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14.7.3.5 Unbalanced Load or Negative Sequence Protection
The protection against unbalanced load due to negative phase sequence (NPS below) of the
current is necessary in electrical rotating machines. Because the NPS current has a rotation
direction opposite to the positive phase sequence, thermal overheating of the rotor can occur. The
NPS current will namely induce in the rotor currents with double frequency, which can produce
large thermal loss in the iron core of the rotor. This results in different field intensities in the
magnetic laminated cores. Points with particularly high field intensities - "hot spots" - lead to local
overheating.
This protection function also prevents reconnection of overheated machines and handles situations
of intermittent NPS current.
Fig. 74
Start Value (Is):
Heating Parameter (K):
Reset Time (t rst):
Time Decreasing Rate:
Parameter
NPS Start Value (Is)
Heating Parameter (K)
Reset Time (t rst)
Timer Decreasing Rate
The threshold setting range is expressed in primary current Ampere [A], up to 0.40 In where In is
the Nominal Primary Current value entered in the General Settings.
Current threshold for negative sequence condition detection.
Heating parameter to vary time delay for Trip condition
The function remains in BLOCK status until the reset time has elapsed
(e.g. to block the possibility of re-closing a motor).
Parameter to vary thermal memory effect.
Values
Default
0.2 – 0.40 p.u. di In
20
2.0 - 30,0
2
0 - 2000
20
0 - 100
0
Step
Unit
Explanation
1
A
Current threshold for
negative sequence detection
0.1
s
Heating parameter
1
s
Time to reset BO after
a trip
1
%
Parameter to vary thermal
memory effect
85
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