The protection element also provides a non-sensitive area to avoid instability due to small asymmetries and unbalances that can generally be present in the network system. This condition is represented by the characteristic reproduced below, where the hatched area is the tripping zone.
When short time delay protection is used with autoreclose, the scheme is normally arranged
to block the instantaneous protection after the first trip. Therefore, if the fault persists after reclosing, time-graded protection will give discriminative tripping with fuses or other protection
devices, resulting in the isolation of the faulted section. However, for certain applications,
where the majority of the faults are likely to be transient, it is not uncommon to allow more
than one instantaneous trip before the instantaneous protection is blocked.
Some schemes allow a number of re-closings and time graded trips after the first
instantaneous trip, which may result in the burning out and clearance of non-permanent
faults. Such an approach may also be used to allow fuses to operate in teed feeders where
the fault current is low.
Any decision to apply the autoreclose function would be influenced by all data known on the
frequency of transient faults (for instance feeders which consist partly of overhead lines and
partly of underground cables). When a significant proportion of the faults are permanent, the
advantages of autoreclose are small, particularly since re-closing on to a faulty cable is likely
to aggravate the damage.
5.2 Description of the function
5.2.1 Autorecloser activation
The autoreclose function is activated using “AUTOMAT. CTRL/ PROTECTION G1” menu.
The same settings apply for the Menu PROTECTION G2.
The autoreclose function of the relay is available only if the following conditions are verified:
− The auxiliary contact of the CB status 52a must be connected to the relay.
Refer to the “AUTOMAT. CTRL/Inputs” menu
− The trip output relay RL1 must not be latched to the earth and/or phase protection
function.
Refer to the “AUTOMAT. CTRL/Latch functions” menu
NOTE: If the auxiliary supply is lost during an autoreclose cycle, the
autoreclose function is totally disabled.
In addition to Autoreclose settings, the user will be able to fully link the autoreclose function
to the protection function using the menus “PROTECTION G1/Phase OC” and
“PROTECTION/E/Gnd”.
5.2.2 Logic Inputs
The autoreclose function has four inputs that can be assigned to the autoreclose logic.
These inputs can be opto-isolated inputs configured for that under the “AUTOMAT. CTRL”
menu. External contacts can then be wired to be used as an input and influence the
autorecloser scheme. These 4 inputs are:
− one external CB fail,
− two external starting orders,
− one external blocking order.
The following table gives the “AUTOMAT.CTRL/Inputs” menu assigned to the autoreclose
logic input. The second column presents the menu disabling the function if not assigned in
the “PROTECTION G1/Autoreclose” menu (Setting = No).
