Differential protection of alternator or Merz-Price circulating current protection system is a only protection system that protects alternator stator from burning through at least 80% of it.
For alternator the most severe out come of fault is the damage of stator. If some how the stator coil is burn out, welded together then there is no solution but to replace it causing longer down time. Other
There are two types fault might occur in a alternator stator, namely phase to phase fault or phase to ground fault. In case of phase to ground fault, if one phase is grounded or shorted with alternator body/ frame, it will form a close circuit with NER or neutral earth resistance of alternator. That is fault current will flow through earthed phase – alternator frame- neutral earth resistance of alternator. For phase to phase fault a local circuit will develop in which the fault current will flow.
To protect alternator stator we have to fight two types of fault – 1.phase to ground fault, 2. Phase to phase fault.
Differential protection of alternator or Merz-Price circulating current protection system
Differential circulating current protection scheme or Mertz Price circulating current scheme is a most popular protection scheme for alternator stator protection. It function on the concept of compareing the two currents in and out of stator coil. In normal condition the two current will be same, if fault occurs there will be some difference , and Merz price circulating current scheme works by detecting this difference or differential current.
Connection and function of Differential protection of alternator
In each phase there are two identical Current transformer.This two CT is in two sides of to be protected zone. So for three phase there are six no of CT’s. The secondaries of all CT’s are connected in star and other winding of each CT’s set are connected through cable called pilot cable.Three coil are connected in pilot cable. They are connected in a equipotential point of pilot cable.
How it functions : Merz-Price circulating current protection system of alternator
Under normal condition there will be no current difference in two sides of protected zone, hence Two ct’s in a phase will conduct same current. Thus currents from two ct’s will be balanced as current entering in same point of same value from opposite direction is vectorically summed up as zero.(From Kirchhoff current law concept)
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