Raptor
80
CT Magnetisation
This function permits the application of an increasing voltage in
the secondary, with the primary open, until the current
transformer is saturated, showing the knee point. Whilst the process is being carried out,
the current and voltage values are measured. As soon as the knee point is detected (in
agreement with a certain criterion), it is displayed.
Knowing the knee point is very important in all CTs, whether they are measurement or
protection CTs, but especially in the latter as if there is a fault in the system, the primary
current increases a great deal more than the nominal current, and it is essential for the
CT to be able to produce the necessary voltage in its secondary bushings to maintain the
consistency of its transformation ratio, so that the protection devices receive real
information about what is occurring on the primary side. As the CT burden remains
constant (although there are protection devices with saturable inputs whose impedance
varies with the current) and the current increases a great deal, the voltage required to
maintain the ratio also rises a great deal above that defined in its nominal power.
There are different Standards to define the knee point. As usual, there is an IEC standard,
commonly used all over the world, and the ANSI standard* that even contains two
definitions of knee point. In any case, the three standards define very similar value points.
This function uses the IEC criterion to calculate and show the Knee point. This criterion is
defined as: “Point at which a 10% increase in voltage over the previous value causes a
50% increase in current over the previous value”
* The ANSI Standard is divided into two, the ANSI 30º and the ANSI 45º, and the criterion is defined
as "Point at which, on a graphical representation of I, logarithmic type on both axes, the slope of the
tangent at the point reaches a value of 30º or of 45º depending on the Standard applied".
To Next Page
Loading...
Need help?
General
