Sel 411L - Figure 3.58 Power System Used for CT Selection Example

P.3.103
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
CT Selection Procedure
To meet these requirements, the CT must satisfy Equation 3.39 or Equation 3.40.
IEC Applications Equation 3.39
ANSI Applications Equation 3.40
where:
➤ ESAT is the CT excitation voltage corresponding to the IEC
maximum permissible error
➤ VSTD is the CT ANSI terminal voltage rating
➤ IF is the fault current in secondary amperes resulting from an
external fault at the remote terminal (distance elements) or the
maximum external fault current (current differential elements)
➤ X/R is the ratio of the primary system reactance over resistance
at the remote terminal (equal to 2
πf • T
P
, where T
P
is the
primary system time constant)
➤ RCT is the CT internal resistance
➤ ZB is the total connected secondary impedance including the
relay and CT cable (for ground faults, the cable resistance
includes the return path for the current)
➤ k is a dimensioning factor (determined by factory testing)
➣ k = 6 for distance elements (mho and quadrilateral elements)
➣ k = 7.5 for current differential elements
The procedure for determining CT requirements is summarized as follows:
Step 1. Calculate the maximum fault current (I
F
) for three-phase faults
and single-phase-to-ground faults at both line terminals.
Step 2. Calculate the X/R ratio for three-phase faults and single-phase-
to-ground faults at both line terminals.
Step 3. Apply Equation 3.39 or Equation 3.40 to determine whether
the selected CT is suitable for three-phase and single-phase-to-
ground faults in distance applications. Use k = 6 and the remote
terminal fault data.
Step 4. Repeat Step 3 to determine whether the selected CT is suitable
for three-phase and single-phase-to-ground faults in current
differential applications. Use k = 7.5 and the data for the
terminal with the greatest fault current.
The SEL-411L CT saturation.xls can also be used as a visualization tool for
evaluation of CT performance for the differential element. See
https://www.selinc.com/SEL-411L/#ct for more information.
CT Selection Example
Figure 3.58 Power System Used for CT Selection Example
E
SAT
I
F
XR 1+R
CT
Z
B
+
k
------------------------------------------------------------
V
STD
I
F
XR 1+Z
B
k
--------------------------------------

System Voltage (Vs) = 110 kV
System Short Circuit Current (Isc) = 31.5 kA
System Z0/Z1 ratio = 1
System X over R ratio = 38
Z
LINE Impedance = (0.089 + j 0.726) Ω primary
Line Z0/Z1 ratio = 3
Remote Bus
SEL-411L