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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)
