How do you calculate knee point voltage?

Step 1: calculating knee point voltage Vkp

Vkp = {2 x Ift (Rct+Rw)}/ k

Vkp = required CT knee point voltage

Ift = max transformer through fault in ampere

Rct = CT secondary winding resistance in ohms

Rw = loop impedance of pilot wire between CT and the

K = CT transformation ratio

Step 2: calculate Transformer through fault Ift

Ift = (KVA x 1000)/(1.732 x V x Impedance)

KVA = transformer rating in kVA

V = transformer secondary voltage

Impedance = transformer impedance

Step 3: How to obtain Rct

To measure when CT is produce

Step 4: How to obtain Rw

This is the resistance of the pilot wire used to connect the 5th class X CT at the transformer star point to the relay

in the LV switchboard. Please obtain this data from the Electrical contractor or consultant. We provide a table to

serve as a general guide on cable resistance.

Example:

Transformer Capacity : 2500kVA

Transformer impedance : 6%

Voltage system : 22kV / 415V 3phase 4 wire

Current transformer ratio : 4000/5A

Current transformer type : Class X PR10

Current transformer Vkp : 185V

Current transformer Rct : 1.02½ (measured)

Pilot wire resistance Rw : 25 meters using

6.0mm sq cable

= 2 x 25 x 0.0032

= 0.16½

Ift = (kVA x 1000) / (1.732 x V x impedance)

= (2500 x 1000) / (1.732 x 415 x 0.06)

= 57,968 round up 58,000A

Vkp = {2 x Ift (Rct+Rw) } / k

= {2 x 58000 (1.02+0.16) } / 800

= 171.1½