Out Of 3 Logic; Figure 3.5-4 Feeder Fault On Interconnected Network - Siemens 7PG2114 Application Manual

A
Considering the D-G feeder fault shown in fig. 2.6-4: the current magnitude through breakers C and D will be
similar and their associated relays will similar prospective operate times. To ensure that only the faulted feeder is
isolated G FWD must be set to be faster than C. Relay G will thus Trip first on FWD settings, leaving D to operate
to clear the fault. The un-faulted Feeder C-E maintains power to the load.
Relays on circuits C and D at the main substation need not be directional to provide the above protection scheme.
However additional directional elements could be mapped to facilitate a blocked overcurrent scheme of busbar
protection.
At A and B, forward looking directional elements enable sensitive settings to be applied to detect transformer
faults whilst reverse elements can be used to provide back-up protection for the relays at C and D.
By using different settings for forward and reverse directions, closed ring circuits can be set to grade correctly
whether fault current flows in a clockwise or counter clockwise direction i.e. it may be practical to use only one
relay to provide dual directional protection.
3.5.1

2 Out of 3 Logic

Sensitive settings can be used with directional overcurrent relays since they are directionalised in a way which
opposes the flow of normal load current i.e. on the substation incomers as shown on fig. 2.6-4. However on
occurrence of transformer HV or feeder incomer phase-phase faults an unbalanced load current may still flow as
an unbalanced driving voltage is present. This unbalanced load current during a fault may be significant where
sensitive overcurrent settings are applied - the load current in one phase may be in the operate direction and
above the relay setting.
Where this current distribution may occur then the relay is set to CURRENT PROTECTION>PHASE
OVERCURRENT> 67 2-out-of-3 Logic = ENABLED
Enabling 2-out-of-3 logic will prevent operation of the directional phase fault protection for a single phase to earth
fault. Dedicated earth-fault protection should therefore be used if required.
©2010 Siemens Protection Devices Limited
C
E
Load

Figure 3.5-4 Feeder Fault on Interconnected Network

7PG2113/4/5/6 Solkor Applications Guide
B
D
Fault 1
G
Chapter 7 Page 26 of 49