Thermal Protection For Cables - ABB REF610 Technical Reference Manual

REF610 REF610
5.1.4.4.
30
Feeder Protection Relay
Technical Reference Manual - ANSI Version
The set pickup value of element 50N can be set to be automatically doubled in a
pickup situation, i.e. when the object to be protected is being connected to a
network. Consequently, a set pickup value below the connection inrush current level
can be selected for the element. A pickup situation is defined as a situation where
the ground-fault current rises from a value below 0.12 x 51N to a value above 1.5 x
51N within less than 60 ms. The pickup situation ends when the current falls below
1.25 x 51N and remain below for at least 200 ms. The selection is made in SGF4.
Consider carefully when using the automatically doubled setting for
the set pickup value of element 50N.
Element 50N can be set out of operation in SGF3. This state will be indicated by
dashes on the LCD and by "999" when the set pickup value is read via serial
communication. It is possible to block the tripping of an ground-fault element by
applying a digital input signal to the relay.
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 a 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:
2
⎛ ⎞
(
I
θ = ⎟ × −
1 e
⎜
×
⎝ ⎠
1 05 . FLA
θ (equals TH LEVEL)
= thermal capacity
I = phase current value
FLA = set full load current
t = time (in minutes)
τ
= time constant (in minutes)
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.
)
− τ
×
t /
100 %
1MRS755535
(1)