Answer / srinivasan venkatesh.

to calculate the CT Vk .

eg
CT Acc. class - 5p20
sec. ct - rated = 1 A
Rct - 3 Ohms
Burden 15VA

Vk = 20((Rct+Burden))

= 20((3+15 )
= 360V

in the above VK ,Imag critieria is less than 5% of the CT
secondary current. ie - 500mA ( equal to or less than) .

Answer / arvind negi

Calculation:
1) Fault Current at Primary Side of CT (kA) = MVA/*100(%
Z*1.732*KLV) 7.55
2) Fault Current at Secondary Side of CT, If (A)=
6.29
3) Cable Lead Resistance, RL (Ohm)= 1.02


4) Knee Point Voltage Vk (Volt) = 2If(RCT+2RL)
101.2


REMARKS : -
Knee Point Voltage of CT provided by CGL for REF CT = 115V
and for Differental CT = 200V are in order.

Answer / muhammad ehsan

I am not familiar with the term saturation test but it seems
from the above posts like an exciting current test to
obtain the knee-point of the CT. With this test you can also
obtain the exciting curve (mag-curve) of the CT.

You'll need a variable voltage, (easiest obtain from a
variac - must be able to obtain the suitable voltage) a
voltmeter, an ammeter, a pencil and paper.You'll have to
measure the voltage and the current while you do the test.
Apply the voltage to the secondary winding with the primary
and other windings being open-circuited. Increase the
voltage until you reach a point where a small increase in
voltage results in a big increase in current.
Now slowly decrease the voltage to a few measuring points,
while you measure the current. (write it down) Something like:

205V - 1.1A
200V - 0.6A
190V - 0.238A
180V - 0.149A
170V - 0.114A
150V - 0.08A
125V - 0.061A
100V - 0.049A
50V - 0.028V
25V - 0.017A
0V - 0A

You have to decrease your voltage slowly to zero volt to
demagnetize the CT-core.

BEWARE The knee-point voltage will depend on the size and
class of the CT, and it can be very high on some CTs
(several hundreds of volts) Be very cautious, you will be
working on live equipment during the test.

Now you can just plot the current and voltage values on a
graph. This is the mag-curve of the CT. IEC classify the
knee point voltage as the point where a 10% increase in
voltage results in a 50% increase in current.

Answer / ramesh

to calculate knee point voltage of current transformer by
formal
Vknee point = Isecandary(Rct+ Rcable+ Rburden)
where
Rct= resistenc of current transformer
Rcable =resistenc of cable =2Rcable
Rburden =burden of relay
IF Vknee point< manufacture voltage of current transformer
current transformer is good
IF Vknee point> manufacture voltage of current transformer
current transformer must be change

Answer / mayank trivedi

PRIMARY WDG IS KEPT OPEN. AND VOLTAGE IS APPLIED AFTER
CALCULATING THE KNEE POINT VOLTAGE. I.E BY THE GIVEN FORMULA.

Vk= If(Rct + 2Rl)

SUPPOSE THE KNEE POINT VOLTAGE IS 300 AND SPECIFIED
MAGNETISING CURRENT IS ABOUT 0.65 AS GIVEN ON NAME PLATE.

THAN 10 % VOLTAGE IS APPLIED ON SECONDARY WINDING OF 300V
I.E 30 VOLTS AND THAN INCREASED IN 10 NUMBER OF STEP.

AT FULL KNEE POINT VOLT THE MAG CURRENT SHOULD BE WITHIN
RANGE. THAN ST IS SAID TO PASS THE THE.

THE MAG CURRENT IS MEASURED BY CONNECTING A AMMETER IN SERIES.

Answer / rajkumar

Knee Point Voltage Vk = IFsc * K * (Rct+(2*RL)+Rr)
Where,
IFsc - Fault current at the secondary of CT
IFsc->IFprimary/CT Ratio
IFprimary -> Fault MVA at primary/(1.732*Primary Voltage)
K - Dimensioning Factor
Rct - CT Resistance
RL - Lead Resistance
Rr - Resistance of relay to which CT is connected

Knee point voltage is the voltage at which 10% raise in secondary voltage will result in 50% raise in current..

Answer / lakshmikanthan

V(kp) = 2 * IF(A) * (RCT(Ω) + RL(Ω) + RB(Ω)) / CT
If= fault current
Rct= resistance of CT winding secondary
Rl= lead resistance
Rb= relay resistance for measuring terminal
CT= ct ratio

Answer / narendra solanki

The knee-point voltage will depend on the size and class of
the CT, and it can be very high on some CTs Be very
cautious, you will be working on
live equipment during the test.

Now you can just plot the current and voltage values on a
graph. This is the mag-curve of the CT. IEC classify the
knee point voltage as the point where a 10% increase in
voltage results in a 50% increase in current.

Answer / jayachandran

Vk=A*N*0.021*1.6

A= cross sectional Area,
N= No. of Secondary Turns

This is only if you know the cross sectional area and number of secondary turns.

Answer / s.arunkumar

For calculating knee point voltage there are two methods,
they are
1.Low method
2.High method


1.low impedance method
Vk=24In(Rct+2Rl)

2.High impedance method
Vk=4IsRs

Then, Rs=If/Is(Rct+Rl)
where
Is,In-Rated Sec. Current of CT
Rct-CT resistance
Rl-Max. one way lead reistance from CT to Relay

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