Satto untimely of differential relays in MV / 2 About As proposed a few days ago and mentioned in the title, we performed the tests and verified that it is misleading to believe that a residual current in three phase systems is manifested in a circuit only when the return of the same residual current in the middle of the transformer can be implemented through a fourth wire (neutral) or through the protective conductor or through the ground, and the same in the presence of an imbalance of load impedances, as failures.
fact occurred with the calculation in parallel with two branches of a three-phase system without neutral, if the impedances of the two routes in parallel two sets are different from each other just for their parts reactive, such as a reciprocal arrangement between the different phases and / or for different distances between the phases, each branch can be seen significantly affected by a residual current (which can trigger a differential relay rated at a few amperes). The residual current (the carrier 3io), which involves a branch is equal and opposite to that flowing through the second branch. It is assumed the same triad of symmetrical current input and output parallel. No zero-sequence component in either input or output by the parallel. The
study developed is well suited to the case of a MV / LV powered by two separate pipes from a cabin of delivery. Easily understood in terms of examining the behavior of a ring of MT and delivery of a cabin feeds several MV / LV.
I think the interesting observation, because I had never read the texts of electrical systems of the possibility of a zero-sequence component of current could be established even without the presence of failure in a pipeline and that the "fourth line" is found in class in parallel. This applies to a branch on the other and vice versa.
The intensity of the residual current can be calculated to some extent.
Una posa attenta dei cavi unipolari MT può evitare l’intervento dei dispositivi differenziali.
Anche l’utilizzo di cavi tripolari evita l’intervento dei dispositivi differenziali.
Progetti accurati possono eliminare pesantissime interruzioni nell’alimentazione di stabilimenti produttivi sensibili.
Auspichiamo che la Guida CEI 11-35 si occupi in futuro anche di questi importanti aspetti.
