ABB RELION 620 Series Technical Manual page 388

Section 4
Protection functions
382
The ambient temperature value is added to the calculated final temperature rise
estimation. If the total value of temperature is higher than the set trip temperature level, the
PICKUP 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
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:
2
 
 
I
 
∆Θ = p *   * T ⋅
ref
 
 
I
ref
 
 
 
GUID-27A879A9-AF94-4BC3-BAA1-501189F6DE0C V2 EN
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)
Temperature rise setting (temperature rise setting (°C) with the steady-state current
Tref the set value of the
I )
ref
Weighting factor p setting (weighting factor for the short time constant)
p the set value of the
Δt time step between the calculation of the actual temperature
Short time constant setting (the short heating / cooling time constant)
the set value of the
t
1
Long time constant setting (the long heating / cooling time constant)
the set value of the
t
2
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
Long time constant τ . The higher the value of the Weighting factor p setting, the larger is
2
∆ t 
 

−
I I

 
1 ( )
1 − e τ + 1 − p ⋅ 


 
I
ref


 

1MAC504801-IB E
2
 ∆ t
 

−

 
2
 ⋅ T ⋅ 1 − e τ
ref


 


 

Technical Manual
(Equation 25)
and
1
620 series ANSI