Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 134 (504
)
A996A
Initial data for the set-up of the thermal image:
500 mm
2
cross sectional 66 kV copper cable is installed into ground. Its 1s permissible
short circuit current is 71.4 kA and its insulation is XLPE. The cables screen circuit is open
and the laying of the cable is flat. Its current carrying capacity is 575A in 65
̊ C and 680A
in 90 ̊ C. Reference temperature for ground installation is 15 ̊ C.
First let’s calculate estimate of the time constant
τ
from the known 1s short circuit current
related to In. (If manufacturer has not informed the time constant it can be estimated from
given short circuit withstandability current, which is usually 1s value). TOLF function uses
this same method for estimating the heating time constant.
τ
Cable
=
1s
60s ×
�
I
1s
I
N
�
2
=
1s
60s ×
�
71400A
680A
�
2
= 183.75 min
Rest of the settings are found from the initial data for the cable:
In = 680 A, Tmax = 90 ̊ C, Tamb = 15 ̊ C, Tref = 15 ̊ C and k
SF
= 1.0
As can be noted from
the results, when the
cable has been loaded
with stable current for
time which is five times
of the time constant
τ
end temperature of
68.35
̊ C is reached.
This represents 71 % of
the thermal capacity
used. According to the
data sheet with this
current temperature
should be around 65
̊ C
and can be seen that the
model is now 3 degrees
overprotecting.
Figure 3.2.11-73 Thermal image response with nominal load when the installation is
according to the presumptions.