Functions
2.12 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
91
2.12 Unbalanced Load (Negative Sequence) Protection (ANSI 46)
The unbalanced load protection detects asymmetrical loads of three-phase induction machines. Unbalanced
loads create a counter-rotating field which acts on the rotor at double frequency. Eddy currents are induced on
the rotor surface, leading to local overheating at the transition between the slot wedges and the winding bun-
dles. Another effect of unbalanced loads is overheating of the damper winding. In addition, this protection func-
tion may be used to detect interruptions, faults, and polarity problems with current transformers. It is also useful
for detecting 1-pole and 2-pole faults with magnitudes lower than the load currents.
2.12.1 Functional Description
Determining the Unbalanced Load
The unbalanced load protection of 7UM62 filters the fundamental harmonic components from the phase cur-
rents into their symmetrical components. These are used to evaluate the negative-phase sequence system, i.e.
the negative phase-sequence current I
2
. If the negative phase-sequence current exceeds a parameterized
threshold value, the trip time starts. A trip command is transmitted as soon as this trip time has expired.
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 2-23).
Thermal Characteristic
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. In this context, the current-time-area is calculated constantly to ensure a correct consid-
eration of various load cases. As soon as the current-time-area ((I
2
/I
N
)
2
· t) has reached the K asymmetry factor,
the thermal characteristic is tripped.
Limitation
To avoid overfunctioning of the thermal tripping stage during asymmetrical short circuits, the input current I
2
is
restricted. This limit is either 10 · I
2adm.
or the setting value of the I
2
>> stage (addr. 1706), whichever is smaller.
Above this current value the tripping 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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