ABB REF 610 Technical Reference Manual page 22

REF 610
22
Feeder Protection Relay
Technical Reference Manual - ANSI Version
Table 4.1.4.4-1 Thermal level at constant currents
I/FLA
0.5
0.3
0
When the thermal capacity exceeds the trip level, the element will generate a trip
signal. The operate time, i.e. the time from when the element picks up 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. 4.1.4.4.-1...Fig. 4.1.4.4.-3.
τ
=
t x ln
The operate time is calculated as follows:
where
FLA = set full load current
Ip = prior load current
t = 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 element will trip within a safe time span in case
of an overload. The calculated thermal level will approach the thermal level of the
cable.
Element 49 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.
Note!
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.
Note!
The thermal level can be reset or changed via serial communication, which will
generate an event code.
Thermal level (%)
22.7
8.2
0
⎧
( )
2
⎪
I
−
⎨
FLA
( )
2
⎪ ⎩
I
FLA
1MRS 755535
( )
⎫
2
I
⎪
p
⎬
FLA
⎪ ⎭
−
. 1 1025