Calculate Short Circuit Current at Sub Panel (End user side)

November 30, 2024 3 Comments


EXAMPLE:

  • Calculate short circuit current at Electrical Equipment Panel / Distribution Board at end user side.
  • HT Power is received by 11KV RMU and given to 11/0.415KV,1000KVA Transformer by 1 no of 3Cx185 Sq.mm 11KV HT Cable of 25 Meter Length.
  • Resistance and Reactance of HT cable are 0.21 Ώ / KM and 0.1 Ώ / KM
  • Transformer impedance is 6.25%.
  • LT Side of Transformer is connected to Main LT Panel by 8 no of 3.5Cx300 Sq.mm cable of 70 Meter.
  • Resistance and Reactance of 3.5Cx300 Sq.mm, XLPE cable are 0.129 Ώ / KM and 0.071 Ώ / KM.
  • Main LT Panel is connected to Sub Panel-1 by 1 no of 3.5Cx300 Sq.mm cable of 80 Meter.
  • Resistance and Reactance of 3.5Cx300 Sq.mm, XLPE cable are 0.129 Ώ / KM and 0.071 Ώ / KM.
  • Main LT Panel is connected to Sub Panel-2 by 1 no of 3.5Cx95 Sq.mm cable of 80 Meter.
  • Resistance and Reactance of 3.5Cx95 Sq.mm, XLPE cable are 0.409 Ώ / KM and 0.072 Ώ / KM.
  • Calculate Short circuit Current at Sub Panel-1 & Sub Panel-2.

1

CALCUALTION

  • Short Circuit Current is calculated at various points of Power Distribution Networks, At Power Receiving Point (11KV), At Main Panel Point (0.415V) and At Sub Distribution Point (0.415V)

(1) SHORT CIRCUIT CURRENT AT 11 KV PANEL / RMU:

  • Assume that 11KV Side System Fault MVA is 350 MVA.
  • Shor Circuit Current at 11KV System = Fault MVA / 1.732 x System Voltage at Fault Point.
  • Shor Circuit Current at 11KV System = 350 / 1.732 x 11.
  • Shor Circuit Current at 11KV System = 18.37 KA.

 (2) SHORT CIRCUIT CURRENT AT TRANSFORMER:

  • To calculate Short Circuit Current at Transformer Side, First We need to sum Impedance of the system up to Transformer (Impedance of HT Source +Impedance of HT Cable + Impedance of Transformer).
  • Transformer Capacity is 1000 KVA
  • Base MVA = TC Size / 1000
  • Base MVA = 1000 / 1000
  • Base MVA = 1 MVA or
  • Base KVA = 1000 KVA
  • % Impedance of Source = Base MVA x100 / System Fault MVA
  • % Impedance of Source = 1 x100 / 350
  • % Impedance of Source = 0.29 Ώ / KM.
  • CALCULATE % RESISTANCE & REACTANCE OF CABLE ( 185 SQ.MM HT CABLE)
  • % Resistance of Cable =Base KVA (TC Capacity) x Cable Resistance x No of Run x Length of Cable / (System Voltage in KV)²x10.
  • % Resistance of Cable =1000 x 0.21 x 1 x 25 / 11x11x10
  • % Resistance of Cable =0.004339 % Ώ
  • % Reactance of Cable =Base KVA (TC Capacity) x Cable Reactance x No of Run x Length of Cable / (System Voltage in KV)²x10.
  • % Reactance of Cable =1000 x 0.1 x 1 x 25 / 11x11x10
  • % Reactance of Cable =0.002066 % Ώ
  • % Impedance of Cable =√ R² + X²
  • % Impedance of Cable =√ (0.004339) ² + (0.002066) ²
  • % Impedance of Cable =0.00481 % Ώ.
  • Considering 10% Tolerance in Transformer Impedance
  • % Impedance of Transformer = Impedance of Transformer – 10% Variation of Transformer Impedance
  • % Impedance of Transformer = 6.25 -(6.25X10%)
  • % Impedance of Transformer =5.63 % Ώ.
  • Total % Impedance up to Transformer = % Impedance of Source +%Cable Impedance + % Transformer Impedance
  • Total % Impedance up to Transformer = 0.29+0.00481+5.63 % Ώ.
  • Total % Impedance up to Transformer = 5.92% Ώ.
  • Fault KVA on LT Side of Transformer = Base MVA of Transformer x100 / Total % Impedance up to Transformer.
  • Fault KVA on LT Side of Transformer = 1 x100 / 5.92.
  • Fault KVA on LT Side of Transformer = 16.9 KVA.
  • Short Circuit Current at Transformer LT Side= Fault KVA / 1.732 x System Voltage at Fault Point.
  • Short Circuit Current at Transformer LT Side= 16.9 / 1.732 x 0.415
  • Short Circuit Current at Transformer LT Side= 25.32 KA

