Thermal counter
T2PTTR applies the thermal model of two time constants for temperature
measurement. The temperature rise in degrees Celsius (°C) is calculated from the
highest of the three-phase currents according to the expression:
∆Θ
∆
=
⋅ −
+
−
(
)
⋅
−
p
I
I
T
e
p
ref
ref
t
*
*
2
1
1
1
τ
II
I
T
e
ref
ref
t
⋅
⋅ −
−
2
2
1
∆
τ
(Equation 36)
ΔΘ
calculated temperature rise (°C) in transformer
I
measured phase current with the highest TRMS value
I
ref
the set value of the
Current reference
setting (rated current of the protected
object)
T
ref
the set value of the
Temperature rise
setting (temperature rise setting (°C)
with the steady-state current I
ref
)
p
the set value of the
Weighting factor p
setting (weighting factor for the short
time constant)
Δt
time step between the calculation of the actual temperature
t
1
the set value of the
Short time constant
setting (the short heating / cooling
time constant)
t
2
the set value of the
Long time constant
setting (the long heating / cooling
time constant)
The warming and cooling following the two time-constant thermal curve is a
characteristic of transformers. The thermal time constants of the protected
transformer are given in seconds with the
Short time constant and Long time
constant settings. The Short time constant setting describes the warming of the
transformer with respect to windings. The
Long time constant setting describes the
warming of the transformer with respect to the oil. Using the two time-constant
model, the protection relay is able to follow both fast and slow changes in the
temperature of the protected object.
The
Weighting factor p setting is the weighting factor between Short time constant
τ
1
and
Long time constant τ
2
. The higher the value of the
Weighting factor
p setting, the larger is the share of the steep part of the heating curve. When
Weighting factor p =1, only Short-time constant is used. When Weighting factor p =
0, only
Long time constant is used.
Protection functions
1MRS759142 F
392
REX640
Technical Manual