Looking for some insight as to what happened at a local water plant. A scheduled outage from the utility was scheduled for maintenance. Plant operators stopped all their VFD's and across the line starters. They did not power down the drives, just stopped them before the outage, so they where setting there idle, powered up, with no run signal. After everything was stopped, maintenance gave the poco the okay to take the outage. Power company pulled the 4160V cut outs to their 300KVA Delta Wye 277/480 pad mount one at a time, did their work, and closed the cut outs 30 minutes later one at a time. No issues..... checked voltage at the transformer.... poco lets maintenance know they're done and can restart the plant. Maintenance walks into the plant to find a 75HP 480V ABB ACQ580 with 2 blown input fuses, and a 50HP 480V Toshiba P9 with 2 blown internal input fuses. Both drives where replaced with spares and the plant was up and running. Condition of the drives is unknown but assumed blown rectifiers. Other drives in the facility where not affected. Water plant blames poco...... Come to find out, a week prior to that, one of the bayonet fuses on the primary side of the pad mount was loose from the factory and had been arching to the point of failure, and one leg of the primary was lost resulting in the plant voltage going crazy and drives kicking out. Poco replaced the bayonet the same day, but didn't have the correct size, so had to take the above outage a week later to install the correct size fuse. Alot of finger pointing going around, plant maintenance says facility should have been completely disconnected from utility via main disconnect before power down, and that the fact that the 4160V cut outs where pulled and closed one at a time caused the issue. Poco explains to the facility that they could have a primary fuse blow or line snap during any storm that would, and probably has, recreate above scenario, and the drives should protect themselves and fault out. Open to any ideas as to what may have caused the issue. No line side reactors on either drive. No ferro resonance issue at the transformer. Drives are both about 2 years old.
