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Blocking Logic Selectivity Settings Menu
Blocking Logic Selectivity Settings Menu
(Displayed
briefly as
Blocking Settings
on the relay menus) is a
control menu that enables the users to utilize the
Blocking Logic Selectivity
scheme on a number of relays
that are protecting the same circuit, and are installed
back-to-back. Applying the
Blocking Logic Selectivity
scheme on systems eliminates the need for time
grading selectivity schemes, which, by their nature,
prolong the clearance times of faults and can be
applied with relatively small number of steps of
selectivity.
The principle of the
Blocking Logic Selectivity
scheme is
a follows: when a fault current flows through the
current transformers of two or more relays which are
located in series regarding to the primary circuit, the
upstream relay is blocked by the downstream relay by
means of a dedicated input. That is achieved utilizing a
pilot line, which conducts an AC or DC signal from the
downstream relay to the input of the upstream relay.
Regardless of the number of the relays on the system
that sense the fault, each relay is blocked by the
nearest downstream relay instantly and independently;
that results the only relay that is not blocked to trip –
which is automatically the nearest relay to the fault
location. Note that the relay that is nearest to the fault
location is not blocked, for that the next downstream
relay does not read any current.
The signal conduction needed for the utilization of
blocking logic selectivity can be achieved by conducting
DC or AC signals directly between relays that are
located relatively close; when the distances grow, an
optical communication interface application may be
needed.
It is determined by the options on the menu (shown in
the middle and lower pictures) if a relay will be blocked
against generating trip signals when it receives a signal
on its blocking input. If the current reading of a relay is
exceeding the threshold current value of a protection
function at the instant it receives the blocking signal,
the relay checks the blocking settings and decides
whether to intervene or behave neutral. If the blocking
setting of that protection function is set as Active, then
the relay will not generate any tripping signal;
otherwise, it will start the pre-adjusted protection
procedure.
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