Blocked Overcurrent Protection Schemes - Siemens 7SR10 Argus Series Manual

2.5.1 Blocked Overcurrent Protection Schemes

A combination of instantaneous and DTL elements can be used in blocked overcurrent protection schemes.
These protection schemes are applied to protect substation busbars or interconnectors etc. Blocked overcurrent
protection provides improved fault clearance times when compared against normally graded overcurrent relays.
The blocked overcurrent scheme of busbar protection shown in Figure 2.5 illustrates that circuit overcurrent 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 overcurrent 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.
Power
flow
51-1
50-1
(0ms)
F2
Figure 2-6
Typically a time delay as low as 50ms on the incomer 50-1 element will ensure that the incomer is not tripped for
outgoing circuit faults. However, to include for both equipment tolerances and a safety margin a minimum time
delay of 100ms is recommended.
This type of scheme is very cost effective and provides a compromise between back-up overcurrent busbar
protection and dedicated schemes of busbar protection.
Instantaneous elements are also commonly applied to autoreclose schemes to grade with downstream circuit
reclosers and maximise the probability of a successful auto-reclose sequence – see section 4.
© 2018 Siemens Protection Devices Limited
INCOMER
F1
50-1
(0ms)
Blocking outputs for circuit faults
Blocking Scheme Using Instantaneous Overcurrent Elements
7SR10, 7SR11, 7SR12 Applications Guide
Block 50-1
51-1
50-1 (100ms)
50-1
(0ms)
50-1
(0ms)
Chapter 7 Page 13 of 41