Thermal Protection For Cables - ABB REF 610 Technical Reference Manual

REF 610 REF 610
5.1.4.4.
28
Feeder Protection Relay
Technical Reference Manual
current rises from a value below 0.12 x I
than 60 ms. The start situation ends when the current falls below 1.25 x I
remain below for at least 200 ms. The selection is made in SGF4.
Consider carefully when using the automatically doubled setting for
the set start value of stage I
Stage I >> can be set out of operation in SGF3. This state will be indicated by
0
dashes on the LCD and by "999" when the set start value is read via serial
communication. It is possible to block the tripping of an earth-fault stage by
applying a digital input signal to the relay.
Stages I > and I
0
function.

Thermal protection for cables

The thermal protection detects long-time overloads during normal operation.
Prolonged overloading results in the thermal stress capacity of the cable being
exceeded, which degrades the insulation of the cable, which in turn may cause a
short circuit or an earth fault. The heating up of the cable follows an exponential
curve, the levelled-out value of which is determined by the squared value of the load
current. The thermal protection may equally well be used to protect dry-type
transformers, capacitor banks, busbars and overhead lines, for instance.
The thermal protection stage continuously calculates the thermal capacity used as a
percentage of the cable's total thermal capacity. The thermal capacity is calculated
as follows:
2
⎛ ⎞
(
I
θ −
×
⎜ ⎟ 1 e
=
×
⎝ ⎠
1.05 I
θ
θ
= thermal capacity
I = phase current value
I = set full load current
θ
t = time (in minutes)
τ
= time constant (in minutes)
When one or several phase currents exceed the set full load current, I
start. At the same time, the thermal capacity will start to increase at a rate depending
on the current amplitude and the prior load of the cable.
When the thermal capacity, influenced by the thermal history of the cable, exceeds
the set alarm level, θ
>, the stage will generate an alarm signal. The thermal alarm
a
can be used to avoid unnecessary tripping due to a beginning overload. The thermal
level at various constant currents are presented in the table below:
> to a value above 1.5 x I
0
>>.
0
>> can be set to be blocked by the auto-reclose
0
)
τ
−
×
t /
100 %
1MRS755310
> within less
0
> and
0
(1)
, stage θ> will
θ