Thermal counter
49T 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
GUID-27A879A9-AF94-4BC3-BAA1-501189F6DE0C V2 EN
(Equation 14)
ΔΘ calculated temperature rise (°C) in transformer
I
measured phase current with the highest TRMS value
Iref the set value of the
Current reference setting (rated current of the protected object)
Tref 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.
Section 4
1MAC059074-MB A
Protection functions
278
615 series ANSI
Technical Manual
Summary of Contents for Relion 615 series
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