Accuracy of Instrument Transformers
Current transformers are usually either “measuring” or “protective” types, these
descriptions being indicative of their functions.
Accuracy of Measuring CT.
A desirable characteristic of a measuring CT is that it should “saturate” when the primary
current exceeds the percentage of rated current specified as upper limit to which the
accuracy provisions apply. This means that at these higher levels of primary current, the
secondary current is less than proportionate. The effect of this is to reduce the possibility to
which any measuring device connected to the CT secondary is subjected to current
overload. It will help to protect the measuring instruments from being damaged due to
overloading.
The principal requirements of a measuring CT are that, for primary currents up to 120% or
125% of the rated current, its secondary current is proportional to its primary to the degree
of accuracy as defined by its class without exceeding allowable limit of ratio and phase angle
error.
This defines the maximum permissible current error at the rated current for a particular
accuracy class. The standard accuracy classes for the measuring CTs may be one of 0.1, 0.2,
0.5, 1, 3 and 5.
Instrument Security Factor
ISF or instrument security factor of current transformer is defined as the ratio of instrument
limit primary current to the rated primary current. The instrument limit primary current of
metering CT is the value primary current beyond which CT core becomes saturated.
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