Instantaneous Overcurrent (50/50G/50N); Figure 3.4-1 General Form Of Dtl Operate Characteristic - Siemens 7PG2114 Application Manual

7PG2113/4/5/6 Solkor Applications Guide
3.4

Instantaneous Overcurrent (50/50G/50N)

Each instantaneous element has an independent setting for pick-up current and a follower definite time lag (DTL)
which can be used to provide time grading margins, sequence co-ordination grading or scheme logic. The
"instantaneous" description relates to the pick-up of the element rather than its operation.

Figure 3.4-1 General Form of DTL Operate Characteristic

Instantaneous elements can be used in current graded schemes where there is a significant difference between
the fault current levels at different relay point. The Instantaneous element is set to pick up at a current level above
the maximum Fault Current level at the next downstream relay location, and below its own minimum fault current
level. The protection is set to operate instantaneously and is often termed 'Highset Overcurrent'. A typical
application is the protection of transformer HV connections – the impedance of the transformer ensuring that the
LV side has a much lower level of fault current.
The 50-n elements have a very low transient overreach i.e. their accuracy is not appreciably affected by the initial
dc offset transient associated with fault inception.
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