GF Coordination Y-G/Y-G Solid ground XFMR

[SindhuK]

Hello Everyone,

I have a 3MVA 34.5kV/480V Y-Y solid ground transformer, the primary and secondary neutral terminals are tied together and grounded near the transformer. The primary cable from the 34.5kV switchgear feeder is 4/0AWG 3-wire (no neutral wire). On 480V side I have a switchboard with 4000A main and 2500 A feeder breakers LSIG type with ground fault protection feature.

My questions are:

1] Does the high side ground fault relay need to be time coordinated with low-side 480V breaker ground fault curves?
2] How does the primary side relays see the ground faults on 480V bus, as a phase fault or as a ground fault ?
3] Not having a neutral connection between high side switchgear and the transformer primary, will effect the zero sequence current flow?

Thanks in advance.

 

[ScottyUK]

If you download the old GE white paper "The Whys of the Wyes" it will answer these questions and more. It is old but still very relevant. Find it in the Transformers section at

https://www.geindustrial.com/white-papers

 

[arrehman1]

1] Does the high side ground fault relay need to be time coordinated with low-side 480V breaker ground fault curves?

Yes absolutely. The highside protection should coordinate the lowside protection... the curves should be stacked and the highside curve should backup the lowside curve. Well, this is how it's done at my company.

For example, if a LG fault occurs on the low side feeder, we should expect the low side feeder protection (relay) to pickup and trip the feeder breaker (or switchgear) on the low side. For a Wye-grounded Wye-grounded transformer, the highside relay will see a line-to-ground fault on the low side (search for zero-sequence impedance diagram for transformer connections). So the curves need to stacked and the highside need to act as a backup protection for faults on the low side.


2] How does the primary side relays see the ground faults on 480V bus, as a phase fault or as a ground fault ?

When learning about something, it's sometimes really difficult to find a place to start. So for this question, do some researching on "zero-sequence impedance diagram for transformer connections"

To be very clear, the high side relay will see a ground fault on the low side for a wye-grounded wye-grounded transformer connection.

3] Not having a neutral connection between high side switchgear and the transformer primary, will effect the zero sequence current flow?

Your question will be answered by researching "zero-sequence impedance diagram for transformer connections"

But to give you a clear answer, for a wye-grounded and wye-(ungrounded) transformer connection, the highside relay will not see a ground fault on the low side.

Good luck!

http://GeneralPAC.com

- A platform to ASK, LEARN, SHARE, & GROW in power protection, automation, and controls.
The Wye Wye Transformer Connection video:

http://gpac.link/1GdGRSQ

 

[dpc]

You will end up with a high zero sequence impedance looking from the low side due to the lack of a grounded conductor between the primary source and the primary wye point. You are dependent on the impedance of the earth, which will be high. If the zero sequence impedance is high enough, you will need to consider if the transformer has a 5-legged core or 3-legged core. With a 3-legged core, you may get flux flowing through the tank and this will create a "phantom delta" winding to lower the Zo.

 

[jghrist]

You may not have a separate neutral on the 34.5 kV cable, but you will have a grounded cable shield. If it is grounded on both ends, it will provide a metallic path for ground current. The zero-sequence impedance will be lower if you have a separate neutral.