Protection issues for under-impedance relay used as starting supervision for large synchronous motors

Some petrochemical plants (low density polyethylene type) always require the use of big synchronous motors to feed large power compressors (typically from 10 to 30 MW).In certain supply network conditions, it could happen that the motor undergoes a critical starting which takes several tens of seconds to be completed. "Critical" means that the starting time is higher than the time the rotor can withstand in blocked condition: therefore, it is necessary to assess the actual motor rotation to be sure that the motor is being self-ventilating during its acceleration and can survive the critical starting.By means of numerical simulations, it is demonstrated that the solution of using an under-impedance relay (ANSI code 21) for starting supervision, as is proposed in technical literature since many years, gives some troubles for the correct and safe protection of the starting of the motor, with respect to the most common solution of adopting a supervision switch relay (ANSI code 14) based on rotor speed measurement.