ABB RELION 620 Series Technical Manual page 334

Section 4
Protection functions
328
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
and Long time constant τ
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.
GUID-E040FFF4-7FE3-4736-8E5F-D96DB1F1B16B V1 EN
Figure 162: Effect of the Weighting factor p factor and the difference between the
two time constants and one time constant models
The actual temperature of the transformer is calculated by adding the ambient
temperature to the calculated temperature.
Θ = ∆ + Θ Θ
amb
GUID-77E49346-66D2-4CAB-A764-E81D6F382E74 V2 EN
Θ temperature in transformer (°C)
ΔΘ calculated temperature rise (°C) in transformer
Env temperature Set setting or measured ambient temperature
Θ set value of the
amb
. The higher the value of the Weighting factor p setting, the
2
1MRS757644 F
1
(Equation 16)
620 series
Technical Manual