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Calculation of V For 1.6 Mva Transformer Neutral (6.6Kv/0.433Kv)

This document calculates the knee voltage (Vk) for a 1.6 MVA transformer neutral with a 6.6kV/0.433kV rating using a 2500/1 CT. It first calculates the fault level and secondary fault current. It then determines the resistances of the CT and cable. Finally, it calculates that the knee voltage must be greater than 218.193V to limit the current in the neutral to less than 30mA.

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210 views2 pages

Calculation of V For 1.6 Mva Transformer Neutral (6.6Kv/0.433Kv)

This document calculates the knee voltage (Vk) for a 1.6 MVA transformer neutral with a 6.6kV/0.433kV rating using a 2500/1 CT. It first calculates the fault level and secondary fault current. It then determines the resistances of the CT and cable. Finally, it calculates that the knee voltage must be greater than 218.193V to limit the current in the neutral to less than 30mA.

Uploaded by

kapil
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOC, PDF, TXT or read online on Scribd
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CALCULATION OF Vk FOR 1.6 MVA TRANSFORMER NEUTRAL (6.6KV/0.

433KV)

CT ratio 2500/ 1 CL PS

VK > 2If (RCT + 2RL)

FAULT LEVEL CALCULATION

6.25% OF IMPEDANCE.

Fault Level for 1.6 MVA with 6.25% of Z is

1.6 MVA x 100 1.6 MVA x 100


= -------------------------- = -------------------
% OF IMPEDENCEx1.732x0.433kV 6.25 x 1.732x0.433kV

Fault KA = 34.135 KA

Fault current (If ) at Secondary side

34135
If = -----------
2500

If = 13.654 A

Cable resistance (RL )

RL for one Kilo meter = 14.5  (Considering 3Cx2.5 SQ.MM cable)

14.5
RL for 60 meters (RL) = ---------- X 60 = 0.87
1000

2RL = 1.74 

CT Resistance (Rct)

RCT = 2.5 per 1000 turns.

2.5
RCT for 2500 turns = ---------X 2500 = 6.25 
1000

505071571.doc
RCT = 6.25 

KNEE VOLTAGE (VK)

VK > 2If (RCT + 2RL)

VK > 2 x 13.654 x (6.25 +1.74)

VK > 218.193

FOR 2500/1 CT VK > 219 V

I mag < 30 mA at VK / 2

505071571.doc

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