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1MRS 755310
4.1.4.4.
Feeder Protection Relay
Technical Reference Manual
It is possible to block the tripping of an earth-fault stage by applying a digital input
signal to the relay.
Note!
Stages I
> and I
>> can be set to be blocked by the auto-reclose function.
0
0
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
×
(
-------------------- -
=
×
1.05 I
θ
where
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:
Table 4.1.4.4-1 Thermal level at constant currents
I/I
n
1.0
0.9
0.8
0.7
0.5
0.3
0
t τ ⁄
–
)
×
1 e
–
100%
Thermal level (%)
90.7
73.5
58
44.4
22.7
8.2
0
REF 610
, stage θ> will
θ
21
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