Transformer Neutral CT Sizing

DOC NO : HZL-BTN-ELE-D HZL-BTN-ELE-DS-SY-031 S-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 /

A

Ratio

=

1

Accuracy class

= 5P10

Burden ( VA )

= 15

VA

IDMT Overcurrent Relay

= 0.1

VA

Cable Length ( m )

= 150

m

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

= 5.54

Ω/km

Loop Resistance ( R L) (Ω)

= 1.66

((Ω Ω)

Loop Burden ( VA )

= 1.66

VA

Total Meter Burden ( VA )

= 0.1

VA

Total Burden on CT ( VA )

= 1.76

VA

1.2 Burdens of relays of relays connected :

2

1.3 Selected burden is more than requ requir ired ed..

HENC HENCE E SAFE.

Core II : 1.0 CT sizing calculation for132kV ‐ Restricted Earth Fault Protection 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 

Through Fault current at secondary I fs

1 of 18

1 A

kV

= 89.820359281437 89.8203592814371 1 x 1000/(1.732 1000/(1.732 x6.6 = 7857.5 A = 7.857

A

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

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

= 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 ( R L) (Ω)

= 1.1080

Ω

= 1.1080

VA

= 2

Ω

Loop Burden ( VA )

R

Inte Intern rna al resi resist sta ance nce of CT seco secon ndary dary ( Ω) 1.3 Required Knee point Voltage point Voltage Vk 

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

V

≥ 500

V

2 of 18

L