Detecting Wye Side Faults Through a Delta (on a Delta-WyeGnd transformer)

[paddymurphy]

I have a 2500kVA 13.8kV delta - 480Y/277 WyeGnd service transformer that serves a building. The owner is putting 2MW of PV in a field beside the building, which will be connected at the main switchgear (on the 480Y/277 side of the transformer). So the secondary feeder from the transformer and the PV feeder will end up connecting at the same location in the building's main switchgear.

My concern is that for a single line to ground fault on the high side (delta side) of the transformer, the next upstream protective device on the 13.8kV system will open, but the DG could still keep feeding the fault. I would need to somehow detect the fault, and disconnect the DG. And (as is the case with everything) it needs to be done in an economical manner.

Any advice??

 

[Marmite]

Do you have any VT's on the 13.8kV side?
A single line diagram would be useful.

Regards
Marmite

 

[waross]

Your local utility will have conditions for supplying power to their system. Some of the conditions will be the type of protection that they will require.
Connecting and feeding power into the grid without prior consultation and agreement with the utility is a really bad idea.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

There are protective relays that can be used, but most require a high side PT.

 

[stevenal]

The post and the title don't match. If you have a single line to ground fault on the high voltage delta side, any sources on the wye side will not feed the fault.

 

[HamburgerHelper]

If there is a ground source on the high side, the dg will feed positive and negative sequence currents through the delta to the fault.

 

[paddymurphy]

My local utility says that they would need "zero sequence overvoltage detection", commonly known as 3V0. Can anyone explain how that works?

 

[cranky108]

3V0 = Va + Vb + Vc (vector addition).

 

[davidbeach]

Anymore it's just a bit of math in any decent relay. Provide the relay with a 3-phase wye set of VTs and set the 59N and you're good to go.

 

[cranky108]

Come on david and do it the old way with the broken delta PT's.

 

[davidbeach]

A necessary evil back in the day. Heck, you could even get away with a single line-to-ground VT and apply a simple 27/59 scheme on it, but just because you can do something doesn't mean you should. ;-)

 

[cranky108]

Ask the utility to provide an example of there perfered methods of detecting zero sequence voltage. There is no reason they can't provide that.

And ask for there recommended time to disconnect to prevent reclosing out of phase.

 

[Kiribanda]

paddymurphy,
When the project proposal is made normally the Utility will do an impact study on their system due to 2MW PV installation. Back feeding during a Utility
ground fault is one concern. That is why Utility always recommends to have the step-up transformer with WYE grounded on high side and DELTA on your side
so that during a Utility ground fault still their system is effectively grounded and tripping can be effected. Now in your case it looks like due to
one reason or the other the Utility has not done their home work. They are asking you to provide a scheme to detect a ground fault in their system.
Having said that using 3V0 is one simple method. You have to have three VTs where primaries are connected in WYE grounded on Utility DELTA side and the
secondary's of the three VTs are connected in BROKEN DELTA method and in turn it is connected to simple overvoltage relay (59N). When there is a
solid ground fault on DELTA side you will get full 3 times zero seq voltage = 3xsingle phase to ground voltage. The relay is alarmed/ trip
with a little time delay. In some applications (per G59 UK code it is called Vector shift protection).