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Knee Point Voltage Calculation: Data

This document provides details to calculate the knee point voltage for overcurrent protection of a 2000 KVA, 11/0.433kV transformer with 6.25% impedance and a full load current of 105A. It lists the make and type of relay used, its current transformer ratio of 120/5A, and class PS. The calculation shows that given the resistance of the current transformer secondary winding, resistance of pilot cables, and rated current of the relay, the knee point voltage is 328V.

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PRATHAM SARAN
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402 views1 page

Knee Point Voltage Calculation: Data

This document provides details to calculate the knee point voltage for overcurrent protection of a 2000 KVA, 11/0.433kV transformer with 6.25% impedance and a full load current of 105A. It lists the make and type of relay used, its current transformer ratio of 120/5A, and class PS. The calculation shows that given the resistance of the current transformer secondary winding, resistance of pilot cables, and rated current of the relay, the knee point voltage is 328V.

Uploaded by

PRATHAM SARAN
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOC, PDF, TXT or read online on Scribd
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Knee Point Voltage Calculation

Data :

Transformer ratio : 11/0.433KV


Rating : 2000 KVA
%Z : 6.25%
Full Load Current I : 105Amp.
Relay :

Make : Alstom
Type : DTH-31
CTR : 120/5A
Class : PS

Calculation :

Knee point Voltage Vk = 40 I (Rct + 2RL)

Where I = Relay rated Current(A)

Rct = Resistence of CT
(secondary winding – ohms)

RL = Resistence of each pilot from the relay


To the CTs (Ohms)

Considering length of 10mtrs of 2.5 sqmm. Cable whose resistance is 7 ohms/Km.

RL =7 x 10 = 0.07 ohms
1000

Assuming CT resistance = 2 ohms

So Vk = 40 * 5 (1.5 + 2 (0.07)

= 200 (1.5+0.14)
= 200 * 1.64
= 328V

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