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MR-REF & MR-REF3 Technical Manual
Issue 3, November 2013
1.1 Principle of operation
In the simple case above, for a current I
p
flowing through both CT’s will produce an equal secondary current. This
will flow around the loop and the voltage X-Y will remain zero and the MREF relay would not operate.
In the event of an internal (in zone) fault I
f
, only CT (A) would “see” this fault. The other CT (B) would appear as an
open circuit. Thus any secondary current in CT (A) would flow between X & Y operating the MREF.
In Practice, should this case become extremely large, one CT may saturate, in effect representing a shorted
secondary. In this case, the circulating current would produce a voltage across X/Y. Calculations must be made to
ensure stability under these circumstances, these are described in P&B application Note MR901
Unbiased Differential
Three MR-REF relays or one MR-REF3 are required to achieve full protection. Each with stabilizing resistors and
possibly a voltage dependent non-linear resistor.
