Design considerations when applying large compressor synchronous motors to an SVC controlled weak power supply system

Three 7000 HP synchronous motors driving 360 r/min hydrogen compressors were applied to a refinery with a utility supply. A process requirement that the motors ride through an upstream fault which could be cleared in seven cycles required a large flywheel which exacerbated starting times. The pulsation frequency of the motors coincides with the response time of the static VAr compensator controls, leading to a potential for positive feedback destabilizing the compensator. To analyze the problems requires developing a transient electrical model of the motor, and a dynamic torque pulsation model of the motor-compressor-flywheel system. The results from the models are combined to establish the watts and VArs at the pulsation peaks and valleys. The use of readily available commercial loadflow programs to calculate the magnitude of the voltage variations on the supply stream is thus possible