Θ
final
ref
ref
I
I
T
=
⋅
2
(Equation 15)
I
highest measured phase current
I
ref
the set value of the
Current reference
setting
T
ref
the set value of the
Temperature rise
setting (temperature rise (°C) with the
steady-state current I
ref
The ambient temperature value is added to the calculated final temperature
rise estimation. If the total value of temperature is higher than the set operate
temperature level, the
START
output is activated.
The
Current reference setting is a steady-state current that gives the steady-state
end temperature value
Temperature rise. It gives a setting value corresponding to
the rated power of the transformer.
The
Temperature rise setting is used when the value of the reference temperature
rise corresponds to the
Current reference value. The temperature values with
the corresponding transformer load currents are usually given by transformer
manufacturers.
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 16)
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
1MRS757644 H
Protection functions
620 series
Technical Manual
345
