32
I
eq
k.I
θ
>
5.3 Thermal Overload Protection
Thermal overload protection can be used to prevent damages to the equipment of the electrical plant.
A prolonged overloading causes excessive heating, which may result in deterioration of the
insulation, or in extreme cases, insulation failure.
Load current is used to calculate the heating and cooling effect of the equipment to be protected.
The highest phase current is automatically used as input information for the thermal model.
The thermal overload protection can be set with both alarm and trip stages,
θ
Trip % and
θ
Alarm
%, with 5% below the set % for resetting.
The heating within any plant equipment, such as cables or transformers, is of resistive type (I²R x t).
Thus the thermal time characteristic used in the relay is based on current squared, integrated over
time.
Protection equipment is designed to operate continuously at a temperature corresponding to its full
load rating, where heat generated is balanced with heat dissipated. Over-temperature conditions
occur when currents in excess of rating flow for a certain period of time. It can be shown that
temperatures during heating follow exponential time constants and a similar exponential decrease of
temperature occurs during cooling.
In order to apply this protection element, the thermal time constant (T
θ
) of the plant equipment to
be protected is therefore required.
The calculation of the Time to trip is given by:
T
trip
=
Time to trip (in seconds)
T
θ
=
Thermal time constant of the protected element (in seconds)
K
=
I
eq
=
Equivalent current corresponding to the RMS value of the largest phase current.
I
θ
>
= Full load current rating given by the national standard or by the supplier.
k
= Factor associated to the thermal state formula.
θ
= Initial thermal state. If the initial thermal state = 30% then
θ
= 0.3
θ
trip
=
Trip thermal state. If the trip thermal state is set at 100%, then
θ
trip = 1
The settings of these parameters are available in the menus:
PROTECTION G1/G2 – Thermal OL
The calculation of the thermal state is given by the following formula:
θ
being calculated every 20ms.
θ
τ+1
=
K
2
(1-e ) +
-t
T
θ
θ
τ
e
-t
T
θ
T
trip
= T
θ
ln
-
-
θ
θ
trip
K
K
2
2
(
)
Valid when:
K
2
>
θ
K
2
>
θ
trip
