Composite Error (εc) in Current Transformers (CTs)

In high-voltage power systems, accuracy of current measurement is crucial—especially when used for protection and metering. One important parameter that defines the performance of a Current Transformer (CT) is the Composite Error, denoted as εc.

🔹 What is Composite Error (εc)?
Under steady-state conditions, composite error is defined as:
The r.m.s. (root mean square) value of the difference between:

• The instantaneous primary current, and
• The instantaneous secondary current multiplied by the rated transformation ratio,

with proper polarity (as per the terminal marking conventions of the CT).

🔹 The composite error εc is generally expressed as a percentage of the r.m.s. values of the primary current:
where,
kr is the rated transformation ratio;
Ip is the r.m.s. value of the primary current;
ip is the instantaneous value of the primary current;
is is the instantaneous value of the secondary current;
T is the duration of one cycle.