 (3) SHORT CIRCUIT CURRENT AT MAIN LT PANEL:

  • To calculate Short Circuit Current at Main LT Panel, we need to sum Impedance of the system up to Main LT Panel (Total Impedance up to Transformer LT Side + Impedance of LT Cable).
  • Total % Impedance up to Transformer LT Side= 5.92% Ώ.
  • CALCULATE % RESISTANCE & REACTANCE OF CABLE (300 SQ.MM)
  • % Resistance of Cable =Base KVA (TC Capacity) x Cable Resistance x Length of Cable / (System Voltage in KV)²x10.
  • % Resistance of Cable =1000 x 0.129 x 1 x 70 / 0.415 x 0.415 x10
  • % Resistance of Cable =0.66 % Ώ
  • % Reactance of Cable =Base KVA (TC Capacity) x Cable Reactance x No of Run x Length of Cable / (System Voltage in KV)² x10.
  • % Reactance of Cable =1000 x 0.071 x 8 x 70 / 0.415×0.415×10
  • % Reactance of Cable =0.36 % Ώ
  • % Impedance of Cable =√ R² + X²
  • % Impedance of Cable =√ (0.66) ² + (0.36) ²
  • % Impedance of Cable =0.747 % Ώ.——————————–(B)
  • Total % Impedance up to Main LT Panel = % Impedance up Transformer +%Cable Impedance
  • Total % Impedance up to Main LT Panel = = 5.92+0.747 % Ώ.
  • Total % Impedance up to Main LT Panel = = 6.662% Ώ. ————————–(B)
  • Fault KVA on Main LT Panel= Base MVA x100 / Total % Impedance.
  • Fault KVA on LT Side of Transformer = 1 x100 / 6.662.
  • Fault KVA on LT Side of Transformer = 15.01 KVA.
  • Short Circuit Current at Main LT Panel = Fault KVA / 1.732 x System Voltage at Fault Point.
  • Short Circuit Current at Main LT Panel = 15.01 / 1.732 x 0.415
  • Short Circuit Current at Main LT Panel = 20.88 KA.
  • Short Circuit Current at Main LT Panel = 21 KA

(4) SHORT CIRCUIT CURRENT AT SUB PANEL-1:

