CT Knee Point Voltage Calculation

DOC NO :

TRANSFORMER SECONDARY SIDE NEUTRAL CT

Core I : 1.0 CT sizing calculation for 33kV ‐ Definite Time Ground O/C Protection (51NS) 1.1 Basic Data: Ratio

= 900

Accuracy class

= 5P10

Burden ( VA )

= 15

VA

IDMT Overcurrent Relay

= 0.1

VA

Cable Length ( m )

= 150

m

Cable Size of CT connected

= 4

Sqmm

Lead resistance at 75ºC for 4mm 2 copper cable

= 5.41

Ω/km

Loop Resistance ( RL) (Ω)

= 1.62

(Ω)

Loop Burden ( VA )

= 1.62

VA

Total Meter Burden ( VA )

= 0.1

VA

Total Burden on CT ( VA )

= 1.72

VA

/ 1 A

1.2 Burdens of relays connected :

1.3 Selected burden is more than required.

HENCE SAFE.

Core II : 1.0 CT sizing calculation for 33kV ‐ Restricted Earth Fault Protection (64R) 1.1 Basic Data: Ratio

=

900 / 1 A

Primary Voltage ( kV )

=

33

kV

Secondar Voltage ( kV )

=

6.6

kV

Accuracy Class

=

PS

Transformer rating

=

8

Impedence of transformer

=

8.35

Maximum through fault, MVA

= (8 x 100 ) /8.35 =

Through Fault current at primary I f

95.81

MVA %

MVA

= ( 95.81 x 1000 ) / (√3 x 33 ) If

Through Fault current at secondary I fs

=

1676.29 A

= ( 95.81 x 1000 )/( √3 x 6.6x 900 ) Ifs

1 of 18

=

1.863

A

DOC NO : TRANSFORMER SECONDARY SIDE NEUTRAL CT 1.2 Burdens of Relays connected : Restricted Earth Fault Relay

= 1.00

VA

Cable Length ( m )

= 150

m

Cable Size of CT connected (Sqmm)

= 4

Sqmm

Lead resistance at 75ºC for 4 mm 2 copper cable

= 5.41

Ω/km

Loop Resistance ( RL) (Ω)

= 1.6230

Ω

= 1.6230

VA

= 2

Ω

Loop Burden ( VA )

RL

Internal resistance of CT secondary ( Ω) Rct 1.3 Required Knee point Voltage Vk

≥ (2 x Ifs(Rct+2RL)) volt ≥ 2 x1.87 x (2+ 2 x 1.623)

Vk

≥ 19.55

V

≥ 500

V

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