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Protective current transformer classes for transient performance

Views : 1030
Update time : 2024-09-11 14:21:56
NOTE A larger variety of curves is given in the Technical Report IEC 61869-100 TR.  



2B.1.3 C-O-C-O duty cycles 

The transient dimensioning for auto-reclosure duty cycles has to be done separately for each cycle according to the equations given above. 

For cores having a high secondary time constant (typically TPX cores), there is no significant flux declination after t’. 

For cores having a low secondary time constant (typically TPY and TPZ cores), the secondary linked flux declines exponentially with the secondary time constant  Ts during the fault repetition time. In this case, no analytical formula exists for the time argument t in the term for the first cycle, and several case differentiations may be necessary.  

Fig. 2B.7 shows a typical case where saturation is reached after  t’al. The flux is limited to saturation flux before  t’ is reached. It declines to a value which is low enough to remain below saturation up to  t’’al. Ignoring saturation (shoved by curve, the declined flux starts from a higher level at the beginning of the second cycle. This example demonstrates the interdependency between the core dimensioning in the first and in the second cycle, and the determination of Ktd. 

NOTE 1 The formula for the C-O-C-O-cycle, which was given in the preceding standard IEC 60044-6, ignores saturation within the first cycle and leads in many cases to unnecessarily high Ktd values. See Fig. 2B.7. 

It is therefore recommended to draw a graph similar to the one in Fig. 2B.7, in order to make oneself familiar with the actual situation.
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