Menghitung Short Circuit calculation Menggunakan MVA method (manual)

Maaf saya tulis dalam bahasa inggris, karena ini saya belajar dari literatur berbahasa inggris.



Utility: 150KV, 1000 MVAsc
Transformer 1: 170 MVA, 150/13.8KV, 15% Z


13.8KV Bus
Generator: 100MVA, X"d = 0.2
Transformer 2: 30 MVA, 13.8/6.6KV, 15% Z


6.6KV Bus
Transformer 3: 2MVA, 6.6KV/400V, 10% Z
Motor 1: 10 MVA (Lumped), 20% Z


400V Bus
Motor 2: 1000 KVA (Lumped), 20% Z
Motor 3: 600 KVA (Lumped), 10% Z






In the event of a short circuit, the sources of short circuit current are
1. Utility
2. Generators
3. Motors


Static loads such as heaters and lighting do not contribute to short circuit.
"EquivalentMVA" are:
Transformers and Motors

Generators


Cables and Reactors



So, here are the results of MVAsc:
Utility: MVAsc = 1000MVA
Transformer 1: MVAsc = 170 / 0.15 = 1133.33 MVA


13.8KV Bus
Generator: MVAsc = 100 / 0.2 = 500 MVA
Transnformer 2: MVAsc = 30 / 0.15 = 200 MVA


6.6KV Bus
Transformer 3: MVAsc = 2 / 0.1 = 20 MVA
Motor 1: MVAsc = 10 / 0.2 = 50 MVA


400V Bus
Motor 2: MVAsc = 1 / 0.2 = 5 MVA
Motor 3: MVAsc = 0.6 / 0.1 = 6 MVA

Now we calculate the upstream contribution :
At Transformer 1:
MVAsc @ 150KV = 1000 MVA
MVAsc @ 13.8KV = 1/ (1 / 1000 + 1 /1133.33) = 531.25 MVA


At Transformer 2:
MVAsc @ 13.8KV = 531.25 + 500 = 1031.25 MVA
MVAsc @ 6.6KV = 1/ (1 / 1031.25 + 1 / 200) = 167.51 MVA


At Transformer 3:
MVAsc @ 6.6KV = 167.51 + 50 = 217.51 MVA
MVAsc @ 400V = 1/ (1 / 217.51 + 1 / 20) = 18.31 MVA


At 400V Motors
Motor 3: MVAsc = 18.31 x 5 / ( 5 + 6 ) = 8.3 MVA
Motor 4: MVAsc = 18.31 x 6 / ( 5 + 6 ) = 9.98 MVA
The fault MVAsc @bus 400V = 18.31 + 5 + 6 = 29.31MVAsc
The three phase If = 29.31/(1.732*(0.4)) = 42.3 kA.




Now we come to fault single phase to ground :
For single phase faults, positive sequence, negative sequence and zero sequence
impedances need to be calculated.
If = 3 (I1 + I2 + I0)


Examining the circuit in above, at the 400V Bus, on Transformer 3 contributes to the
zero sequence current.


For transformers, the negative sequence and zero sequence impedance are equal to the positive sequence impedance.


Z1 = Z2 = Z0 or
MVA1 = MVA2 = MVA0
@bus 400V;
1 / MVAsc =1/3 (1 / MVAsc1 + 1 / MVAsc2 + 1 / MVAsc0)
1 / MVAsc = 1/3 (1 / 29.31 + 1 / 29.31 + 1 / 20 )
1/ MVAsc =


MVAsc = 1/ = 25.4 MVAsc



If= 25.4 / (1.732 x 0.4) = 36.6 kA
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