Page 107 - Practical Power System and Protective Relays Commissioning

Chapter 10

Traditional and Electronic

Current Transformers Theory
Testing and Commissioning

10.1 INTRODUCTION
A current transformer (CT) is used to transfer the primary current to a sec-
ondary current proportional with the primary current. The secondary current
is suitable for the measuring and protection devices.
Fig. 10.1 shows the construction, components, and types of some CTs
used in most substations throughout the world.

10.2 CURRENT TRANSFORMER EQUIVALENT CIRCUIT

A CT consists of an iron core with two windings. The primary winding mea-
sures the primary current and the second winding connected with the second-
ary load (also called a burden). The current in the primary circuit produces
an alternating flux in the core and this flux produces an emf in the secondary
circuit that produces the secondary current, see Fig. 10.2. Refer to Fig. 10.3
for a current phasor diagram for CTs.
As shown in Fig. 10.3, the primary current is the summation of the sec-
ondary current (I s ), this corresponds to primary side plus the excitation cur-
rent (I e ) which has two components: I m to magnetize the core and I c for iron
and hysteresis losses.

10.3 CURRENT TRANSFORMER MAGNETIZATION CURVE
Refer to Fig. 10.4 for the CT magnetization curve. The protection CT is
working between the ankle point and the knee point and beyond, but the
measuring CT usually operates in the region of the ankle point. The knee
point voltage is defined as the point at which an increase in voltage by 10%
will increase the magnetizing current (I e ) by 50%. Above this voltage, the
CT will be saturated.

Practical Power System and Protective Relays Commissioning.
DOI: https://doi.org/10.1016/B978-0-12-816858-5.00010-1 105
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