Arrester issues on URD system
Got a 7200/12470 Grd Y system fed from the local utility here for a private customer through some CTs and a transfer switch for the customers generators.
The promblem comes when they do a monthly test of the transfer switch and run the generators. They cut over to the generator power and off the utility feed during the test and all is well. But when they switch back to utility it blows arresters all over the place.
Had a big pow wow with engineers and inspected the entire system for abnormalities and all I could find was utility feed was a 3 phase 4 wire system. The generator feed was a 3 wire system with no neutral hook up on the generators at all. This system is in an airport and Im told the all grounds and neutrals are made up to with ground grid for the strips which Im very sure are adiquate.
This system has been in place and worked fine for 20+ years and now all this is happening. And cant seem to find the source of the problem. I nor my company has the proper meters to monitor the system during the tests to see any voltage spikes or anything. Ferroresonance has been discussed and dismissed. As well as the harmonics of the system.
The arresters blowing are 10kv some distribution some riser. Some porcelin, some poly. Theres more hands in this system then just mine so some work done is iffy. Engineers are saying we need to go through and install 15kv MOV arresters and discard all the 10kv stuff. And allow me to clarify that the arresters dont blow like you would normally see them. the bottoms stay intact and they blow out the sides and usually buck phases in the process. (Was standing next to a 3 phase pad mount during a test and it bucked all 3 phases and bulged out the cabinet something fierce! lol)
Anyone have any ideas or suggestions? This is a crucial system that has to maintain its reliability at all times but the engineers and myself are stumpped..