Table of Contents
Advertisement
Example 6
How to set the
phase unbalance
and phase
reversal unit
16
The phase unbalance and phase reversal unit in
SPAM 150 C is based on the measurement of
the difference between the three phase currents.
The difference between the highest and the low-
est phase current is compared in per cent of the
highest one.
The benefits with this arrangement is a lower
sensitivity to frequency variations and harmon-
ics in the currents. Furthermore the phase re-
versal protection is separated as a stand-alone
unit not dependent of the asymmetry protec-
tion.
In a full broken phase condition, the unbalance
reading of SPAM 150 C is 100%. This can be
compared to the reading of 57.8% with a relay
measuring unbalance on a negative phase se-
quence current basis (e.g. SPAM 110). Thus
when comparing n.p.s. values to current differ-
ence values, a conversion factor of 0.578 is used.
Example:
We want to have an unbalance unit sensitivity
of about 15% in terms of negative phase se-
quence current on a motor. The unbalance pro-
tection should operate with an operating time
of 10 s at the 15 % amount of unbalance.
We calculate:
15% (NPS) = 15 / 0.578 ≈26 % (∆I )
Thus we can set ∆I> =25% on the relay
From the trip time chart we can see that a trip
time of the desired 10 s at 25% is achieved by
using a time setting t∆ of 60 s. The unbalance
unit will not operate for unbalance levels below
the set 25% but at this level the trip time is
10 s and for higher degrees of unbalance the
time decreases down to 1 s at a full single phas-
ing condition.
The phase unbalance unit is selected to be op-
erative with the switch SGF/5 =1 and the phase
reversal protection is made operative with switch
SGF/6 = 1.
Finally the output relay is selected with switches
SGR1/5 or SGR2/5.
120
100
t
80
s
60
50
40
t∆
s
30
120
100
20
80
60
10
40
8
6
5
20
4
3
2
1
10
20
30
Figure. 12 The trip time diagram for the phase
unbalance unit.
40
50
60
70 80 90 100
∆I/ IL
[%]
Hide quick links:
Advertisement
Table of Contents
