Grounding transformer continous current rating
Grounding transformer continous current rating
(OP)
Hi,
I would like to know how to determine the continuous current rating for a grounding transformer.
I know the short-circuit current, so the transformer will be rated for this current for a short time, but i don't know what will be the continuous current under unbalance condition.
Thanks






RE: Grounding transformer continous current rating
The continuous leakage current through the grounding transformer would be due to the triplen harmonics in the system including the third harmonics due to generator design (if the same happens to be part of the system).
If it is LV system and unbalanced load is intended to be supplied, that is a different subject.
RE: Grounding transformer continous current rating
RE: Grounding transformer continous current rating
The kVA rating= V(l-n)(kV)X In (rated)
hope this helps.
RE: Grounding transformer continous current rating
RE: Grounding transformer continous current rating
The IEEE C62.92-2 / Table-1 gives different factors for short time rating of grounding transformers. For example, for 10sec duration, it comes to 9.5% as against 3% in IEEE 32.
Any idea.
RE: Grounding transformer continous current rating
Hi and thank for your reply
I forgot to mention, this is for a MV distribution system, and this system as to be solidly grounded, so the transformer impedance will be little. I think a lot of continuous current will be derived from voltage unbalance. This will create a zero sequence voltage, and a zero sequence current in the transformer. I'm not sure how to determine that.
any idea?
RE: Grounding transformer continous current rating
raghun, the standard C62.92-2 is for generator grounding. I have the std32-1972, which gives 3% for 10 sec, but i wasn't sure if it was enough. Maybe the C62.92-4, which is for distribution give another standard rating for 10 sec, or 5 sec?
Thanks
RE: Grounding transformer continous current rating
RE: Grounding transformer continous current rating
RE: Grounding transformer continous current rating
I hope this example helps. Suppose your system is a 12kV system and your max neutral current demand is 180A (this value you can get by doing a vectorial sum of the phase currents, simulation or from the relays at the station). If the transformer is per ANSI/ IEEE then it should be able to withstand 6000A for 10s which is 24MVA for 10s (if transformer is zig-zag), so the continuous rating of the transformer would be 24MVA x 0.03 which is 720kVA. So you would size the transformer at 720kVA in this instance. Of course you would want to leave some wriggle room for future growth so you would apply a factor to the 720kVA.
RE: Grounding transformer continous current rating
Hi,
Let me reformulate my question in another way
The grounding transformer will be connected to a 27.6kV distribution line, 3 phases 4 wires. I don't have the neutral current, and i don't think utility will give me this info. The only thing i know is that the voltage unbalance will be 3 % maximum on the distribution line (our installation included). With this info, and with the known zero sequence impedance of the grounding transformer, let say 8 ohms, how can i determine the maximum continuous current that will appear in the neutral of the transformer? Can I?
Thanks for your help
RE: Grounding transformer continous current rating
RE: Grounding transformer continous current rating
As opmgr1 says, there is not enough data to calculate the required size for continuous unbalanced current.