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Response of a CT in saturated state

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    The emergence of static relays leads to revision of protection behaviour as a whole in the case of strong currents: as the CT saturates beyond a certain threshold, the first reaction is often to avoid this by raising the threshold. However, this results in both additional costs (more efficient, larger, more space consuming CT) and in the risk of excessive temperature rise of the relays.

     

    On the contrary, saturation plays a useful role for the «measurement» function since primary current image accuracy is only useful up to the value of the rated current I1n. Beyond this point, the measurement ceases to be of any use and saturation must occur for a low current (2 to 3 I1n) in order to limit the secondary current and protect the measuring instruments.

     

     

    It is thus necessary to know the response of the CT in saturated state to ensure the protection device works properly when the primary current exceeds rated current strength, particularly for the high values which appear if a short-circuit occurs.

     

    In theory, induction in the core reaches a plateau at the saturation bend, thus limiting current strength at the secondary. In actual fact the experiment performed will show that current strength at the secondary slightly increases and that protection relay operation is quite satisfactory.

     

    experiments - wiring

     

    A current i1 is injected in the CT primary, and the current supplied by the secondary in a load Z containing a relay R and a resistance is analysed (see fig. ).

     

    The currents at the secondary l2 are given, according to the current supplied at the primary (represented by the

     

    parameter N = I1/I1n ) for various loads Z and various CTs 

     

    Reference: Schneider

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