6 WORKED EXAMPLES
The power system and the line parameters used in these examples are as follows:
● Single circuit operation between Green Valley and Blue River
● System voltage = 230 kV
● System frequency = 50 Hz
● System grounding = solid
● CT ratio = 1200/1
● Line length = 100 km
● Line positive sequence impedance Z1 = 0.089 + j 0.476 ohm per km
● Bus fault level = 40 kA
● Primary time constant = 120 ms
Important notes to be considered
● For calculating the CT requirements, the bus bar short time symmetrical fault rating should be considered
as the bus fault level.
● If only indicative X/R ratios are available, the circuit breaker’s DC breaking capacity is used to derive the
primary time constant and therefore the primary system X/R. It is derived from the circuit breaker
manufacturer’s practical primary time constants. These vary between 50 ms (66 kV and 132 kV breakers)
and 120 ms (220 kV and 400 kV breakers). 150 ms is a practical figure for generator circuit breakers.
● 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
6.1
CALCULATION OF PRIMARY X/R RATIO
Primary X/R up to the Green Valley busbar:
= 2
p
f
´
primary time constant
= 2
p
´ 50
´ 0.12 37.7
6.2
CALCULATION OF SOURCE IMPEDANCE
Source impedance magnitude
:
= 230 kV / (1.732
´
40 kA) =
3.32
ohms
Source angle:
= tan-1 (37.7) = 88.48°
Hence:
Zs = 0.088 + j3.317
6.3
CALCULATION OF FULL LINE IMPEDANCE
Z1:
= 0.089 + j0.476 ohms / km
Z
L
:
= 8.9 + j47.6 = 48.42 ohms angle 79.4°
Chapter 16 - Current Transformer Requirements P543i/P545i
424 P54x1i-TM-EN-1
To Next Page
Loading...
Need help?
General
