Areva MiCOM P125, Technical Manual

Page: 196 / 362

P12y/EN TD/D32 Technical Data
Page 28/28 MiCOM P125/P126 & P127
12.2 Thermal Overload Curves
The thermal time characteristic is given by:
( ) ( )
( )
² ²
² ²
p
FLC
t
I I
kxI I
e
−
−
=





 −
τ
Where:
t = Time to trip, following application of the overload current, I
τ
= Heating and cooling time constant of the protected plant equipment
I = Largest phase current
I
FLC
= Full load current rating (relay setting 'Thermal Trip')
k = 1.05 constant, allows continuous operation up to < 1.05 I
FLC
I
P
= Steady state pre-loading current before application of the overload
The time to trip varies depending on the load current carried before application of the
overload, i.e. whether the overload was applied from "hot" or "cold".
Curves of the thermal overload time characteristic are given in Technical Data.
The mathematical formula applicable to MiCOM Relays is the following








θ −
θ
=
² trip ² K
² - K²
In Te Trip t
Where :
t Trip = Time to trip (in seconds)
Te = Thermal time constant of the equipment to be protected (in seconds)
K = Thermal overload equal to Ieq/k I
θ
> with:
Ieq = 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
alarm = Initial thermal state.
θ
trip = Trip thermal state
The settings of these parameters are available in the various menus. The calculation of the
thermal state is given by the following formula:





 −
τ





 −
+ τ
Θ +








−






> Θ
= Θ
Te
t
Te
t
²
eq
1
e e 1
kxI
I
θ
being calculated every 100ms.