Currents and voltages to lower magnitudes and to

currents and voltages to lower magnitudes, and to provide galvanic isolation between the power network and
the relays or other instruments connected to the transformer secondary windings. The ratings of the secondary
windings of transducers are standardised (5A or 1A), so that a degree of interchangeability among different
manufacturers’ relays and meters can be achieved. The function of transducers is to provide current and voltage
signals to the relays (and meters) which are accurate reproductions of the corresponding primary quantities.
Although modern transducers do so quite well in most cases, transformation errors introduced by the
transducers should be considered when performance of relays is assessed.
1.2.1.1 Current Transformers A current transformer is used to transform the primary current in terms of its
magnitude and phase to a secondary value such that under normal conditions the secondary value is
proportional to the primary value. A current transformer consists of a primary winding, a magnetic core and a
secondary winding. The alternating current flowing in the primary produces a magnetic field in the core, which
then induces current flow in the secondary winding circuit. A primary objective of current transformer design is
to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current bears an
accurate relationship to the primary current.
The primary winding is connected in series with the network, which means that the primary and secondary
currents are stiff and completely unaffected by the secondary burden. The currents are the primary quantities
and the voltage drops are only of interest regarding exciting current and measuring cores. It is important to
short circuit the secondary windings or connect them to a low impedance in order to avoid high voltage across
the windings. Excessive current through the primary would cause overheat of the transformer and also leads to
saturation of the second winding affecting their accuracy.
If the excitation current could be neglected, the transformer would reproduce the primary current without errors
and the following equation should apply between the primary and secondary currents:
(1)
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