Differential Highset (87Hs) Settings - Siemens 7SR242 Duobias Technical Manual

2.1.4 Differential Highset (87HS) Settings

The element operates on the differential current measured by the relay.
The 87HS element is generally applied as an unrestrained differential element to provide fast tripping for heavy
internal faults.
I I
W1 W2
Fig. 2-3: Differential Highset Characteristic
87HS Setting (1 to 30 x In)
The 87HS element is set as low as possible but not less than the maximum three phase through fault current and
not less than half the peak magnetizing inrush current.
For almost all applications a setting of 7 or 8 x In has shown to be sufficiently sensitive for internal faults as well
as providing stability during external faults and transient system conditions.
A Differential Highset Setting of 7 x In will be stable for a peak magnetizing inrush levels of 14 x rated current.
Smaller transformers generally will have lower impedance and therefore greater three phase through fault levels
and magnetizing inrush currents. A setting of 8 x can be used as CT saturation is reduced as system X/R is
usually very low and the peak level of magnetising current does not usually ever exceed 16 x rating.
87HS Delay Setting
A 5ms setting is recommended to compensate for transient overreach.
87HS Inrush Action
87HS Overfluxing Action
These functions are set to 'Disabled' unless specifically required by the application.
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