Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 358 (504
)
A996A
Thermal image for the TOLT function is calculated according to equation described below:
θ
t%
= ��θ
t - 1
- �
I
MAX
I
N
× k
SF
× k
AMB
�
2
× e
-
t
τ
1
/ τ
2
� + �
I
MAX
I
N
× k
SF
× k
AMB
�
2
� × 100%
, where
θ
t%
= Thermal image status in percent of the maximum thermal capacity available
θ
t-1
= Thermal image status in previous calculation cycle (the memory of the function)
I
MAX
= Measured maximum of the three TRMS phase currents
I
N
= Current for the 100 % thermal capacity to be used (pick-up current in p.u., with this
current t
max
will be achieved in time
τ
x 5)
k
SF
= Loading factor (service factor) coefficient, maximum allowed load current in per
unit value depend of the protected object or cable/line installation
k
AMB
= Temperature correction factor either from linear approximation or settable 10
point thermal capacity curve.
τ
1
= Thermal heating time constant of the protected object (in minutes)
τ
2
= Thermal heating time constant of the protected object (in minutes)
e = Euler’s number
t = Calculation time step in seconds (for NP900 IED:s 0.005s)
The basic operating principle of the thermal replica is based into that the nominal
temperature rise is achieved when the protected object is loaded with nominal load in
nominal ambient temperature. When the object is loaded with nominal load for time equal
its heating constant tau (
τ ), 63% of the nominal thermal capacity is used. When the
loading continues until five times this given constant the used thermal capacity indefinitely
approaches to 100% but never exceeds it. With a single time constant model cooling of
the object follows this same behavior reversible to the heating when the current feeding is
completely zero.