Auxiliary Earthing Transformer rating requirements.

[electrical429]

As per IEEE Std C57.32 section 6.4.2 extract below Earthing Transformer continuous rating must be 3% of 10s thermal rated current.

We will be installing ZNyn11 Auxiliary Earthing transformer for the purposes of limiting 132/33 kV YNd11 power transformer 33 kV side earth fault current to 1000 A and providing site LVAC supplies. Aux. ETx will be rated at 1000A for 10s. Site LVAC load is quite small, ~200 kVA. If you go by IEEE Std C57.32 we would require Aux. Tx thermal rating of:

Instr = 1000 A for 10s
Instr per phase = 1000A / 3 = 333 A
Snstr = 3 x 33000 V/sqrt(3) x 333 A = 19.033 MVA
Based on IEEE Std C57.32 continuous rating of transformer should be 3% x 19033 kVA = 517 kVA

What happens if install 200 kVA Aux. Tx rated at 1000 A for 10s? I've seen designs where 200 and 300 kVA Aux. Earthing transformers were rated at 1000A for 10s.

 

[RRaghunath]

You need a primary winding rated for 517kVA (to be able to carry the required earth fault currents for 10s) and the LV winding need not be rated to 517kVA (and can be rated to a lower value) as your load is only 200kVA or 300kVA.
I am sure transformer designer would have done the same while providing you the specified transformer.

 

[electrical429]

10s rating is 19.033 MVA. 517 kVA is continuous rating. My question is where does 3% in the IEEE standard comes from? And does "unless otherwise specified" means that you can choose any continuous rating independent of thermal rating figure (19.033 MVA).

 

[prc]

1) The 3 % comes from transformer design. Copper temperature at the end of 100 % current for 10 sec will be same as at the end of 3 % current continuously.
2) Please refer to IEEE standard 62.92.4-2014 Guidance for the application of grounding transformer for a typical calculation for sizing of grounding transformer.
3) If you are planning ZNyn connection for grounding tarnsformer and planning to load the secondary of grounding transformer with unbalanced loads, please provide a stabilizing delta winding as extra.
4) You can reduce the grounding transformer current by providing a neutral resistor to the neutral of grounding transformer.