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
.
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
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






RE: Zigzag transformer calculation
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).
RE: Zigzag transformer calculation
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.
RE: Zigzag transformer calculation
Lot of thanks!
Best regards in the New Year
RE: Zigzag transformer calculation
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
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.