Thermal Protection For Cables - ABB REF 610 Technical Reference Manual

1MRS 755535
4.1.4.4.
Feeder Protection Relay
Technical Reference Manual - ANSI Version
It is possible to block the tripping of an ground-fault element by applying a digital
input signal to the relay.
Note!
Elements 51N and 50N can be set to be blocked by the auto-reclose 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 ground 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 element continuously calculates the thermal capacity used as
a percentage of the cable's total thermal capacity. The thermal capacity is calculated
as follows:
⎛ ⎞
I
θ
= ------------------------ -
⎝ ⎠
×
FLA
1.05
where
θ
1)
= thermal capacity
I = phase current value
FLA = set full load current
t = time (in minutes)
τ = time constant (in minutes)
θ equals TH LEVEL
1)
"When one or several phase currents exceed the set full load current, FLA, element
49 will pickup. 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, 49ALARM the element will generate an alarm signal. The
thermal alarm 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/FLA
1.0
0.9
0.8
0.7
2
t τ ⁄
–
× ( ) ×
1 e – 100%
Thermal level (%)
90.7
73.5
58
44.4
REF 610
21