Transformer Neutral Ct Sizing

DOC NO : HZL-BTN-ELE-DS-SY-031

TRANSFORMER LV NEUTRAL CT Core I : 1.0 CT sizing calculation for 132kV ‐ Definite Time Ground O/C Protection 1.1 Basic Data: 800 /

Ratio

=

Accuracy class

= 5P10

Burden ( VA )

= 15

VA

IDMT Overcurrent Relay

= 0.1

VA

Cable Length ( m )

= 150

m

A

1

1.2 Burdens of relays connected :

2 Lead resistance at 70ºC for 4mm copper ( Ω/km ) = 5.54

Ω/km

Loop Resistance ( RL) (Ω)

= 1.66

(Ω)

Loop Burden ( VA )

= 1.66

VA

Total Meter Burden ( VA )

= 0.1

VA

Total Burden on CT ( VA )

= 1.76

VA

1.3 Selected burden is more than required.

HENCE SAFE.

Core II : 1.0 CT sizing calculation for132kV ‐ Restricted Earth Fault Protection (64R) 1.1 Basic Data: Ratio

= 1000 /

Voltage ( kV )

= 6.6

Accuracy Class

= PS

Transformer rating

= 7.5

MVA

Impedence of transformer

= 8.35

%

Maximum through fault, MVA

= 7.5 x 100/8.35 = 89.82 MVA

Through Fault current at primary I f

= 89.8203592814371 x 1000/(1.732 x6.6 ) = 7857.5 A

Through Fault current at secondary I fs

= 7.857

1 of 18

1 A

kV

A

DOC NO : HZL-BTN-ELE-DS-SY-031

TRANSFORMER LV NEUTRAL CT 1.2 Burdens of relays connected : Restricted Earth Fault Relay

= 1.00

VA

Cable Length ( m )

= 100

m

2 Lead resistance at 70ºC for 4 mm copper ( Ω/km ) = 5.54

Ω/km

Loop Resistance ( RL) (Ω)

= 1.1080

Ω

= 1.1080

VA

Loop Burden ( VA ) R

L

Internal resistance of CT secondary ( Ω) = 2 1.3 Required Knee point Voltage Vk

≥ (2 x Ifs(Rct+2RL)) volt ≥ 2 x7.86 x (2+ 2x1.108) ≥ 66.26

V

≥ 500

V

2 of 18

Ω