We have the odd problems relivening the older 11 kV polyloom cables in our network, after either transformer maintenance or upgrade.
These are 11 kV single core screened cables.
These circuits are usually fitted with ddo's and to avoid the inrush surge, we either liven 1Ø first, then the other 2Øs at the same time with 2 sets of fuse sticks.
Or the other way around, liven 2Øs first, then the third phase.
We isolate the secondary transformer fuses to avoid single phasing of motors, do a secondary voltage check before inserting the LV fuses.
Ok easiest would be to drop the feeder briefly, insert ddo's and liven all 3Øs at once, this usually avoids the ferro resonance problem. Of course our controlroom doesn't like that so it's out of the question.
I have heard of putting a resistive load on the TF secondary. like a 2 kW heater to "soften" the inrush to the transformer, Not had much luck with it yet.
Usually it's BANG !! and one ddo blows up, we have a cable fault on the polyloom and put in a generator(s) for the time being if we can't backfeed the 11 kV.
There are lots of theoretical explanations of above topic on Google but has any one practical experience with this fenomena and the best way to minimise the blow ups?
The best way is of course replace the cables, they are over 30 years old and are not armoured, only screened with a thin Cu sleeve and then black PVC sheeth.
Thanks, Ray
