2 Functions
70
7UM61 Manual
C53000-G1176-C127-3
2.10 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
Unbalanced load protection detects unbalanced loads of three-phase induction ma-
chines. Unbalanced loads create a counter-rotating field which acts on the rotor at
double frequency. Eddy currents are induced at the rotor surface leading to local over-
heating in rotor end zones and slot wedges. Another effect of unbalanced loads is
overheating of the damper winding. In addition, this protection function may be used
to detect interruptions, short circuits and polarity problems with current transformers.
It is also useful in detecting 1-pole and 2-pole faults with magnitudes lower than the
load currents.
2.10.1 Functional Description
Unbalanced Load
Determination
The unbalanced load protection of 7UM61 filters the fundamental component from the
phase currents and splits it into symmetrical components. It evaluates the negative-
phase sequence system, the negative phase-sequence current I
2
. If the negative
phase-sequence current exceeds a set threshold value, the trip timer starts. A trip
command is transmitted on timeout.
Warning Stage If the value of the continuously permissible, negative phase-sequence current I2> is
exceeded, after expiry of a set time T WARN a warning message “I2> Warn” is
issued (see Figure 2-16).
Thermal Character-
istic
The machine manufacturers indicate the permissible unbalanced load by means of the
following formula:
The asymmetry factor depends on the machine and represents the time in seconds
during which the generator can be loaded with a 100 % unbalanced load. This factor
is typically in a range between 5 s and 30 s.
The heating up of the object to be protected is calculated in the device as soon as the
permissible unbalanced load I2> is exceeded. The current-time area is calculated
constantly to ensure correct consideration of different load cases. As soon as the
current-time-area ((I
2
/I
N
)
2
· t) has reached the K asymmetry factor, the thermal charac-
teristic is tripped.
Limitation To avoid overfunctioning of the thermal tripping stage during asymmetrical short cir-
cuits, the input current I
2
is restricted. This limit is either 10 · I
2adm.
or the setting value
of the I
2
>> stage (Addr. 1701), whichever is smaller. Above this current value the trip-
ping time of the thermal function is constant. In addition the thermal memory is limited
to 200% of the tripping temperature. This avoids prolonged cooling after a delayed
short circuit tripping.
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