To carry out the selectivity study between two low and medium voltage circuit-breakers, the data below must first be put into a logarithm diagram (referring to a single reference voltage).
1. Transformer:
- Connection curve (inrush);
- Rated current;
- Short-circuit current at the LV busbars;
- Short-circuit withstand capacity of the transformer.
2. Distributor utility:
- Maximum current and time limits which can be set for the protections required.
At this point, the trip curve of the main low voltage circuit-breaker must be traced so that:
- Protection of the transformer against overload is verified (threshold I1 of protection function L close to the rated current of the transformer);
- It is selective with the other low voltage circuit- breakers on the load side.
Once the LV protection has been defined, the curve of the medium voltage circuit-breaker voltage can be traced so that:
- It protects the transformer against overloads (this protection is usually ensured by the low voltage circuit-breaker);
- It lies above the inrush current curves of the transformer;
- It lies below the representative point of the thermal withstand (this protection can be carried out by the low voltage circuit-breaker, but any short-circuit between the low voltage circuit-breaker and the terminals of the transformer remains unprotected);
- It lies below the limits set by the distributor utility.
(*) The use of a thermometric equipment allows improving the protection of the transformer against overload.
Before facing the problem of selectivity between the medium and low voltage circuit-breakers, it is necessary to clarify the functions of these circuit-breakers.
The MV protection on the supply side of the transformer must:
- Protect the transformer against short-circuit;
- Protect the transformer against faults on the supply side of the main LV circuit-breaker (if a dedicated protection is not provided);
- Not intervene when the transformer is supplied with voltage (inrush current);
- Be set so as to satisfy the limits imposed by the distributor utility;
- Be set so as to be selective with the protections on the supply side (if requested).
The LV protection on the load side of the transformer must:
- Protect the transformer against short-circuit and overload; (*)
- Be set so as to be selective with the protections on the load side.