©2015 Siemens Protection Devices Limited
Chapter 7 Page 12 of 64
2.1.3 Biased Differential (87BD) Settings
The 87BD elements provide differential protection for phase and earth faults. The limiting factors for protection
sensitivity are dictated by the need to ensure protection stability during load or through fault conditions.
Magnitude restraint bias is used to ensure the relay is stable when the transformer is carrying load current and
during the passage of external (out of zone) fault current. As the bias current increases the differential current
required for operation increases.
Harmonic bias is used to prevent relay operation during magnetising inrush current into one winding when the
transformer is first energised.
2
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I
W2
W1
O
p
e
ra
te
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d
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ia
s
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Figure 2-2: 87BD Characteristic
87BD Initial Setting (0.1 to 2.0 x In)
This setting is selected to ensure stability in the presence of CT and relay errors when low levels of bias current
are present i.e. low load levels.
This is the minimum level of differential current at which the relay will operate. Typically this setting is chosen to
match the on load tap-change range. For example if the tap change range is +10% to –20%, a setting of 0.3In is
selected.
87BD 1
st
Bias Slope Setting (0.0 to 0.7)
Steady state unbalance current will appear in the differential (operate) circuit of the relay due to the transformer
tap position, relay tolerance and to CT measurement errors. The differential current will increase with increasing
load or through fault current in the transformer so, to ensure stability, the differential current required for operation
increases with increasing bias current. The bias slope expresses the current to operate the relay relative to the
biasing (restraint) current.
The Bias slope setting chosen must be greater than the maximum unbalance, it is selected to ensure stability
when through fault or heavy load current flows in the transformer and the tap changer is in its extreme position.
The recommended setting is 1 x the tap change range. As the protection is optimised around the centre tap
position then using the total tap change range includes for a 100% safety margin, this provides contingency for
CT and relay tolerances. For example if the tap change range is +10 to –20%, the overall range is 30% so a 0.3x
setting is chosen.
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