ABB SPAM 150 C, Техническая информация

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6
Example 1.
Protecting a
circuit breaker
controlled motor
in an isolated
network
Given data of the squirrel cage motor to be pro-
tected:
Rated power P
nm
4500 kW
Rated voltage U
nm
3300 V
Rated current I
nm
930 A
Starting current I
s
6.2 x I
nm
Starting time t
s
11 s
Stall time permitted
under cold conditions
19 s
Ambient temperature 20
°
C
C.T. current ratio 1000 /1A
Network earth-fault current at
fully developed fault (100 % e/f )
10 A
Earth-fault sensitivity required 20 %
Calculation of settings:
Due to the ambient temperature <40
°
C, the full
load current (FLC) is increased by 5%:
FLC = 1 .05 x 930 A
The relay setting I
θ
is then
I
θ
/ I
n
= = 0.98
The motor is directly started, .i.e. p = 50 %.
The setting t
6x
is selected from the time/cur-
rent characteristic corresponding to a hot mo-
tor condition. This will enable one hot / two
cold starts.
First the ratio between the starting current and
the full load current is calculated :
I
s
/ I
θ
= I
s
/ ( 1.05 x I
nm
) = 6.2 / 1.05
≈
5.9
A setting t
6x
≈
25 s is selected, permitting a start-
ing time which is a few seconds longer than the
given motor starting time.
As the safe stall time is longer than the start-up
time no speed switch arrangements are needed.
For a single startup from a cold condition, the
thermal unit would trip only after about 25 sec-
onds, which is longer than the permitted 19 sec-
onds. A single startup is instead protected by
the startup supervision, i.e. I
s
2
x t
s
. The start-
ing current is set directly to the value I
s
= 6.2 x
930 A x 1/1000
≈
6.1 x In, whereas the startup
time is set at about 10% above the normal
startup time in order to give a safety margin for
the operation. Thus t
s
is set at 11 s x 1.1
≈
12 s.
The prior alarm setting can be selected e.g.
Θ
a
=80...90 % for a reasonably early alarm.
Fig. 1. How to find the setting t
6x
from the hot
thermal curve
As the start-up consumes 11 s / 25 s
≈
45% of
the full thermal capacity, the restart inhibit level
Θ
i
must be set lower than 55%, e.g. at 50%.
The cooling factor k
c
is set e.g. at 4 because the
motor is a normal, totally enclosed unit, cooled
by a fan on the the rotor shaft.
If we want the high-set overcurrent to double
as a running stall protection, the basic setting
should be lower than the startup/stall current
and be doubled during start. Thus this element
can be set to a value of 75 ...90 % of I
s
:
I>> = 0.75 x 6.2 x 930 A x 1 / 1000
≈
4.3 x 1A.
Doubling during the start-up -> SGF/2 =1.
Generally a setting as low as 75% should be giv-
ing good results but if the inrush current should
cause trippings during startup, a higher setting
must be chosen.
For the earth-fault a core balance transformer
with a CT ratio of 100 / 1 A is used.
The earth-fault protection should detect an
earth-fault of 20% of the fully developed fault
current, i.e.: 20% x 10 A = 2.0 A
I
0
= 2 A x 1 A / 100 A = 2.0% x 1 A
50
10
20
5
1
2
3
4
30
40
1 2 3 4 5 10 6 8 I/I θ
1.05
t 6x
[s]
120
60
30
20
15
10
5
2.5
1.05 x 930 A x 1 A
1000 A x 1A