Ensuring availability of a large Adjustable Speed Drive for process gas compressor application rated 11 kV, 15.5 MW (20778 hp)

Voltage source Inverter (VSI) technology, suitable for synchronous motor applications, now exists with high power ratings producing high output voltage (ges11 kV) with significant benefits over the widely used load commutated inverter (LCI) drives. The paper covers an existing VSI technology applied for the first time at a higher operating voltage. This 15.5 MW, 11 kV VSI Adjustable Speed Drive (ASD) installation is used to soft start three synchronous motors and speed control of one of these for a process gas compressor application. Challenges faced during commissioning are described and associated solutions are presented. A failure analysis of the ASD during commissioning is presented. An ASD design review was undertaken which has resulted in an improved alternative to the conventional HV busbar design. A protection co-ordination study incorporating incoming switchgear and internal drive protection functions is presented. Improved protection schemes and settings are described to obtain faster coordinated fault clearance and improved ASD availability. The paper provides operating experience since the modified ASD was placed in service in January 2006. Key recommendations are made to enhance the reliability, availability and performance of medium voltage drive systems. Also, suggestions are made for additions to the 2006 IEEE 1566 medium voltage ASD standard.