7SR18 Commissioning & Maintenance Guide
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2.1 BIASED DIFFERENTIAL (87BD, 87HS)
Meas
Earth
2
nd
Harm
Local
Remote
Ia
Line
Ia
Prim
Ia
Nom.
Ia
Sec
I EF
Der.
I
Seq.
Comp
Ia
Op.
Ib
Op.
Ic
Op.
Loc
Rel.
Ic
Loc
Rel.
Ib
Loc
Rel.
Ia
Rmt
Rel.
Ic
Rmt
Rel.
Ib
Rmt
Rel.
Ia
2
nd
Harm
Ib
Line
Ib
Prim
Ib
Nom.
Ib
Sec
2
nd
Harm
Ic
Line
Ic
Prim
Ic
Nom.
Ic
Sec
Meas
Earth
I EF
Der.
I
Seq.
Comp
2
nd
Harm
Ic
Line
Ic
prim
Ic
Nom.
Ic
sec
2
nd
Harm
Ib
Line
Ib
prim
Ib
Nom.
Ib
sec
2
nd
Harm
Ia
Line
Ia
prim
Ia
Nom.
Ia
sec
Therm
Therm
Therm
Therm
Therm
Therm
Ib
Rest.
Ia
Rest.
Ic
Rest.
Local
Relay
Remote
Relay
Figure 2.1-1 Biased Differential
Current Inputs: I
A,
I
B
, I
C
,
Disable: 46, 49, 50, 51, 50N, 51N, 50BF,
Map Pickup LED: 87BD, 87HS - Self Reset
The differential elements are subjected to CT multipliers, Vector Group Compensation and Zero Sequence filters
when applied to power transformers. The complexity of these features can cause confusion during testing and
lead to incorrect relay settings being applied. It is recommended that the accuracy of the differential elements are
tested by secondary injection with simplified differential settings applied to avoid ambiguity before reinstating the
required site settings which can be tested more thoroughly by primary injection followed by final checking with the
protected transformer on load.
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