Definition in the basics of the current transformer
In the Basics of current transformers, these Transformers are regarded as a piece of equipment consisting of a pair of mutually coupled windings mounted around a core usually of magnetic material search transformers are normally used to step down high current flowing in the primary winding to lower levels suitable for feeding to measuring and protective equipment.
Equivalent circuits with regards to the basics of current transformers
In the basics of current Transformers, these transformers behave similarly to all two winding Transformers and may be represented by the well-known circuit shown in figure 2.3(a) which is based on the winding directions and current and voltage polarities shown in figure 2.3(b).
Because the connected burdens are normally of low impedance, the secondary output VA and voltage are relatively low, typically of a maximum of 20VA and 20V for a current transformer with a secondary winding rated at 1 ampere. The voltage across the primary winding, because of the turns ratio, is not therefore for likely to exceed a fraction of 1 volt, a negligible value relative to the rated voltage of the primary circuit. For this reason, the equivalent circuit of a current transformer can be simplified to that shown in figure 2.4. This circuit can be made to represent any current transformer operating with any burden and primary current under either steady state or transient conditions.
It will be seen that the exciting current Ie is dependent on the exciting impedance, presented by Rl in parallel with lm and the secondary E.M.F (Es) is needed to drive the secondary current Is through the total secondary circuit impedance. Because the secondary current of a current transformer may vary over a wide range, that is from zero under no load conditions to very large values but when there is a fault on the primary circuit the secondary E.M.F and excitation current may also vary greatly and in this respect the behavior is very different from that of voltage transformers.
Because of the non-linearity of the excitation characteristics of the magnetic materials used for current transformer cores, the exciting impedance of a given current transformer is not constant, both the magnetizing inductance Lm and loss resistance Rl varying with the core flux needed to provide the secondary E.M.F es. Allowance may be made for this non-linearity, if necessary, when determining the behavior of a particular Transformer under specified conditions, calculation than being done using step-by-step or other methods. If great accuracy is not required, however, simplifications can be affected by assigning constant values, the averages over a cycle, to Rl and Lm.
An alternative method of representing a current transformer is to employee the concept of mutual inductance Mps. By definition and based on the conventions used above:
es = -Mps di/dt
when there is zero current in the secondary winding. It has been shown that the circuit shown in figure 2.5 is equally as satisfactory a model as that shown in figure 2.4.
The self-inductance, Lss of a secondary winding is given by:
Lss = Ls-Mps Ns/Np
in which Ls is the leakage inductance used in the circuit of figure 2.4. It is clear therefore that the value of Ls for a given Transformer may be determined experimentally by measuring itself Lss and mutual inductancesMps.
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