ABB REF 610 Technical Reference Manual page 29

1MRS755310
Feeder Protection Relay
Technical Reference Manual
Table 5.1.4.4.-1 Thermal level at constant currents
I/I
θ
1.0
0.9
0.8
0.7
0.5
0.3
0
When the thermal capacity exceeds the trip level, θ
signal. The operate time, i.e. the time from when the stage starts until it trips, is
determined by the time constant, τ, and depends on the cable (cable cross section
area and cable rated voltage). The time constant is provided by the cable
manufacturer. For a 22 kV cable, the typical time constant is 20 minutes. For operate
times, see Fig. 5.1.4.4.-1...Fig. 5.1.4.4.-3. The operate time is calculated as follows:
(
⎧
( )
2
−
I I / I I
⎪
θ
p
= × τ
⎨
t ln
( )
2
−
⎩ ⎪
I I / 1 1025 .
θ
I = phase current value
I = set full load current
θ
I = prior load current
p
t = operate time (in minutes)
τ
= time constant (in minutes)
ln = natural logarithm
At power up, the thermal level will be set to 75 percent of the thermal capacity of
the cable. This will ensure that the stage will trip within a safe time span in case of
an overload. The calculated thermal level will approach the thermal level of the
cable.
Stage θ> can be set out of operation in SGF3. This state will be indicated by dashes
on the LCD and by "999" when the set full load current is read via serial
communication.
At an alarm level below 75 percent, connecting the auxiliary supply to
the relay will cause a thermal alarm due to the initialization of the
thermal level to 75 percent of the thermal capacity of the cable. The
thermal level can be reset via the HMI during power up.
The thermal level can be reset or changed via serial communication,
which will generate an event code.
)
⎫
2
/
⎪
θ
⎬
⎭ ⎪
REF 610 REF 610
Thermal level (%)
90.7
73.5
58
44.4
22.7
8.2
0
>, the stage will generate a trip
t
(2)
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