Basler BE1-CDS240, Instruction Manual, Page: 574 / 580

Saturation Factor Defined from the ANSI C Classification, Ignoring R
S
In Figure D-8, the CT terminal voltage increases linearly with the secondary current along the V
T
=Z
B
* I
S
line where
Z
B
is the total CT burden (leads plus relays for a particular fault and connection). A terminal
voltage (
V
T
) corresponds to the maximum fault current (
I
F
). This voltage is lower than the maximum
voltage ( V
C
) that the CT can support. Saturation will occur for secondary currents in excess of IF
s
where
the corresponding terminal voltage crosses the accuracy class limit
VC
(point C in Figure D-8). We can
define a measure of the degree of saturation with the saturation factor:
D-24 BE1-CDS240 Settings Calculations 9365200990 Rev F
IFs
IF
SF

By examination of triangles OAB and OCD, the same saturation factor can be expressed as:
VC
VT
SF

Note: In this definition, the voltage drop in the
CT
internal resistance is ignored.
Figure D-8. CT Terminal and Excitation Voltages
This first definition of saturation relates the CT terminal voltage to the accuracy class of the CT (effective
class in the case of multi-ratio CTs). It is practical and easy to calculate since it requires only readily
available data. An application is considered reasonably secure when
SF
is less than 0.5
Saturation Factor Defined from the CT Excitation Curve and V
exc
@ IC = 10 A
The definition of the saturation factor given above appears to be conservative because it assumes the
worst case ratio error. However, a closer look is required since it neglects the CT internal resistance. It
corresponds to an excitation voltage on a curve passing through point Q in Figure D-8 at which the
excitation current is 10 amperes (the maximum error allowed by the accuracy class definition). The
R
CT
*
100
term represents the voltage drop across the CT internal resistance. An alternative
SF
that takes the
internal CT resistance into account can be defined on the excitation curve, as:
` 10
`
V
Ve
SF
