Instantaneous Over-Current (50/50G/50N); Blocked Over-Current Protection Schemes - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
2.3

Instantaneous Over-current (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 2.3-1 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 Over-current'. 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.
2.3.1

Blocked Over-current Protection Schemes

A combination of instantaneous and DTL elements can be used in blocked over-current protection schemes.
These protection schemes are applied to protect substation busbars or interconnectors etc. Blocked over-current
protection provides improved fault clearance times when compared against normally graded over-current relays.
The blocked over-current scheme of busbar protection shown in Figure 2.3-2 illustrates that circuit over-current
and earth-fault protection relays can additionally be configured with busbar protection logic.
The diagram shows a substation. The relay on the incomer is to trip for busbar faults (F1) but remain inoperative
for circuit faults (F2).
In this example the over-current and earth-fault settings for the incomer 50-1 element are set to below the
relevant busbar fault levels. 50-1 time delay is set longer than it would take to acknowledge receipt of a blocking
signal from an outgoing circuit.
Close up faults on the outgoing circuits will have a similar fault level to busbar faults. As the incomer 50-1
elements would operate for these faults it is necessary to provide a blocking output from the circuit protections.
The 50-1 elements of the output relays are given lower current settings than the incomer 50-1 settings, the time
delay is set to 0ms. The output is mapped to a contact. The outgoing relay blocking contacts of all circuits are
wired in parallel and this wiring is also connected to a BI on the incomer relay. The BI on the incomer relay is
mapped to block its 50-1 element.
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