×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Zigzag transformer calculation
2

Zigzag transformer calculation

Zigzag transformer calculation

(OP)
Dear All.
Im so sorry, Im open new thread with more or less same name.
Zigzag grounding transformer make me crazy smile.
Data of zagzag transformer (about 20 years old).
Data from transformer ( sent me by mobile, from site smile )
S=2078kVA
I rated= 150A
I rated for 3 hours=181A
Short circuit in neutral 1500A for 5sec.
Impedance per phase in Ohm 27.8.
Rated voltage 24kV

according to my understading, max ZS current is
24/1.73/27.8=499A, same calculation as for NGR.
2078/24/1.73=150A it's neutral rated current

But what is 1500A for 5 sec, simple 3-ph symetrical short circuit to ground 3x500A??

Thanks in advance


 

RE: Zigzag transformer calculation

(OP)
Dear 1x1x1x

Lot of thanks!
Best regards in the New Year

RE: Zigzag transformer calculation

It is not clear what is S= 2078 KVA  stands for.May be the grounding transformer has a delta or star secondary to feed auxiliary load of 2078 kVA.

Normally zig-zag grounding transformer will take only magnetising current.But when an LG fault occurs  on say B phase, a fault current of 3I moves from fault point on the line to grounding transformer and moves  I to ground through B phase   of grounding transformer, leaving 2 I to B terminal of feeding transformer which will come out of R & Y terminals, I each, enter in to R &Y phase of grounding transformer,add up at its neutral to 3 I completing circuit through ground. You can see this circuit diagram in J & P Transformer Book 12th or earlier editions.( not in 13th edition)

In this case 3 I =1500 A, decided based on system requirements. To limit fault current to this value, grounding transformer requires 27.8 ohms as leakage impedance.Transformer designer makes zig-zag windings such a way that they  contribute a leakage impedance of 27.8 ohms when 500 A each flows through the windings of grounding transformer during an LG fault.( Fault current /3)

RE: Zigzag transformer calculation

I guess there are some complementation and correction:
A zigzag transformer is designed to provide a low impedance path for zero sequence current flow. In a condition-ground fault zero sequence currents can flow to ground at the point of fault and return through the neutral of the zigzag transformer.
The impedance of the zigzag transformer in a balanced three-phase system is relatively high, so when there is NO fault to ground only a small magnetizing current flows in the windings. It means that Irated= 150A refers the grounding transformer originally dimensioned for delta or star secondary to feed auxiliary load of 2078 kVA.

 max ZS current is 24/1.73/27.8=499A, same calculation as for NGR.


2078/24/1.73=150A it's phase current of auxiliary load system.
It can support 181A phase current for 3 hours.

It must support 1500/3 =500 A phase current during 5sec.

Check:
% N3= k* 181^2 * 3* 60 * 60 ; % Joule- 3-phase energy zigzag can support  during 3 hours
% N5 = k * 500^2 * 5; % Joule- 3-phase energy in 5 seconds
% N5/k = N3/k = 181^2 * 3 * 60 * 60 = 500^2 * t;
t = 181^2 * 3 * 60 * 60 / 500^2; .
t= 1415.28 sec ( > 5s); % time phase windings can support 500A.

the plots for the winding currents for 3-phase load  should be 150A rms,  and zero A for neutral.







 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources