- 77 -
I
Θ
> Thermal Overload Protection Menu
Thermal Overload Protection is symbolized as I
Θ
> in
IEC Standards, and coded as 49 in IEEE/ANSI
Standards.
I
Θ
> symbol indicates that the protection function
measures the phase RMS current and behaves
according to the thermal overload protection formula.
IEC 60255-8 compliant thermal overload formula is
given below.
where
t trip time (minutes),
T
e
thermal constant (minutes),
I RMS load current (A),
I
Θ
set current (A),
k trip threshold multiplier (-),
%Θ
p
overload pre-heating (%),
%Θ
trip
overload trip heating (%).
Thermal overload protection is a unique function of
protection which is usually utilized to provide
overloading protection for power transformers, power
cables, motors and overhead lines at current range of
(1.0- 1.5) I
r
.
By combining I
Θ
> thermal overload protection and I>
overcurrent protection functions in appropriate
schemes within the boundaries of (1.0- 1.5) I
r
,
providing the optimum balance between maximum
supply continuity and accurate protection for electrical
power systems is easier.
Thermal overload protection plays a critical role in
modern systems where transient overcurrent and high-
order harmonics are observed frequently. Thermal
overload protection function provides maximum supply
continuity by giving the system sufficient time to
recover transient overcurrent, which do not perform a
significant stress on components and need not to be
cleared instantaneously; while providing a solid
protection by monitoring “hidden” effects of high-order
harmonics on elements of the system and intervening
when necessary.
To Next Page
Loading...
Need help?
General
