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132/33kV YNd transformers in petrochem

132/33kV YNd transformers in petrochem

132/33kV YNd transformers in petrochem

(OP)
Hi,

I recently came across YNd1 and YNd11 transformers at 132/33kV (or similar) in new petrochemical installations. The 132kV system is solidly earthed at the star points of the generator transformers. The 33kV system is resistance earthed using zig zag transformers.

Question 1: What are the benefits of using this configuration instead of the more common one with Dyn11 or Dyn1 transformers?
Question 2: If the 33kV system supplies transformers only, does it have to be resistance earthed or solidly earthed is acceptable?

Thanks in advance...

RE: 132/33kV YNd transformers in petrochem

High voltage system is usually solid grounded in order to reduce the insulation stress in a ground fault case. Also, the protection has to clear the fault in very short time. The Generator step-up transformer neutral is usually insulated and connected to ground by means of a surge arrestor. A grounding disconnecting switch could connect directly to grounding if it is required.
The medium voltage may work with one ground fault for a while then is better to reduce ground fault current. The system may be designed so that the ground potential is still reduced [not the full grounded phase potential] using zig-zag transformer and other.
 

RE: 132/33kV YNd transformers in petrochem

At 33kV the solidly-earthed system fault level may well be too high for circuit breakers to open, or at least that may have been the case when the 33kV system standards were defined perhaps 70 or 80 years ago.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: 132/33kV YNd transformers in petrochem

(OP)
Thanks for the responses!

And how does a star secondary (resistance earthed) compare with a delta secondary (resistance earth via zig zag transformer) at 33kV? What's the best technical solution?

RE: 132/33kV YNd transformers in petrochem

In case of a delta secondary, the zero sequence component of the fault current / unbalance current does not flow in the system. It circulates within the delta connected winding only. This configuration is used when the load is sensitive to harmonics. The zig-zag transformer provides a neutral reference which would otherwise have been absent in a delta system and this becomes quite essential to protect the transformer earth faults very near to the neutral.  
Star secondary at 33 kV is used predominantly used in a 3 phase 4 wire system. Resistane earthing serving the purpose of limiting the fault current.
   

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