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RFD TEHNICAL DESCRIPTION.doc KON-215-60-33.2-v.1.0-E
current protection. As input values of the
function we are using effective values of all
three phase currents. Using the measured
values we calculate minimal and maximal values
i.e. the unbalance current.
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max
minmax
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LL III
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Transformer thermal overload
protection (49T-Thermal Ov.)
Protection against thermal overload of
transformers is intended for thermal protection
of the transformer. The principle of operation is
based on the function of the thermal model with
two time constants, which are used for heating
and cooling the transformer. In both cases, the
protection algorithm is used to calculate the
temperature of the exponential curve. The term
used for this is:
When the calculated value of the current
assumes a value greater than or equal to the set
value it will generate the predefined signals.
Trip circuit failure protection
(50BF-CBFP)
After the general trip warrant generation, it is
possible that the trip circuit does not trip for
some reason. Circuit failure reasons can be
various, from line braking towards tripping coil,
to a fault in the circuit itself. In order to cut power
supply to the failed area, the trip circuit failure
protection is activated, whose task is to control
whether the trip based on warrant is realized,
and if not to either trip the breaker through the
second tripping circuit, or to trip the
corresponding next trip circuit that supplies
power to the failure. The trip circuit supervision
is also possible based on signal contact, and
current size through the switch. If the value ON
is chosen for parameter CB-contact, the switch
state function deduces based on the state of the
signal switch. If the chosen value is OFF, the
function deduces the state of the switch based
on measured current through the switch. If that
current exceeds the parameter set by function
Current threshold (0,05ln=default), the function
deduces that the switch remains closed. After
the expiration of time function set by parameter
t-BF, the function will generate the trip signal on
the corresponding relay output. Given that at
RFD devices we have two trip circuits only the
primary one is monitored and if he is not tripped
function generates a trip signal.
Trip circuit supervision
(74TC-Trip C.S.)
In case of trip circuit failure, the trip command
will not cause breaker tripping. Such a state is
extremely dangerous; therefore trip circuit
supervision is used in order to alert personnel to
failure as soon as possible. Trip circuit control is
realized using adequate binary inputs.
Depending on binary input state, the function
deduces whether the trip circuit is in sound or
erroneous state. Two connectivity schemes are
possible –with one or two binary inputs. When
supervising the trip circuit with one binary input
(TCS2), the binary input shall be in lead state,
with a working trip circuit with the breaker on
and off, required that the TRIP contact is not
closed. In order to avoid trip circuit signal failure
on protection pickup, a definite time
characteristic time delay is integrated.
TC failure signal will be generated after
expiration of the time delay. It is necessary to
set the time delay so it is longer than the
duration of TRIP relay closure. Such
connectivity keeps the TC failure signal even in
case the TRIP relay contacts remain
permanently closed after tripping.
Figure 10. Primary trip circuit failure supervision
As with the device RFD there is a switch at
primary and secondary side of the transformer
so there are two control tripping circuit which
