Rockwell Automation 865 Differential Protection Relay - Manual (Page 152)

8-10 Applications
865-UM001A-EN-P – July 2009
CTs on LV side:
500/5 5P10
(Max. Short circuit current is 4400 A = 8.8 x 500 A)
Winding resistance R
CT
= 0.28 Ω
(RCT depends on the CT type, INCT and power rating. Let's say
that the selected CT type, 500 A and an initial guess of 15 VA yields
to 0.28 Ω)
Safety factor c = 3.
(Transformer differential, Δ.)
Differential current setting of the non-stabilized stage ΔI>>: I
SET
= 9 x I
N
R
L
= 0.008 ΩTypical burden of an Allen-Bradley relay current
input.
R
WHV
= 0.138 Ω Wiring impedance of high voltage side.
(2x16 m, 4 mm 2 )
R
WLV
= 0.086 Ω Wiring impedance of low voltage side.
(2x10 m, 4 mm
2
)
The needed CT power on HV side will be
( )
6 . 14 5 07 . 0 008 . 0 138 . 0 07 . 0
100 10
84 9 4
2 = ⋅
⎥⎦
⎤
⎢⎣
⎡
− + + ⋅
⋅
⋅ ⋅
>
N
S
VA
⇒ 15 VA is a good choice for HV side.
And on the LV side
( )
2 . 15 5 28 . 0 008 . 0 086 . 0 28 . 0
500 10
440 9 3
2 = ⋅
⎥⎦
⎤
⎢⎣
⎡
− + + ⋅
⋅
⋅ ⋅
>
N
S
VA
⇒ 15 VA is a good choice for LV side.