ABB RELION 620 Series Technical Manual page 354

Section 4
Protection functions
348
Thermal level calculator
The module calculates the thermal load considering the TRMS and negative-sequence
currents. The heating up of the motor is determined by the square value of the load
current.
However, in case of unbalanced phase currents, the negative-sequence current also
causes additional heating. By deploying a protection based on both current
components, abnormal heating of the motor is avoided.
The thermal load is calculated based on different situations or operations and it also
depends on the phase current level. The equations used for the heating calculations
are:
2

I
  
 K
θ =  + ×
 
B 2
k I k I
 ×
r
  

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2

I
  

K
θ =  + ×
 
A 2
k I k I
 ×
 r  

GUID-9C893D3E-7CAF-4EA6-B92D-C914288D7CFC V2 EN
I TRMS value of the measured max of phase currents
I set Current reference , FLC or internal FLC
r
I measured negative sequence current
2
Overload factor
k set value of
Negative Seq factor
K set value of
2
Weighting factor
p set value of
time constant
t
The equation θ is used when the values of all the phase currents are below the
B
overload limit, that is, k x I
phase currents exceeds the overload limit.
During overload condition, the thermal level calculator calculates the value of θ
background, and when the overload ends the thermal level is brought linearly from θ
to θ with a speed of 1.66 percent per second. For the motor at standstill, that is, when
B
the current is below the value of 0.12 x I
t
−
e τ
θ = θ ×
02
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θ initial thermal level when cooling begins
02
2

I

t /
2
− τ
( )
 1 e p %
× − ×

× 
r


2

I

t /
2
− τ
( )
 1 100 %
e
× − ×

× 
r 

. The equation θ is used when the value of any one of the
r A
, the cooling is expressed as:
r
1MRS757644 F
(Equation 17)
(Equation 18)
in
B
A
(Equation 19)
620 series
Technical Manual