Big enough usually works. Perhaps a more meaningful question would elicit a more meaningful answer. What are you using it for? What do you want to accomplish? Why zig-zag instead of some other configuration? Etc? Etc?
I got 30MVA YNd1 stepdown transformer 132/33kV. The primary side is YN dan the secondary side is delta. The primary side neutral will be solid earth meanwhile secondary side I need zig-zag transformer to create artificial earthing (later will connect to NER). By doing this my earthfault relay will be able to function when single phase fault occurred.
But I do no know how to size my zigzag transformer in my single line diagram.
Size the Zig-zag to carry the NER current.
You may use three off the shelf 33 kV distribution transformers connected in or a wye:delta for grounding. Some advantages are:
Lower initial cost.
Replacements are readily available.
Special order is not required.
The current limiting resistor may be inserted from either the primary wye point to ground or in series with the delta secondary. This gives a little flexibility when selecting the resistor or impedance.
Bill
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"Why not the best?"
Jimmy Carter
Normally earthing transformers are designed to carry the maximum permissible earth fault current for 30s. At the end of 30s the copper winding temperature must not exceed 250C. Normal practice is to limit the earth fault current to the full load current of the transformer. You have a choice of specifying a self protecting high impedance earthing transformer which will limit the fault current to 524A by virtue of its own impedance, or you can specify a low impedance type which uses an external earthing resistor. You are showing neutral switchgear on your SLD including a bypass direct to earth. Lose the switchgear and go for a solid connection either direct to earth or via a resistor, otherwise you will complicate the specification of the transformer. It's not clear what other information you require to "size" your transformer, other than the normal system parameters and the information you have from this and earlier posts. You may want to consider providing an auxiliary winding to supply substation LVAC services. This is normal practice in the UK.
Regards
Marmite
so, from the above info, Can I say this 'the stated KVA rating for zig zag transformer is actually to stating the kVA rating for auxilary winding size'? (purpose of auxiliary winding is to supply power to substation LVAC services)
azamfaez77,
What maximum Ig (ground fault current)do you think your NER will be subjected to? Since you indicated that your NER is rated for 30 seconds (this is new to me, the usual ratings are 10 seconds or 1 minute), recommend that you rate your grounding trafo to withstand the ground fault current longer than your NER (>30s), say 1 minute. Your power multiplier for a 1 minute fault is 7% (3% for a 10 seconds rating).
Here's my guess (not enough data for me):
Assuming a full winding voltage appearing across your NER, your fault current will be 33kV/sqrt(3)/36.3 ohms = 525 amps! Your zigzag transformer winding current would be 1/3 of the fault current = 525/3 = 175 amps. Zigzag transformer rating will be sqrt(3)X 33,000 X 173 X 7% = 700 kVA.
Fault current magnitude and duration, plus auxiliary loading if applicable are the main factors in sizing the transformer. Assigning a rating in the traditional sense is not appropriate as the transformer has to be adequately sized to cope with the short term thermal heating plus the mechanical forces resulting from the passage of fault current. This results in a transformer that is more generously proportioned than the "kVA rating" would suggest.
Regards
Marmite
some utilities use transformer with connection Yn/d or Yn/Yn/d for transmission systems of 230,138, 88, and 69 kV.
88 or 69kV delta are grounded by zig-zag transformer.
zig-zag transformer is sized for 3 < X0/X1 <10.
That is to say that the short-circuit current phase to ground should be equal to or less than the three-phase short circuit. When X0/X1> 10 the system tends to become high-impedance grounded and lightning arresters (unfaulted phases) during a fault-to-ground are damaged due to overvoltage.
It is time to change the zig-zag.
