Grounding transformer continous current rating

[tem1234]

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

 

[RRaghunath]

tem1234,
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.

 

[jghrist]

The current in each winding of the grounding transformer will be the zero-sequence current. If you add the phase load currents vectorially, the sum is three time the zero-sequence current.

 

[opmgr1]

The grounding transformer on delta connected system is not design to facilitate unbalance conditions unless due to a fault and as such these transformers are designed to carry rated current for a limited time and is used in conjunction with relay 51G to detect earth faults.

The kVA rating= V(l-n)(kV)X In (rated)

hope this helps.

 

[opmgr1]

A point to also note is that if the grounding transformer you are using is a ziz-zag transformer then the rating stated above can be further reduced by a factor of sqrt(3). IEEE Std32-1972 requires the transformer to continuously carry 3% of the rating for a 10 sec. rated unit, and 7% for a 1 min. rated unit. If your transformer id designed per ANSI/ IEEE then you can use the MVA and Voltage parameters to determine the magnitude of the neutral current tnat the transformer can handle continuously and based on your system parameters you can work backwards to determine the percentage imbalance that the system can withstand before the 51G/N operates.

 

[RRaghunath]

opmgr1,
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.

 

[tem1234]

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?

 

[tem1234]

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

 

[jghrist]

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.

You have the cause and effect reversed. The load imbalance creates zero-sequence current in the transformer. The zero-sequence current creates zero-sequence voltage.

 

[opmgr1]

The 3% stated in the thread is the continuous rating. The short time rating of the 10s rated unit is 33 times the continuous rating. This is approx. 3 times the short time rating of the transformer used for neutral grounding at the generator unit. An interesting point to note is that the grounding transformer described in the previous thread is 3-phase and the generator neutral grounding transformer is single-phase. Ring a bell?

 

[opmgr1]

team1234,
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.

 

[tem1234]

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

 

[opmgr1]

In the absence of the neutral current data the only other way is to calculate the ground fault current and apply the IEEE Std 32 to this value to get the continuous neutral current demand. There is not enough information in your thread to make this calculation.

 

[jghrist]

Is this grounding transformer being installed because it is a utility requirement for a distributed generation application? If so, the utility shouldn't expect you to size it to supply an unlimited load unbalance on their feeder. I would propose sizing the transformer for the fault current and protect it so that it will trip off line if the continuous unbalanced current exceeds the transformer rating.

As opmgr1 says, there is not enough data to calculate the required size for continuous unbalanced current.