Overvoltage on transformer primary 2

[MattRTB]

A 500kva threephase padmounted distribution transformer ( loopfeed with 3,8.5kv elbow arresters) primary voltage 13.8kv DELTA and 277/480volt WYE SECONDARY was energized with no load and while single phase switching and upon closing the last cutout one of the MOV elbow arresters failed with a loud bang indicating an overvoltage ..... could a cable fault on the downstream 480volt cable have caused this or does it indicate a damaged primary winding.

 

[waross]

My first though is a faulty arrester.
Was the arrester on the phase that was closed last?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

A Single Line to Ground fault causes voltage of unfaulted phases to rise.
Thus, if you noticed an SLG fault in 480V cable, that could explain the Surge Arrester operation on primary.

 

[wcaseyharman]

I remember the old timers at my company talking about issues with ferroresonance and single phase switching on underground systems . I’ll have to look back at the older books, but I do know it was a problem with single phase switching on underground circuits with minimal load and could cause very high and very distorted voltages. I think it was more common on 35kV systems but depended on the length of cable attached.
Edit: a quick google search turned up quite a bit on the subject, and it appears that single phase switching on unloaded delta primary transformers is at risk for ferroresonance.

 

[dpc]

Delta-wye transformer? Ferroresonance overvoltage can occur during single phase switching. You might start here:

http://www.powerqualityworld.com/2012/03/ferroresonance-transformer-connection.html

There is a classic IEEE paper from 1965, but it's pay-to-play.

 

[bacon4life]

We try to avoid single phase switching for that configuration to avoid ferroresonance. We purchase each padmount delta/wye transformer with a 3 phase load break switch.

 

[crshears]

In Ontario, Canada, Hydro One specifies that if LV trafos are energized single-phase, the closing device must be a minimum of one pole span away [ commonly a couple hundred to a few hundred feet distant ] for personnel safety; and of course the crew doing the switching always takes the time to ensure that customer employees, other contractors, and MOTP [members of the public] are also clear of the trafo being energized.

If the energizing device is ganged and in good condition, no such restriction is applied. If the single phase closing device is too close to the trafo to satisfy this rule, the source power supply must be taken off potential while the device is closed, even if customer load interruption will take place . . . or at least that's how it was when I retired .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]