  • To calculate Short Circuit Current at Sub Panel-1, We need to sum Total Impedance of the system up to Sub Panel-1 (Total Impedance up to Main LT Panel + Impedance of LT Cable-1).
  • Total % Impedance up to Main LT Panel= 6.662 % Ώ. (Calculated as per (B))
  • CALCULATE % RESISTANCE & REACTANCE OF CABLE-1 (300 SQ.MM)
  • % Resistance of Cable =Base KVA (TC Capacity) x Cable Resistance x Length of Cable / (System Voltage in KV)²x10.
  • % Resistance of Cable =1000 x 0.129 x 1 x 80 / 0.415 x 0.415 x10
  • % Resistance of Cable =5.99 % Ώ
  • % Reactance of Cable =Base KVA (TC Capacity) x Cable Reactance x No of Run x Length of Cable / (System Voltage in KV)² x10.
  • % Reactance of Cable =1000 x 0.071 x 1 x 80 / 0.415×0.415×10
  • % Reactance of Cable =3.27 % Ώ
  • % Impedance of Cable =√ R² + X²
  • % Impedance of Cable =√ (5.99) ² + (3.27) ²
  • % Impedance of Cable =6.82 % Ώ.
  • Total % Impedance up to Sub Panel-1 = % Impedance up Main LT Panel  +%Cable Impedance
  • Total % Impedance = 5.92+6.82 % Ώ.
  • Total % Impedance = 13.49% Ώ.
  • Fault KVA on Sub Panel-1= Base MVA x100 / Total % Impedance.
  • Fault KVA on Sub Panel-1 = 1 x100 / 13.49.
  • Fault KVA on Sub Panel-1 = 7.41 KVA.
  • Short Circuit Current at Sub Panel-1 = Fault KVA on Sub Panel-1 / 1.732 x System Voltage at Fault Point.
  • Short Circuit Current at Sub Panel-1 = 7.41 / 1.732 x0.415.
  • Short Circuit Current at Sub Panel-1 = 10.33 KA.
  • Short Circuit Current at Sub Panel-1 = 11 KA

(5) SHORT CIRCUIT CURRENT AT SUB PANEL-2:

  • To calculate Short Circuit Current at Sub Panel-2, We need to sum Total Impedance of the system up to Sub Panel-2 (Total Impedance up to Main LT Panel + Impedance of LT Cable-2).
  • Total % Impedance up to Main LT Panel= 6.662 % Ώ. (Calculated as per (B))
  • CALCULATE % RESISTANCE & REACTANCE OF CABLE-2 (95 SQ.MM)
  • % Resistance of Cable =Base KVA (TC Capacity) x Cable Resistance x Length of Cable / (System Voltage in KV)²x10.
  • % Resistance of Cable =1000 x 0.409 x 1 x 80 / 0.415 x 0.415×10
  • % Resistance of Cable =19 % Ώ
  • % Reactance of Cable =Base KVA (TC Capacity) x Cable Reactance x No of Run x Length of Cable / (System Voltage in KV)² x10.
  • % Reactance of Cable =1000 x 0.072 x 1 x 80 / 0.415×0.415×10
  • % Reactance of Cable =3.36 % Ώ
  • % Impedance of Cable =√ R² + X²
  • % Impedance of Cable =√ (19) ² + (3.36) ²
  • % Impedance of Cable =19.294 % Ώ.
  • Total % Impedance up to Sub Panel-2 = % Impedance up Main LT Panel +%Cable Impedance
  • Total % Impedance up to Sub Panel-2 = 6.662+19.294 % Ώ.
  • Total % Impedance up to Sub Panel-2 = 25.956% Ώ.
  • Fault KVA on Sub Panel-2 = Base MVA x100 / Total % Impedance up to Sub Panel-2.
  • Fault KVA on Sub Panel-2 = 1 x100 / 25.956.
  • Fault KVA on Sub Panel-2 = 3.85 KVA.
  • Short Circuit Current at Sub Panel-2 = Fault KVA on Sub Panel-2 / 1.732 x System Voltage at Fault Point.
  • Short Circuit Current at Sub Panel = 3.85 / 1.732 x 415.
  • Short Circuit Current at Sub Panel = 5.36 KA.
  • Short Circuit Current at Sub Panel-2 = 6 KA

 CONCLUSION:

  • Shor Circuit Current at 11KV System = 18.37 KA.
  • Short Circuit Current at Transformer LT Side= 25.32 KA
  • Short Circuit Current at Main LT Panel = 21 KA
  • Short Circuit Current at Sub Panel-1 = 11 KA
  • Short Circuit Current at Sub Panel-2 = 6 KA

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