Alstom MiCOM P545 - 6.7 Worked example for CT requirements; 6.7.1 Important notes to be considered; 6.7.2 System data; 6.7.3 Calculation of Primary X;R

Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-85
AP
6.7 Worked example for CT requirements
The Power system and the line parameters (Line length: 100 km) as given in section 3.2 is
use
d here to calculate CT requirements.
6.7.1 Important notes to be considered

Current differential – Both If and X/R are to be calculated for a through fault
 Distance Zone1 reach point case – Both If and X/R are to be calculated for a fault at
Zone1 reach point
 For calculating the CT requirements, the bus bar short time symmetrical fault rating shall
be considered as the bus fault level.
 When there are only indicative X/Rs available, then the circuit breaker’s dc breaking
capacity shall be used to arrive at the primary time constant (and hence the primary
system X/R). The derivation shall be obtained from the circuit breaker manufacturer;
practical primary time constants arrived in such a way vary between 50 ms (applicable
for 66 kV and 132 kV breakers) and 120 ms (applicable for 220 kV and 400 kV
breakers). 150 ms is a practical figure for generator circuit breakers.
6.7.2 System data
In the following exampl
e, the following parameters have been considered for the CT
calculations:
System voltage - 230 kV
System frequency - 50 Hz
System grounding - solid
Single circuit operation between Green Valley and Blue River
CT ratio - 1200/1
Line length - 100 kms
Line positive sequence impedance Z1 = 0.089 + j 0.476 ohm / km
Bus fault level - 40 kA
Primary time constant = 120 ms
6.7.3 Calculation of Primary X/R
Primary X/R till the Green
Valley bus = 2 * pi * f * primary time constant in s
= 2 * pi * 50 * 0.12
Primary X/R till the bus = 37.7
6.7.4 Calculation of Source Impedance Zs
Source Impe
dance Zs = 230 kV / (1.732 * 40 kA)
= 3.32 ohms
Source angle = tan-1 (X/R)
= tan-1 (37.7)
= 88.48 deg
Hence, Zs = 0.088 + j 3.317 ohms