currents and voltages to lower magnitudes and to provide galvanic isolation

Currents and voltages to lower magnitudes and to

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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) The University of Sydney Page 4

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• Electric power transmission, Current transformer