cycling of the uninterruptible power system (UPS) utilization equipment from normal power to generator mode. Upon transfer to generator mode, the generator would inadvertently or prematurely shut down.
Individual power analyzers were connected at the load and generator sides of the automatic transfer switch supplying the telecommunication facility UPS equipment. By manually removing and restoring normal power, the ATS was cycled through several normal to generator mode transfers and retransfers. Steady-state trending values and non-steady state disturbances were continuously recorded and documented at each monitor.
The testing results and analysis:
The steady state measurements showed that the average voltage unbalance was negligible and the frequency did not vary noticeably. The total voltage and current harmonic distortion (THD) was not unusual during the transition periods of the type of loads served. The non-steady state measurements showed that the generator was provided only a 2 seconds time delay to reach no load speed and unexpectedly shutdown approximately 35 seconds after transfer from normal to generator power and the. Further review of the event details showed low frequency oscillatory transients of 723.0 Hz.
A recommendation to provide 30 seconds time delay for the ATS transfer to the generator provided other ATS’s and their related loads a chance to be brought on line prior the UPS. The effect of pre-loading the generator, especially with some inductance stabilized the generator before connecting the UPS. The customer was very pleased to receive a simple solution to a complex problem.