Sir,

In 1250 KVA Delta/Delta connected transformer, Secondary R.phase winding grounded!
Please give me the reason -
Why secondary side of one winding grounded?
What about eath fault trip? and
Why No over current?

 

Hello,

It is the impedance of the transformer (total resistance, which includes 1) pure resistance of the winding wire and more importantly the inductive reactance... which prevents a earth fault/overcurrent, or direct short of the AC transformer when one winding is grounded... (hope this answers part of the question, the other part of the question, e.g., the "why" it is grounded, I will have to get back with a concise explanation...)

RTK

 

Responding to Anonymous' May 26, 12:52 am query:

A) The connection observed is called the 'corner-earthed delta' or 'red-leg' system. (Thread #1026198752 lists other names.) The application to a 480-V Low Voltage distribution system, while not unheard of, is unusual. But, more detail is required to provide an answer to your "Why" question! For instance, the choice was made for economic purposes or to correct electrical design deficiencies.

B) Over-current protection must be provided. If the earth-plane or conductor and phase conductors are in close-proximity then earth-fault current magnitude is essentially that of a ph-to-ph fault. However, if differentiation between the two is essential, or the system is not "effectively" earthed, an earth-fault relay must be provided in the earthing conductor, that is, the connection between the transformer
terminal and earth.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

I was searching the web for a subject reated to the transformers, then I was forwarded to this page. Perhaps you have received the right answers to your questions, if not please let me know.

Barry Mirzaei, P.Eng. (PE)
Canada, Toronto

 

Just out of curiosity, how to you size up the Corner-Grounding cable (how many amperes to carry?) for this Delta-Delta transformer which is protected by an Earth Fault relay?

Is the cable sizing to be based on 10% of the total current output of the transformer at which the EF setting is normally set?

Thank you.

Condor

 

Responding to Condor's Sep 29, 9:34pm query... size is based on the following:

A corner-grounded (also called a phase-grounded) system conductor is always considered as current carrying.

The portion between the transformer terminal to the system ground is called the Grounding Electrode Conductor or GEC!

GEC size should not be based on protective device setting. Instead, GEC size should be based on the size of the largest Ungrounded
Service-Entrance Conductor. The sizing method, as well as exceptions, are detailed in NEC Article 250.66!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])