Zigzag transformer calculation 2

[slavag]

Dear All.
Im so sorry, Im open new thread with more or less same name.
Zigzag grounding transformer make me crazy .
Data of zagzag transformer (about 20 years old).
Data from transformer ( sent me by mobile, from site )
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

 

[1x1x1x]

Hi Slavag,

The attached file shows the phase (winding) currents and the neutral current when there is a bolted fault on the phase A (24 kV supply).

 

[1x1x1x]

Hi Slavag,

Sorry it is getting late, the earlier post only shows the faulted bus currents and their RMS, I forget to attach the plots from the winding currents.

 

[slavag]

Dear 1x1x1x

Lot of thanks!
Best regards in the New Year

 

[prc]

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)

 

[odlanor]

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.