Accuracy of Protection CT
This defines the maximum permissible composite error at the rated accuracy limit primary
current, followed by letter P for protection. The standard prescribed accuracy classes may
be one of 5P, 10P and 15P. A protection CT is designated by accuracy class, followed by
accuracy limit factor, such as 5P10.
On the other hand the reverse is required of the protective type CT, the principal purpose of
which is to provide a secondary current proportional to the primary current when it is
several, or many, times the rated primary current. The measure of this characteristic is
known as the “Accuracy Limit Factor” (A.L.F.). A protection type CT with an A.L.F. of 10 will
produce a proportional current in the secondary winding (subject to the allowable errors)
with primary currents upto a maximum current of 10 times the rated current.
For protection current transformer, the ratio of accuracy limit primary current to the rated
primary current is termed as ALF.
[First we will explain, what is rated accuracy limit primary current?
Broadly, this is the maximum value of primary current, beyond which core of the protection CT starts
saturated. The value of rated accuracy limit primary current is always many times more than the
value of instrument limit primary current. Actually CT transforms the fault current of the electrical
power system for operation of the protection relays connected to the secondary of that CT. If the core
of the CT becomes saturated at lower value of primary current, as in the case of metering CT, the
system fault will not reflect properly to the secondary, which may cause, the relays remain inoperative
even the fault level of the system is large enough.
That is why the core of the protection CT is made such a way that saturation level of that core must be
high enough. But still there is a limit as because, it is impossible to make one magnetic core with
infinitely high saturation level and secondly most important reason is that although the protection
core should have high saturation level but that must be limited up to certain level otherwise total
transformation of primary current during huge fault may badly damage the protection relays.
So it is clear from above explanation, rated accuracy limit primary current, should not be so less, that
it will not at all help the relays to be operated on the other hand this value must not be so high that it
can damage the relays. So, accuracy limit factor or ALF should not have the value nearer to unit and
at the same time it should not be as high as 100
C.T specifications:
• Manufacturer’s name or trademark
• A rated burden at rated current or (VA) [total burden including pilot wires].
• An accuracy class: 5P10 [Protection CT][0.5: Metering Class]
• (ALF) accuracy limit factor an upper limit beyond which the accuracy is not
guaranteed. 20
• Instrument security Factor (I.S.F) [ Metering CT]
• Continuous thermal current rating factor (RF)
• Ratio: Rated Primary / Rated Secondary current ratio 100/5A
• Secondary Resistance
• Short time current (primary)
• Rated Primary Voltage
• Rated frequency (Hz)
• Insulation and Basic impulse insulation level (BIL kV)
Example:
1.15 VA clases 0.5→mettering C.T
2. 15 5p 20 →protection C.T
Rated burden 15VA
Accuracy class 5p
ALF= 20 (20 times the rated current)
0 Comments